ELECTRONIC DEVICES AND CIRCUITS

UNIT-I: DIODES, SPECIAL DIODES AND APPLICATIONS

PN diode – Operation – Voltage-current characteristics – Transition and diffusion capacitance – Reverse recovery time – Diode models – Applications – Half-wave and Full-wave rectifiers and filters – Power supply regulators – Avalanche and Zener breakdown – Zener diodes –Applications – Varactor and optical diodes.

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HISTORY NOTE

Niels Henrik David Bohr (October 7, 1885–November 18, 1962) was a Danish physicist, who made important contributions to understanding the structure of the atom and quantum mechanics by postulating the “planetary” model of the atom. He received the Nobel Prize in physics in 1922. Bohr drew upon the work or collaborated with scientists such as Dalton, Thomson, and Rutherford, among others and has been described as one of the most influential physicists of the 20th century.

Atom

The smallest particle of an element that retains the characteristics of that element

According to the classical Bohr model, atoms have a planetary type of structure that consists of a central nucleus surrounded by orbiting electrons

Protons & neutrons

The nucleus consists of positively

charged particles called protons

and uncharged particles called

neutrons

Electrons

The basic particles of negative charge are called electrons.

Example : Hydrogen atom which has one proton and one electron

Helium atom has two protons and two neutrons in the nucleus and two electrons orbiting the nucleus.

• The Maximum Number of Electrons in Each Shell

Valence Electrons

Electrons that are in orbits farther from the nucleus have higher energy and are less tightly bound to the atom than those closer to the nucleus Electrons with the highest energy exist in the outermost shell of an atom and are relatively loosely bound to the atom. This outermost shell is known as the valence shell, and electrons in this shell are called valence electrons

Ionization

• When an atom absorbs energy, the valence electrons can easily jump to higher energy shells. If a valence electron acquires a sufficient amount of energy, called ionization energy
• The process of losing a valence electron is known as ionization, and the resulting positively charged atom is called a positive ion.
• The escaped valence electron is called a free electron.
• The atom that has acquired the extra electron is called a negative ion

Materials Used in Electronic Devices

• Classified into three groups:
• Conductors
• Semiconductors
• Insulators
• Insulators - An insulator is a material that does not conduct electrical current under normal conditions,

Examples of insulators are rubber, plastics, glass, mica, and quartz.

• Conductors - A conductor is a material that easily conducts electrical current. Most metals are good conductors, which are characterized by atoms with only one valence electron very loosely bound to the atom. conductors.

Examples such as copper (Cu), silver (Ag), gold (Au), and aluminum (Al)

• Semiconductors - A semiconductor is a material that is between conductors and insulators in its ability to conduct electrical current. A semiconductor in its pure (intrinsic) state is neither a good conductor nor a good insulator

Examples such as Antimony (Sb), arsenic (As), astatine (At), boron (B), polonium (Po), tellurium (Te), silicon (Si), and germanium (Ge)

Band Gap

• The difference in energy between the valence band and the conduction band is called an energy gap or band gap.

Current in Semiconductors

Conduction Electrons and Holes

• An intrinsic (pure) silicon crystal at room temperature has sufficient heat (thermal) energy for some valence electrons to jump the gap from the valence band into the conduction band, becoming free electrons. Free electrons are also called conduction electrons
• When an electron jumps to the conduction band, a vacancy is left in the valence band within the crystal. This vacancy is called a hole.

Electron and Hole Current

• Electron Current

When a voltage is applied across a piece of intrinsic silicon the thermally generated free electrons in the conduction band, which are free to move randomly in the crystal structure, are now easily attracted toward the positive end.

This movement of free electrons is one type of current in a semi conductive material and is called electron current

• Hole Current

Although current in the valence band is produced by valence electrons, it is called hole current to distinguish it from electron current in the conduction band.

N-Type and P-Type Semiconductors

• Semiconductive materials do not conduct current well and are of limited value in their intrinsic state.
• This is because of the limited number of free electrons in the conduction band and holes in the valence band Intrinsic silicon (or germanium) must be modified by increasing the number of free electrons or holes to increase its conductivity and make it useful in electronic devices.
• This is done by adding impurities to the intrinsic material. Two types of extrinsic (impure) semiconductive materials, n-type and p-type, are the key building blocks for most types of electronic devices.
• DOPING

Since semiconductors are generally poor conductors, their conductivity can be drastically increased by the controlled addition of impurities to the intrinsic (pure) semiconductive material. This process, called doping, increases the number of current carriers (electrons or holes).

• The two categories of impurities are n-type and p-type.

N-Type Semiconductor

• To increase the number of conduction-band electrons in intrinsic silicon, pentavalent impurity atoms are added, These are atoms with five valence electrons such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb).

Majority and Minority Carriers

• Since most of the current carriers are electrons, silicon (or germanium) doped with pentavalent atoms is an n-type semiconductor (the n stands for the negative charge on an electron). The electrons are called the majority carriers in n-type material.
• Although the majority of current carriers in n-type material are electrons, there are also a few holes that are created when electron-hole pairs are thermally generated. These holes are not produced by the addition of the pentavalent impurity atoms. Holes in an n-type material are called minority carriers.

Donor

• This extra electron becomes a conduction electron because it is not involved in bonding.
• Because the pentavalent atom gives up an electron, it is often called a donor atom

P-Type Semiconductor

• To increase the number of holes in intrinsic silicon, trivalent impurity atoms are added. These are atoms with three valence electrons such as boron (B), indium (In), and gallium (Ga).
• All three of the boron atom’s valence electrons are used in the covalent bonds; and, since four electrons are required, a hole results when each trivalent atom is added. Because the trivalent atom can take an electron.
• The number of holes can be carefully controlled by the number of trivalent impurity atoms added to the silicon.

Majority and Minority Carriers

• Since most of the current carriers are holes, silicon (or germanium) doped with trivalent atoms is called a p-type semiconductor. The holes are the majority carriers in p-type material. Although the majority of current carriers in p-type material are holes,
• There are also a few conduction-band electrons that are created when electron-hole pairs are thermally generated.

acceptor

Since four electrons are required, a hole results when each trivalent atom is added.

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Because the trivalent atom can take an electron, it is often referred to as an acceptor atom

The PN Junction

• A p-type material consists of silicon atoms and trivalent impurity atoms such as boron. The boron atom adds a hole when it bonds with the silicon atoms
• An n-type silicon material consists of silicon atoms and pentavalent impurity atoms such as antimony. As you have seen, an impurity atom releases an electron when it bonds with four silicon atoms.
• If a piece of intrinsic silicon is doped so that part is n-type and the other part is p-type, a pn junction forms at the boundary between the two regions and a diode is created

Formation of the Depletion Region

When the pn junction is formed, the n region loses free electrons as they diffuse across the junction. This creates a layer of positive charges (pentavalent ions) near the junction.

As the electrons move across the junction, the p region loses holes as the electrons and holes combine. This creates a layer of negative charges (trivalent ions) near the junction. These two layers of positive and negative charges form the depletion region

The depletion region is formed very quickly and is very thin compared to the n region and p region

Formation of the Depletion Region

• In the depletion region there are many positive charges and many negative charges on opposite sides of the pn junction. The forces between the opposite charges form an electric field.

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• Barrier potential
• The potential difference of the electric field across the depletion region is the amount of voltage required to move electrons through the electric field. This potential difference is called the barrier potential and is expressed in volts

PN diode

• A modern diode is a two-terminal semiconductor device formed by two doped regions of silicon separated by a pn junction
• a diode is made from a small piece of semiconductor material
• in which half is doped as a p region and half is doped

as an n region with a pn junction and

depletion region in between

• The p region is called the anode
• The n region is called the cathode

Forward Bias

• Apply a dc voltage across it. Forward bias is the condition that allows current through the pn junction.
• Negative side of VBIAS is connected to the n region of the diode and the positive side is connected to the p region
• The resistor limits the forward current to a value that will not damage the diode.
• Bias voltage, VBIAS must be greater than the barrier potential (VB).
• the negative side of the bias-voltage source “pushes” the free electrons, which are the majority carriers in the n region, toward the pn junction.
• This flow of free electrons is called electron current.
• Once in the p region, these conduction electrons have lost enough energy to immediately combine with holes in the valence band
• Now, the electrons are in the valence band in the p region.
• Since unlike charges attract, the positive side of the bias-voltage source attracts the valence electrons toward the left end of the p region.
• The holes in the p region provide the medium or “pathway” for these valence electrons
• The valence electrons move from one hole to the next toward the left

The Effect of Forward Bias on the Depletion Region

• As electrons from the n side are pushed into the depletion region, they combine with holes on the p side, effectively reducing the depletion region. This process during forward bias causes the depletion region to narrow

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• The electrons give up an amount of energy equivalent to the barrier potential when they cross the depletion region. This energy loss results in a voltage drop across the pn junction equal to the barrier potential (0.7 V).
• An additional small voltage drop occurs across the p and n regions due to the internal resistance of the material. For doped semiconductive material, this resistance, called the dynamic resistance, is very small and can usually be neglected

Reverse Bias

• Reverse bias is the condition that essentially prevents current through the diode
• a dc voltage source connected across a diode in the direction to produce reverse bias. This external bias voltage is designated as VBIAS just as it was for forward bias
• positive side of VBIAS is connected to the n region of the diode and the negative side is connected to the p region

The positive side of the bias-voltage source “pulls” the free electrons, which are the majority carriers in the n region, away from the pn junction. As the electrons flow toward the positive side of the voltage source, additional holes are created at the depletion region. This results in a widening of the depletion region and fewer majority carriers.

• the negative side of the voltage source enter as valence electrons and move from hole to hole toward the depletion region.
• This results in a widening of the depletion region and a depletion of majority carriers.
• As the depletion region widens, the availability of majority carriers decreases.

Reverse Current

• The extremely small current that exists in reverse bias after the transition current dies out is caused by the minority carriers in the n and p regions that are produced by thermally generated electron-hole pairs, The conduction band in the p region is at a higher energy level than the conduction band in the n region. Therefore, the minority electrons easily pass through the depletion region because they require no additional energy.

• Reverse Breakdown

Normally, the reverse current is so small that it can be neglected. However, if the external reverse-bias voltage is increased to a value called the breakdown voltage, the reverse current will drastically increase.

• The high reverse-bias voltage imparts energy to the free minority electrons so that as they speed through the p region, they collide with atoms with enough energy to knock valence electrons into the conduction band. The newly created conduction electrons are also high in energy and repeat the process.
• The numbers quickly multiply. As these high-energy electrons go through the depletion region, they have enough energy to go through the n region as conduction electrons, rather than combining with holes.
• The multiplication of conduction electrons just discussed is known as the avalanche effect, and reverse current can increase dramatically if steps are not taken to limit the current. When the reverse current is not limited, the resulting heating will permanently damage the diode

Voltage-Current Characteristic of a Diode

• When a forward-bias voltage is applied across a diode, there is current. This current is called the forward current

Point A corresponds to a zero-bias condition. Point B corresponds where the forward voltage is less than the barrier potential of 0.7 V. Point C corresponds to where the forward voltage approximately equals the barrier potential.

Dynamic Resistance

• The resistance of the forward-biased diode is not constant over the entire curve.
• Because the resistance changes as you move along the V-I curve, it is called dynamic or ac resistance

V-I Characteristic for Reverse Bias

1. When a reverse-bias voltage is applied across a diode, there is only an extremely small reverse current (IR) through the pn junction. With 0 V across the diode.
2. When the applied bias voltage is increased to a value where the reverse voltage across the diode (VR) reaches the breakdown value (VBR), the reverse current begins to increase rapidly.
3. continue to increase the bias voltage, the current continues to increase very rapidly, but the voltage across the diode increases very little above VBR

The Complete V-I Characteristic Curve

Temperature Effects

1. As temperature is increased, the forward current increases for a given value of forward voltage. Also, for a given value of forward current, the forward voltage decreases.
2. The blue curve is at room temperature (25 C) and the red curve is at an elevated temperature (25 C + DT). The barrier potential decreases by 2 mV for each degree increase in temperature.

Diode Current Equation

The Ideal Diode Model

The ideal model of a diode is the least accurate approximation and can be represented by a simple switch. When the diode is forward-biased, it ideally acts like a closed (on) switch. When the diode is reverse-biased, it ideally acts like an open (off) switch,

TRANSITION CAPACITANCE

1. When P-N junction is reverse biased the depletion region act as an insulator or as a dielectric medium and the p-type an N-type region have low resistance and act as the plates.
2. Thus this P-N junction can be considered as a parallel plate capacitor.
3. This junction capacitance is called as space charge capacitance or transition capacitance and is denoted as CT
4. Since reverse bias causes the majority charge carriers to move away from the junction , so the thickness of the depletion region denoted as W increases with the increase in reverse bias voltage.
5. This incremental capacitance CT may be defined as CT = dQ/dV, Where dQ is the increase in charge and dV is the change or increase in voltage.
6. The depletion region increases with the increase in reverse bias potential the resulting transition capacitance decreases.
7. The formula for transition capacitance is given as CT = Aε/d, where A is the cross sectional area of the region, and d is the width.

XC = 1/2πfC

DIFFUSION CAPACITANCE

1. When the junction is forward biased, a capacitance comes into play , that is known as diffusion capacitance denoted as CD. It is much greater than the transition capacitance.
2. During forward biased the potential barrier is reduced. The charge carriers moves away from the junction and recombine.
3. The density of the charge carriers is high near the junction and reduces or decays as the distance increases.
4. Thus in this case charge is stored on both side of the junction and varies with the applied potential. So as per definition change in charge with respect to applied voltage results in capacitance which here is called as diffusion capacitance.
5. The formula for diffusion capacitance is CD = τID / ηVk , where τ is the mean life time of the charge carrier, ID is the diode current and Vk is the applied forward voltage, and η is generation recombination factor.
6. The diffusion capacitance is directly proportional to the diode current.
7. In forward biased CD >> CT . And thus CT can be neglected.

REVERSE RECOVERY TIME

• Reverse recovery time, denoted by trr .
• If the applied voltage should be reversed to establish a reverse-bias situation.
• Diode change instantaneously from the conduction state to the nonconduction state.
• Because of the large number of minority carriers in each material, the diode current will simply reverse and stay at this measurable level for the period of time ts (storage time) required for the minority carriers to return to their majority-carrier state in the opposite material
• when this storage phase has passed, the current will be reduced in level to that associated with the nonconduction state.
• This second period of time is denoted by tt (transition interval).

ts

tt

trr =ts + tt

Diode Applications

• Half-Wave Rectifiers
• Full-Wave Rectifiers
• Power Supply Filters and Regulators
• Diode Limiters and Clampers
• Voltage Multipliers

Half-Wave Rectifiers

• The rectifier converts the ac input voltage to a pulsating dc voltage, called a half-wave rectified voltage

• Half-Wave Mode
• Transformer Coupling

Half-Wave Rectifier Operation

• A diode is connected to an ac source and to a load resistor, RL, forming a half-wave rectifier.
• When the sinusoidal input voltage (Vin) goes positive, the diode is forward-biased and conducts current through the load resistor
• The current produces an output voltage across the load RL, which has the same shape as the positive half-cycle of the input voltage

Transformer Coupling

• Transformer coupling provides two advantages. First, it allows the source voltage to be stepped up or down as needed. Second, the ac source is electrically isolated from the rectifier, thus avoiding a shock hazard in the secondary circuit for lower voltages.

Ripple Factor(Г)

• The ratio of rms value of a.c. component to the d.c. component in the output is known as ripple factor (Г).

PEAK INVERSE VOLTAGE

• It is defined as the maximum reverse voltage that a diode can withstand without destroying the junction. The peak inverse voltage across a diode is the peak of the negative half cycle. For half-wave rectifier, PIV is Vm.

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TRANSFPRMER UTILISATION FACTOR(TUF)

• In the design of any power supply, the rating of the transformer should be determined. This can be done with a knowledge of the d.c. power delivered to the load and the type of rectifying circuit used.

Full-wave rectifiers

• It converts an a.c. voltage into a pulsating d.c. voltage using both half cycles of the applied a.c. voltage. It uses two diodes of which one conducts during one half-cycle while the other diode conducts during the other half-cycle of the applied a.c. voltage. There are two types of full-wave rectifiers viz.
• (i) Full-wave rectifier with center tapped transformer and
• (ii) Full-wave rectifier without transformer (Bridge rectifier).

Bridge Rectifier

• The bridge rectifier has four diodes connected to form a bridge. The a.c. input voltage is applied to the diagonally opposite ends of the bridge. The load resistance is connected between the other two ends of the bridge.
• For the positive half-cycle of the input a.c. voltage, diodes D1 and D3 conduct.
• During the negative half-cycle of the input a.c. voltage, diodes D2 and D4 conduct

D1, D3

D2, D4

Advantages of Bridge rectifier

• The bulky center tapped transformer is not required.
• Transformer utilisation factor is considerably high
• The bridge rectifiers are used in applications allowing floating output terminals, i.e. no output terminal is grounded.

• The bridge rectifier has only one disadvantage that it requires four diodes as compared to two diodes for center-tapped full-wave rectifier

Disadvantages of Bridge rectifier

Filters

• A power supply filter ideally eliminates the fluctuations in the output voltage of a halfwave or full-wave rectifier and produces a constant-level dc voltage.
• Filtering is necessary because electronic circuits require a constant source of dc voltage and current to provide power and biasing for proper operation.
• The filtering concept showing a nearly smooth dc output voltage from the filter.
• The small amount of fluctuation in the filter output voltage is called ripple.

Types of Filters

1. Inductor filter
2. Capacitor filter
3. LC or L-section filter
4. CLC or π-type filter

Inductor filter

• When the output of the rectifier passes through an inductor, it blocks the a.c. component and allows only the d.c. component to reach the load.
• It shows that the ripple factor will decrease when L is increased and RL is decreased.
• The inductor filter is more effective only when the load current is high (small RL).

Capacitor filter

• An inexpensive filter for light loads is found in the capacitor filter which is connected directly across the load
• The property of a capacitor is that it allows a.c. component and blocks the d.c. component. The operation of a capacitor filter is to short the ripple to ground but leave the d.c. to appear at the output when it is connected across a pulsating d.c. voltage

LC or L-section filter

• The ripple factor is directly proportional to the load resistance RL in the inductor filter and inversely proportional to RL in the capacitor filter.
• If the value of the inductance is increased, it will increase the time of conduction. At some critical value of inductance, one diode, either D1 or D2 in full-wave rectifier, will always be conducting.

CLC or π-type filter

• The CLC or p-type filter which basically consists of a capacitor filter followed by an LC section.
• This filter provided a fairly smooth output, and is characterized by a highly peaked diode currents and poor regulation
• The triangular output-voltage wave from the first capacitor
• The output voltage is then approximately that from the input capacitor, decreased by the d.c. voltage drop in the inductor.
• The ripple contained in this output is reduced by the L-section filter

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Voltage Regulators

• While filters can reduce the ripple from power supplies to a low value, the most effective approach is a combination of a capacitor-input filter used with a voltage regulator.
• A voltage regulator is connected to the output of a filtered rectifier and maintains a constant output voltage (or current) despite changes in the input, the load current, or the temperature
• Most regulators are integrated circuits and have three terminals—an input terminal, an output terminal, and a reference (or adjust) terminal
• The input to the regulator is first filtered with a capacitor to reduce the ripple to <10%.
• In addition, most regulators have an internal voltage reference, short-circuit protection, and thermal shutdown circuitry.

• Three-terminal regulators designed for fixed output voltages require only external capacitors to complete the regulation portion of the power supply
• Filtering is accomplished by a large-value capacitor between the input voltage and ground. An output capacitor (typically 0.1 micro F to 1.0 micro F) is connected from the output to ground to improve the transient response.

Zener diode

• A zener diode is a silicon pn junction device that is designed for operation in the reverse-breakdown region.
• The breakdown voltage of a zener diode is set by carefully controlling the doping level during manufacture

Zener Breakdown

• ZB occurs in a zener diode at low reverse voltages. A zener diode is heavily doped to reduce the breakdown voltage. This causes a very thin depletion region. As a result, an intense electric field exists within the depletion region. Near the zener breakdown voltage (VZ), the field is intense enough to pull electrons from their valence bands and create current.
• Zener diodes with breakdown voltages of less than approximately 5 V operate predominately in zener breakdown

• Zener diodes are designed to operate in reverse breakdown. Two types of reverse breakdown in a zener diode are avalanche and zener

Those with breakdown voltages greater than approximately 5 V operate predominately in avalanche breakdown.

Avalanche breakdown

Varactor Diodes

• The junction capacitance of diodes varies with the amount of reverse bias.
• Varactor diodes are specially designed to take advantage of this characteristic and are used as voltage-controlled capacitors rather than traditional diodes.
• These devices are commonly used in communication systems.
• Varactor diodes are also referred to as varicaps or tuning diodes.

• A varactor is a diode that always operates in reverse bias and is doped to maximize the inherent capacitance of the depletion region.
• The depletion region acts as a capacitor dielectric because of its nonconductive characteristic. The p and n regions are conductive and act as the capacitor plates

Optical Diodes

• Three types of optical diodes
• light-emitting diode
• quantum dots
• photodiode

LIGHT-EMITTING DIODE (LED)

• The basic operation of the light-emitting diode (LED) is as follows. When the device is forward-biased, electrons cross the pn junction from the n-type material and recombine with holes in the p-type material
• the difference in energy between the electrons and the holes corresponds to the energy of visible light.
• When recombination takes place, the recombining electrons release energy in the form of photons

• The emitted light tends to be monochromatic (one color) that depends on the band gap (and other factors).
• A large exposed surface area on one layer of the semiconductive material permits the photons to be emitted as visible light. This process, called electroluminescence
• Various impurities are added during the doping process to establish the wavelength of the emitted light.
• The wavelength determines the color of visible light.
• Some LEDs emit photons that are not part of the visible spectrum but have longer wavelengths and are in the infrared (IR) portion of the spectrum.
• LED Semiconductor Materials The semiconductor gallium arsenide (GaAs) was used in early LEDs and emits IR radiation, which is invisible. The first visible red LEDs were produced using gallium arsenide phosphide (GaAsP) on a GaAs substrate. The efficiency was increased using a gallium phosphide (GaP) substrate, resulting in brighter red LEDs and also allowing orange LEDs.

LED Biasing

• The forward voltage across an LED is considerably greater than for a silicon diode. Typically, the maximum VF for LEDs is between 1.2 V and 3.2 V, depending on the material. Reverse breakdown for an LED is much less than for a silicon rectifier diode (3 V to 10 V is typical). The LED emits light in response to a sufficient forward current
• The amount of power output translated into light is directly proportional to the forward current, An increase in IF corresponds proportionally to an increase in light output.
• The light output (both intensity and color) is also dependent on temperature. Light intensity goes down with higher temperature as indicated in the figure.

Quantum Dots

• Quantum dots are a form of nanocrystals that are made from semiconductor material such as silicon, germanium, cadmium sulfide, cadmium selenide, and indium phosphide. Quantum dots are only 1 nm to 12 nm in diameter
• Billions of dots could fit on the head of a pin! Because of their small size, quantum effects arise due to the confinement of electrons and holes; as a result, material properties are very different than the normal material
• Quantum dots can be used to modify the basic color of LEDs by converting higher energy photons (blue) to photons of lower energy.
• The result is a color that more closely approximates an incandescent bulb. Quantum dot filters can be designed to contain combinations of colors, giving designers control of the spectrum.
• medical applications. Water-soluble quantum dots are used as a biochemical luminescent marker for cellular imaging and medical research.

The Photodiode

• The photodiode is a device that operates in reverse bias.
• where Il is the reverse light current. The photodiode has a small transparent window that allows light to strike the pn junction
• The reverse-biased current is produced by thermally generated electron-hole pairs in the depletion region, which are swept across the pn junction by the electric field created by the reverse voltage.
• In a rectifier diode, the reverse leakage current increases with temperature due to an increase in the number of electron-hole pairs
• A photodiode differs from a rectifier diode in that when its pn junction is exposed to light, the reverse current increases with the light intensity.
• When there is no incident light, the reverse current, Il, is almost negligible and is called the dark current. An increase in the amount of light intensity

UNIT-II: BI-POLAR JUNCTION TRANSISTORS AND THYRISTOR

BJT, JFET, MOSFET – Structure, operation, characteristics – UJT, Thyristors and IGBT –Structure, operation and characteristics – Photo-transistors and opto couplers – New semiconductor material – Silicon carbide – Gallium Arsenide.

Bipolar Junction Transistor(BJT)

• A Bipolar Junction Transistor (BJT) is a three terminal semiconductor device in which the operation depends on the interaction of both majority and minority carriers and hence the name Bipolar.
• Current Controlled device
• The BJT is analogous to a vacuum triode and is comparatively smaller in size.
• Applications:
• amplifier and
• oscillator circuits
• as a switch in digital circuits.
• It has wide applications in computers, satellites and other modern communication systems.

BJT Construction

• The BJT consists of a silicon (or germanium) crystal in which a thin layer of N-type Silicon is sandwiched between two layers of P-type silicon. This transistor is referred to as PNP. Alternatively, in a NPN transistor.
• Two Types of BJT
• NPN Transistor
• PNP Transistor
• The three portions of the transistor are Emitter, Base and Collector
• Emitter is heavily doped so that it can inject a large number of charge carriers into the base.
• Base is lightly doped and very thin. It passes most of the injected charge carriers from the emitter into the collector.
• Collector is moderately doped.

Biasing

• A bias arrangement for both npn and pnp BJTs for operation as an amplifier.
• In both cases the base-emitter (BE) junction is forward-biased and the base-collector (BC) junction is reverse-biased. This condition is called forward-reverse bias.

Operation

• The heavily doped n-type emitter region has a very high density of conduction-band (free) electrons
• These free electrons easily diffuse through the forward based BE junction into the lightly doped and very thin p-type base region, as indicated by the wide arrow.
• The base has a low density of holes
• A small percentage of the total number of free electrons injected into the base region recombine with holes and move as valence electrons through the base region and into the emitter region as hole current, indicated by the red arrows.
• free electrons in the metallic base lead and produce the external base current.
• As the free electrons move toward the reverse-biased BC junction, they are swept across into the collector region by the attraction of the positive collector supply voltage

Operation

Ic

Ib

IE

Types of Configuration

• When a transistor is to be connected in a circuit, one terminal is used as an input terminal, the other terminal is used as an output terminal and the third terminal is common to the input and output.

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• Three configurations
• Common base (CB) configuration
• This is also called grounded base configuration. In this configuration, emitter is the input terminal, collector is the output terminal and base is the common terminal
• Common emitter (CE) configuration
• This is also called grounded emitter configuration. In this configuration, base is the input terminal, collector is the output terminal and emitter is the common terminal
• Common collector (CC) configuration
• This is also called grounded collector configuration. In this configuration, base is the input terminal, emitter is the output terminal and collector is the common terminal.

Common base (CB) configuration

Input Characteristics:

• VCB is kept constant at zero volt
• The emitter current IE is increased from zero in suitable equal steps by increasing VEB
• This is repeated for higher fixed values of VCB.
• When VCB is increased keeping VEB constant, the width of the base region will decrease.

Output Characteristics:

• The emitter current IE is kept constant at a suitable value by adjusting the emitter-base voltage VEB.
• Then VCB is increased in suitable equal steps and the collector current IC is noted for each value of IE.

Common base (CB) configuration

VEE

B

C

E

VCC

0 V

Input Characteristics:

VCB is kept constant at zero volt

The emitter current IE is increased from zero in suitable equal steps by increasing VEB

0.7 V

1 V

0.6 V

When VCB is increased keeping VEB constant, the width of the base region will decrease.

Output Characteristics:

The emitter current IE is kept constant at a suitable value by adjusting the emitter-base voltage VEB.

0 V

0 V

1 V

1 mA

1 V

1 mA

2 V

3 V

1.5 V

2 mA

2 mA

1 V

2 V

3 V

Then VCB is increased in suitable equal steps and the collector current IC is noted for each value of IE.

Transistor Parameters

Common emitter (CE) configuration

Input Characteristics:

• VCE is kept constant at zero volt
• The base current Ib is increased from zero in suitable equal steps by increasing VBE
• This is repeated for higher fixed values of VCE.
• When VCE = 0, emitter base junction FWB biased behave like diode, When VCE increased the width of the base region will decrease. Causes the Ib and it reduces

Output Characteristics:

• The base current Ib is kept constant at a suitable value by adjusting the base-emitter voltage VBE.
• Then VCE is increased in suitable equal steps and the collector current IC is noted for each value of IE.

• For larger values of VCE, due to Early effect, a very small change in α is reflected in a very large change in β
• increase in α by about 0.5% results in an increases in β by about 34%.
• The output characteristics have three regions
• saturation region
• cut-off region
• Active region.
• saturation region
• In this region, both junctions are forward biased and an increase in the base current does not cause a corresponding large change in IC
• cut-off region
• The region below the curve for IB = 0 is called the
• both junctions are reverse biased
• Active region.
• emitter-base junction is forward biased and the collector-base junction is reverse biased.
• If the transistor is to be used as a linear amplifier

Transistor Parameters

Common Collector (CC) configuration

Input Characteristics:

• VEC is kept constant at zero volt
• The base current Ib is increased from zero in suitable equal steps by increasing VBC
• This is repeated for higher fixed values of VEC.

Output Characteristics:

• Same as Common emitter characteristics
• The base current Ib is kept constant at a suitable value by adjusting the emitter-collector voltage VEC.
• Then VBC is increased in suitable equal steps and the emitter current IC is noted for each value of IB.

Current Amplification Factor

• In a transistor amplifier with a.c. input signal, the ratio of change in output current to the change in input current is known as the current amplification factor

it is clear that α as approaches unity, β approaches infinity. The CE configuration is used for almost all transistor applications because of its high current gain.

Types of FET

FET

JFET

IG -FET

(Junction Field Effect Transistor)

(Insulated Gate Field Effect Transistor)

Types of FET

FET

JFET

MOSFET

(Junction Field Effect Transistor)

(Metal Oxide Semi Conductor Field Effect Transistor)

n-channel

p-channel

Depletion Type

Enhancement Type

JFET

• The field-effect transistor (FET) is a three-terminal device used for a variety of applications
• The JFET transistor is a voltage-controlled device
• The FET is a unipolar device depending solely on either electron ( n - channel) or hole ( p -channel) conduction.
• Wire leads are connected to each end of the n-channel; the drain is at the upper end, and the source is at the lower end.
• Two p-type regions are diffused in the n-type material to form a channel, and both p-type regions are connected to the gate lead.
• The gate lead is shown connected to only one of the p regions, which are internally connected together
• p- channel JFET
• n- channel JFET

​

n Type

Base

P

P

Gate (G)

Drain (D)

Source (S)

Construction of n channel JFET

Construction of p channel JFET

p Type

Base

n

n

Gate (G)

Drain (D)

Source (S)

Working of n channel JFET

Biasing

VGS

VDS

Source act as a common terminal

0 V

0 V

Depletion region is formed

2 V

VDS = Constant

VGS = - 0.5 V

-0.5 V

pn junction reversed biased , Depletion region penetrates more deeply into the channel

Therefore drain current reduces

VGS = -1 V

- 1 V

- 1.5 V

Pinch off voltage or Threshold voltage

- 1 V

4 V

10 V

VGS = 0 V

IDSS

VP

Saturation Level, VGS = 0 V

VGS = -1 V

VGS = -2 V

VGS = -3 V

VGS = -4 V = VP

VGS = -1 V

VGS = -2 V

VGS = -4 V

Ohmic Region

Working of p channel JFET

VGS

VDS

0 V

0 V

-2 V

1 V

3V

4 V

5 V

-4 V

-10 V

3 V

TRANSFER CHARACTERISTICS

MOSFET - Basic Construction

P Type

Substrate

n

n

Gate (G)

Drain (D)

Source (S)

Construction of n channel Depletion type MOSFET

n

Substrate (ss)

SiO2

n Type

Substrate

p

p

Gate (G)

Drain (D)

Source (S)

Construction of p channel Depletion type MOSFET

p

SiO2

Working of n channel Depletion type MOSFET

VGS = 0 V

VDD

1 V

VGS = -1 V

ID = Is = 5mA

VGS = -3 V

ID = Is = 2 mA

VGS = -6V

ID = Is = 0 mA

VGS = 1 V

ID = Is = 10 mA

MOSFET - Basic Construction

P Type

Substrate

n

n

Gate (G)

Drain (D)

Source (S)

Construction of n channel Enhancement type MOSFET

Substrate (ss)

SiO2

n Type

Substrate

p

p

Gate (G)

Drain (D)

Source (S)

Construction of p channel Enhancement type MOSFET

SiO2

Working of n channel Enhancement type MOSFET

VDD

1 V

5 V

VGS = 0 V

VGS = 3 V

VGS = 5 V

VGS = 7 V

7 V

10 V

15 V

ID = Is = 8 mA

Working of p channel Enhancement type MOSFET

VDD

-1 V

-5 V

VGS = -0 V

VGS = -3 V

VGS = -5 V

VGS = -7 V

-7 V

-15 V

ID = Is = 8 mA

UNIJUNCTION TRANSISTOR

• The UJT is a three-terminal device
• A slab of lightly doped n-type silicon material has two base contacts attached to both ends of one surface and an aluminum rod alloyed to the opposite surface.
• The p – n junction of the device is formed at the boundary of the aluminum rod and the n -type silicon slab. The single p – n junction accounts for the terminology unijunction .
• It was originally called a duo (double) base diode due to the presence of two base contacts
• variety of applications
• Oscillators
• trigger circuits
• sawtooth generators
• phase control
• timing circuits,
• bistable networks, and voltage- or current-regulated supplies

n

p

B1

B2

E

d2 >d1

R2 >R1

10 V

Its acts as a voltage divider

1 V

2 V

5 V

>7 V

Thyristors

• Several types of semiconductor devices are introduced.
• A family of devices known as thyristors are constructed of four semiconductor layers (pnpn).
• Thyristors include the four-layer diode
• The silicon-controlled rectifier (SCR)
• The diac
• The triac,
• The silicon-controlled switch (SCS).

Four-layer diode

The Silicon-Controlled Rectifier (SCR )

• The basic operation of the SCR is different from that of the fundamental two-layer semiconductor diode in that a third terminal, called a gate
• four-layer pnpn structure
• Determines when the rectifier switches from the open-circuit to the short-circuit state forward conduction is to be established,
• The anode must be positive with respect to the cathode
• A pulse of sufficient magnitude must also be applied to the gate to establish a turn-on gate current, represented symbolically by I GT

• Two three-layer transistor structures

• Forward breakover voltage V(BR)F* is the voltage above which the SCR enters the conduction region. The asterisk (*) denotes the letter to be added, which is dependent.
• Holding current I H is the value of current below which the SCR switches from the conduction state to the forward blocking region under stated conditions.
• Forward and reverse blocking regions are the regions corresponding to the open-circuit condition for the controlled rectifier that block the flow of charge (current) from anode to cathode.
• Reverse breakdown voltage is equivalent to the Zener or avalanche region of the fundamental two-layer semiconductor diode.

IGBT (insulated-gate bipolar transistor)

• combines features from both the MOSFET and the BJT that make it useful in high-voltage and high-current switching applications.
• The IGBT has largely replaced the MOSFET and the BJT in many of these applications.
• The IGBT is a device that has the output conduction characteristics of a BJT but is voltage controlled like a MOSFET;
• The IGBT has three terminals: gate, collector, and emitter

• The IGBT has MOSFET input characteristics and BJT output characteristics
• IGBTs exhibit a lower saturation voltage than MOSFETs and have about the same saturation voltage as BJTs.
• IGBTs are superior to MOSFETs in some applications because they can handle high collector-to-emitter voltages exceeding 200 V

Operation

• The IGBT is controlled by the gate voltage just like a MOSFET
• IGBT can be thought of as a voltage-controlled BJT, but with faster switching speeds.
• The input element is a MOSFET, and the output element is a bipolar transistor.
• When the gate voltage with respect to the emitter is less than a threshold voltage, Vthresh,
• The device is turned off. The device is turned on by increasing the gate voltage to a value exceeding the threshold voltage.

​

Operation

• The npnp structure of the IGBT forms a parasitic transistor and an inherent parasitic resistance within the device
• These parasitic components have no effect during normal operation. However, if the maximum collector current is exceeded under certain conditions, the parasitic transistor, Qp can turn on. If Qp turns on, it effectively combines with Q1 to form a parasitic element which a latchup condition can occur.
• In latch-up, the device will stay on and cannot be controlled by the gate voltage. Latch-up can be avoided by always operating within the specified limits of the device.

The Phototransistor

• A phototransistor is similar to a regular BJT except that the base current is produced and controlled by light instead of a voltage source.
• The phototransistor effectively converts light energy to an electrical signal.

Working Principle

• In a phototransistor the base current is produced when light strikes a photosensitive semiconductor base region.
• The collector-base pn junction is exposed to incident light through a lens opening in the transistor package.
• When there is no incident light, there is only a small thermally generated collector-to-emitter leakage current, ICEO; this dark current is typically in the nA range.
• When light strikes the collector-base pn junction, a base current is produced that is directly proportional to the light intensity.
• This action produces a collector current that increases, Except for the way base current is generated,
• The phototransistor behaves as a conventional BJT. In many cases, there is no electrical connection to the base.

• A phototransistor can be either a two-lead or a three-lead device. In the three-lead configuration, the base lead is brought out so that the device can be used as a conventional BJT with or without the additional light-sensitivity feature
• In the two-lead configuration, the base is not electrically available, and the device can be used only with light as the input. In many applications,
• The phototransistor is used in the two-lead version.

Optocouplers

• An optocoupler uses an LED optically coupled to a photodiode or a phototransistor in a single package.
• Two basic types are LED-to-photodiode and LED-to-phototransistor

• A key parameter in optocouplers is the CTR (current transfer ratio).
• The CTR is an indication of how efficiently a signal is coupled from input to output and is expressed as the ratio of a change in the LED current to the corresponding change in the photodiode or phototransistor current.
• It is usually expressed as a percentage
• Optocouplers are used to isolate sections of a circuit that are incompatible in terms of the voltage levels or currents required
• protect hospital patients from shock when they are connected to monitoring instruments or other devices

UNIT-III: AMPLIFIERS

Biasing: base, emitter and voltage divider – DC operating point – BJT small signal model -Analysis of CE, CB, CC amplifiers – Gain and frequency response – MOSFET small signal model – Analysis of CS and source follower – Gain and frequency response – High frequency equivalent model.

Transistor as an AMPLIFIERS

• A load resistor RL is connected in series with the collector supply voltage VCC of CB transistor configuration.
• A small change in the input voltage between emitter and base,causes a relatively larger change in emitter current,
• Here, the voltage amplification Av is greater than unity and thus the transistor acts as an amplifier.

Large Signal,d.c and small signal CE values of current gain

The output characteristics of CE configuration show that in the cut-off region, the values IE = 0, IC = ICBO and IB = – ICBO

Bias Stability

• The quiescent operating point of a transistor amplifier should be established in the active region of its characteristics. Since the transistor parameters such as β, ICO and VBE are functions of temperature, the operating point shifts with changes in temperature
• Need for Biasing
• In order to produce distortion-free output in amplifier circuits, the supply voltages and resistances in the circuit must be suitably chosen.
• These voltages and resistances establish a set of d.c. voltage VCEQ and current ICQ to operate the transistor in the active region. These voltages and currents are called quiescent values which determine the operating point or Q-point for the transistor.
• The process of giving proper supply voltages and resistances for obtaining the desired Q-point is called biasing

​

• Here the three variables hFE, i.e., b, IB and ICO are found to increase with temperature. For every 10°C rise in temperature, ICO doubles itself. When ICO increases, IC increases significantly

​

DC load line

• VCC and RC are fixed and IC and VCE are dependent on RB.

VCC = IC RC + VCE

• The straight line represented by AB is called the d.c. load line
• end point A are obtained by substituting VCE = 0 in the above equation. Then IC = VCC/RC
• The coordinates of B are obtained by substituting IC = 0 in the above equation. Then VCE = VCC.
• the optimum Q-point is located at the midpoint of the d.c. load line AB between the saturation and cut-off regions, i.e. Q is exactly midway between A and B

​

AC Load Line

• After drawing the d.c. load line, the operating point Q is properly located at the center of the d.c. load line. This operating point is chosen under zero input signal condition of the circuit.
• Hence, the a.c. load line should also pass through the operating point Q. The effective a.c. load resistance, Ra.c., is the combination of RC parallel to RL, i.e. Ra.c. = RC || RL.

Thermal Runaway

• IC = βIB + (1 + β) ICO
• β, IB, ICO increase with rise in temperature
• The collector current IC causes the collector–base junction temperature to rise which, in turn, increase ICO, as a result IC will increase still further, which will further rise the temperature at the collector–base junction. This process will become cumulative leading to “thermal runaway.”

Stability Factor (S)

It is defined as the rate of change of collector current IC with respect to the collector–base leakage current ICO, keeping both the current IB and the current gain β constant

• From this equation, it is clear that this factor S should be as small as possible to have better thermal stability.

Stability Factor S’ & S’’

• The stability factor S’ is defined as the rate of change of IC with VBE, keeping ICO and β constant.

​

• The stability factor S’’ is defined as the rate of change of IC with respect to β, keeping ICO and VBE constant

Methods of Transistor biasing

• Fixed bias or base resistor method
• Emitter-feedback bias
• Collector-to-base bias or collector feedback bias
• Collector – emitter feedback bias
• Voltage divider bias, self bias or Emitter Bias

Fixed bias or base resistor method

• The d.c. analysis of the circuit yields the following equation.

​

​

​

​

• Since this equation is independent of current IC, dIB/dIC = 0
• Therefore stability equation becomes S = 1 + β
• Since β is a large quantity, this is a very poor bias stable circuit. Therefore, in practice, this circuit is not used for biasing the base.

Emitter-feedback bias

• The analysis will be performed by first examining the base-emitter loop and then using the results to investigate the collector-emitter loop.
• Base emitter loop

​

Collector emitter loop

Collector-to-base bias or collector feedback bias

• This circuit is the simplest way to provide some degree of stabilization to the amplifier operating point.

As can be seen, this value of the stability factor is smaller than the value obtained by fixed bias circuit. Also, S can be made small and the stability can be improved by making RB small or RC large. If RC is very small, then S = (1 + β), i.e. stability is very poor. Hence, the value of RC must be quite large for good stabilization.

Collector – emitter feedback bias

Voltage divider bias, self bias or Emitter Bias

TWO – PORT Network Parameters

1. Z-parameters or Impedance parameters
2. Y-parameters or Admittance parameters
3. h-parameters or Hybrid parameters.

Z-parameters or Impedance parameters

Y-parameters or Admittance parameters

h-parameters or Hybrid parameters.

Hybrid Model for Two Port Network

Transistor Hybrid Model

• h-parameters are real numbers upto radio frequencies
• they are easy to measure
• they can be determined from the transistor static characteristics curves
• they are convenient to use in circuit analysis and design
• easily convertable from one configuration to other
• readily supplied by manufacturers.

Analysis of transistor Amplifier Using h- Parameters

• Voltage Amplifier
• Current Amplifier
• Power Amplifier

BJT Large Signal Model

Amplifier

DC Supply

Biasing

DC Biasing

BJT

• CE is commonly used

Common Emitter

DC Analysis

AC Analysis

- Operating Point(Q)

- Gain and Frequency response,

Input & Output impedance,

Voltage & Current Gain.

• If you connect a microphone across BE its not amplifying, we need two types of analysis

BJT – Large Signal Model

• Transistor Model

​

Large Signal Model

Small signal Model

DC (biasing)

+AC (Time Varying signal)

Small signal analysis of BJT

• Remove DC supply by ground them
• (V.S--S.Ckt , C.S –O.Ckt)
• Replace coupling & bypass capacitor with S.Ckt
• Replace BJT with small signal model

Steps for Small signal

BJT – CE Amplifier

• B.E fwd biased by VBB, CB Reverse biased by VCC, transistor remain active region
• C1 and C2 are coupling capacitors, provide isolation b/w biasing to input & output side
• Capacitor allows ac signals (short circuit), blocks DC signals, it will act as a open circuit

​

C2

C1

RB

RC

(A) CE amplifier with fixed bias

• Step 1

• Step 2

• Step 3

C2

C1

Vi

AC i/p

AC o/p

Vcc

B

C

E

Vout

hfe Ib

B

C

E

Vout

Input Impedance =

Voltage Gain:

Current gain

Output Impedance

it is the impedance determined with Vi = 0; Ib=0, input voltage source short circuited

Substituting the re model

Vout

Vout

Input Impedance =

Input Impedance =

Current gain

Current gain

Voltage Gain:

Voltage Gain:

Output Impedance

Output Impedance

(B) CE amplifier with unbypassed Emitter resistor

• Step 1

• Step 2

• Step 3

C2

C1

Vi

AC i/p

AC o/p

Vcc

B

C

E

Vout

hfe Ib

B

C

E

Vout

Input Impedance =

Voltage Gain:

Current gain

Output Impedance

it is the impedance determined with Vi = 0; Ib=0, input voltage source short circuited

(C) CE amplifier with Voltage divider Bias

• Step 1

• Step 2

• Step 3

C2

C1

Vi

AC i/p

AC o/p

Vcc

B

C

E

Vout

UNIT-IV: MULTISTAGE AMPLIFIERS AND DIFFERENTIAL AMPLIFIER

Cascade amplifier – Differential amplifier – Common mode and difference mode analysis – FET input stages – Single tuned amplifiers – Gain and frequency response – Neutralization methods, power amplifiers –Types (Qualitative analysis).

Differential amplifier:

• The function of a differential amplifier is to amplify the difference between two signals.
• The output signal in a differential amplifier is proportional to the difference between the two input signals.

Differential amplifier Using Operational Amplifier

• The amplifier has two input signals V1 and V2 and a single output Vo.
• For the purpose of signal analysis, they can be represented by its input resistance Rid, output resistance Ro,
• A is the open circuit voltage gain, Vid = (V1 – V2) is the differential mode input voltage.
• The signal voltage developed at the output of the amplifier is in phase with the voltage applied to the positive input terminal and 180° out of phase with the signal applied to the negative input terminal.
• The V1 and V2 terminals are therefore refered to as non-inverting input and inverting input respectively

CMRR

• The differential amplifier is said to be operated in a common mode configuration when the same voltage is applied to both the inputs V1 = V2.
• main requirements of the differential amplifier is to cancel or reject the noise signal that appears as a common input signal to both the input terminals of the differential amplifier.
• CMRR is defined as the ratio of the differential voltage gain Ad to common mode gain AC and is generally expressed in dB

Differential amplifiers using BJT

• The differential amplifiers using BJT are broadly classified into two types namely (i) differential BJT amplifier with resistive loading and (ii) differential BJT amplifier with active loading.

differential BJT amplifier with resistive loading

• The emitter coupled or source coupled differential amplifier forms the input stage of most analog ICs.
• It is important to note that the performance of a differential amplifier depends on the ideal matching of the transistor pair, Q1 and Q2.
• The amplifier uses both a positive power supply +VCC and a negative power supply –VEE.
• It is to be mentioned that these amplifiers operate even at d.c., because appropriate d.c. level shifting could be obtained without the use of coupling capacitors

SMALL SIGNAL TUNED AMPLIFIERS

• In order to obtain a large overall voltage gain, it is required to use a number of tuned amplifier stages in cascade. These cascaded tuned amplifiers may be classified as
• (i) single tuned amplifiers, (ii) double tuned amplifiers, and (iii) stagger tuned amplifiers.
• Single tuned amplifiers use one parallel resonant circuit as the load impedance in each stage and all the tuned circuits are tuned to the same frequency.

single tuned amplifiers

• Single tuned amplifiers can be further classified as
• (a) capacitance coupled single tuned amplifier and
• (b) transformer coupled or inductively coupled single tuned amplifier.
• capacitance coupled single tuned amplifier
• single tuned amplifier in which the output across the tuned circuit is coupled to the next stage through the coupling capacitor CC. The tuned circuit formed by L and C’ resonates at the frequency of operation.

Neutralization

• The technique used for the elimination of potential oscillations is called neutralization.
• BJT and FET are potentially unstable over some frequency range due to the feedback parameter (YN) present in them.
• If the feedback can be cancelled by an additional feedback signal that is equal in amplitude and opposite in sign, the transistor becomes unilateral from input to output till the oscillations completely stop.
• This is achieved by neutralization.

Hazeltine neutralization method.

• This is a neutralization technique employed in tuned RF amplifiers to maintain stability.

Power Amplifiers

• Power amplifiers are those amplifiers that have the objective of delivering power to a load
• Four classes of BJT power amplifiers:
• class A,
• class B,
• class AB,
• class C
• These amplifier classifications are based on the percentage of the input cycle for which the amplifier operates in its linear region
• Power amplifiers are frequently used as the final stage of a system such as a communications receiver or transmitter to provide signal power to speakers or to a transmitting antenna.

Class A amplifier

• In a Class A amplifier, the transistor is biased such that the output current flows, i.e. the transistor is ON for the full cycle (360°) of the input ac signal

Class B amplifier

• In a Class B amplifier the transistor bias and the amplitude of the input signal are selected such that the output current flows, i.e. the transistor is ON for only one half cycle (180°) of the input ac signal

Class AB amplifier

• In a Class AB amplifier, the transistor operates between the two extremes defined for Class A and Class B amplifiers. Hence, the output signal exists for more than 180° but less than 360° of the input ac signal

Class C amplifier

• In a Class C amplifier, the transistor bias and the amplitude of the input signal are selected such that the output current flows, i.e. the transistor is ON for less than one half cycle (180°) of the input ac signal

Class A amplifier

• A simple transistor amplifier that supplies power to a pure resistance load RL
• Assuming that the static output characteristics are equidistant for equal increments of input base current iB, if the input signal iB is a sinusoidal the output current and voltage are also sinusoidal

Transformer coupled class A audio power amplifiers

• Since the load is not directly connected to collector terminal the d.c. collector current does not pass through it. In an ideal transformer, the resistance of the primary winding is zero. Hence d.c. power loss in the load is zero. Hence, the transformer substitutes the d.c. load with an ac load.
• By taking N2 lesser than N1, n can be made much less than unity and RL can be made to look much bigger than the actual value.

Class B amplifier

• In a Class B amplifier the transistor is biased almost at cut-off, so that it remains forward biased only for one half cycle of the input signal. Hence its conduction angle is only 180°.
• The advantages of Class B as compared with Class A operation are

(i) possible to obtain greater power output

(ii) efficiency is higher

(iii) negligible power loss (as no output current flows) at no input signal.

• For these reasons, in such applications where the power supply is limited, say, operating from solar cells or a battery, the output is usually delivered through a push-pull Class B transistor circuit.

PUSH-PULL Amplifier (Class – B)

• A large amount of distortion introduced by the non-linearity of the dynamic transfer characteristic may be eliminated by the push-pull configuration.
• In this circuit the input excitation is introduced through a center-tapped transformer where two equal voltages which differ in the phase by 180° is produced across the secondary winding. Thus when the signal on transistor Q1 is positive, the signal on Q2 is negative by an equal amount

Advantages:

1. A push-pull arrangement gives less distortion for a given power output.
2. The d.c. components of the collector current oppose each other magnetically in the transformer core, thereby eliminating any tendency towards core saturation leading to non-linear distortion.
3. The effects of ripple voltages contained in the power supply will be balanced out.

Class AB amplifier

• Class AB amplifier overcomes the problem of crossover distortion present in Class B amplifiers, in which a small current flows even at zero input signal level.
• which is essentially the same as that of Class B amplifier, has additional RE resistors referred ton as the emitter-stabilizing resistors.
• This biases the transistor away from Class B slightly towards Class A operation. The transistors Q1 and Q2 are biased such that the Q point of Class AB is placed in between the active region of Class A and cut-off region of Class B.
• When an ac signal is applied to the base, the collector current starts flowing immediately. But there will be a decrease in the output power due to the negative feedback effect.
• The efficiency of Class AB amplifier is greater than Class A amplifier and slightly less than Class B amplifier.

Class C amplifier

• Class C power amplifier is designed to conduct only over a small part of positive half cycle of the input waveform.
• the negative supply voltage VBB connected to the base circuit reverse biases the base- emitter junction so that it will conduct only when input signal exceeds the reverse bias. As a result, the collector current IC will be in the form of pulses.
• Hence the class C amplifier is not used in the audio frequency but used in the radio frequency range.
• The tank circuit connected to the collector of the amplifier restores the sine wave of the input signal, but complex audio waveform and rectangular waveforms cannot be restored.
• Class C amplifiers are designed to ensure small conduction angle in order to maintain high efficiency

• The conduction angle is less than 90° for small input signal and more than 120° for large input signal. Conduction angle more than 120° is too large and amplifier efficiency will decrease. Since large currents overheat the transistor, signal biasing is used to overcome the problem and maintain the constant conduction angle.
• the input signal of the self bias circuit exceeds, average charge of the capacitor C1 increases, thereby increasing the reverse bias of Base-Emitter junction, thus maintaining the constant conduction angle. Thus the effective reverse bias of base emitter junction automatically adjusts to the amplitude of the input signal so that the transistor is switched ON over a constant conduction angle.

UNIT – V : FEEDBACK AMPLIFIERS AND OSCILLATORS

• Feedback amplifier – Concepts of positive and negative feedback – Feedback topologies – positive feedback – Condition for oscillations, phase shift – Wien bridge, hartley, colpitts and crystal oscillators.

Feedback amplifier

• The output quantity (either voltage or current) is sampled by a suitable sampler which is of two types, namely, voltage sampler and current sampler, and fed to the feedback network

• Types of Amplifiers
• Positive feedback amplifier
• Negative feedback amplifier

Positive feedback amplifier

• If the feedback signal Vf is in phase with input signal Vs, then the net Vi = Vs + Vf. Hence, the input voltage applied to the basic amplifier is increased thereby increasing Vo exponentially. This type of feedback is said to be positive or regenerative feedback. Gain of the amplifier with positive feedback is

• The gain of the amplifier with positive feedback is infinite and the amplifier gives an a.c. output without a.c. input signal. Thus, the amplifier acts as an oscillator.
• The positive feedback increases the instability of an amplifier, reduces the band-width and increases the distortion and noise.
• The property of the positive feedback is utilized in oscillators

Negative feedback amplifier

• If the feedback signal Vf is out of phase with the input signal Vs, then Vi = Vs – Vf. So the input voltage applied to the basic amplifier is decreased and correspondingly the output is decreased.
• Hence, the voltage gain is reduced. This type of feedback is known as negative or degenerative feedback. Gain of the amplifier with negative feedback is

​

​

​

​

​

• Negative feedback is used to improve the performance of an electronic amplifier. Negative feedback always helps to increase the bandwidth, decrease distortion and noise, modify input and output resistances as desired

Feedback network

• Most often it is simply a resistive configuration in amplifier circuits. It provides a reduced portion of the output as feedback signal to the input mixer network and it is given as Vf = β Vo where β is a feedback factor or feedback ratio which always lies between 0 and 1.

Mixer Network

• Based on the type of sampling at the output side and the type of mixing to the input side, feedback amplifiers are classified into four topologies as

(1) voltage-series feedback or series shunt feedback

(2) current-series feedback or series series feedback

(3) current-shunt feedback or shunt series feedback

(4) voltage-shunt feedback or shunt shunt feedback

• in feedback amplifiers, series mixing at the input tends to increase the input resistance and shunt mixing tends to decrease the input resistance.
• Voltage series feedback

The amplifier input and output circuit replaced by its Thevenin’s model. In this circuit AV represents the open-circuit voltage gain taking Rs into account. We have considered Rs to be part of the amplifier throughout the discussion of feedback amplifier. Here, the input impedance with feedback is given by Rif = Vs /Ii.

Current Series feedback

• The amplifier input circuit represented by Thevenin’s model and the output circuit by Norton’s equivalent circuit. Here, the input impedance with feedback is given by Rif = Vs /Ii.

Current Shunt feedback

Voltage Shunt feedback

Effect of negative feedback amplifier characteristics

Oscillators

• Any circuit which is used to generate a periodic voltage without an a.c. input signal is called an oscillator.
• To generate the periodic voltage, the circuit is supplied with energy from a d.c. source.
• If the output voltage is a sine wave function of time, the oscillator is called a “Sinusoidal” or “Harmonic” oscillator

Classification of oscillators:

1. According to the waveforms generated:

(a) Sinusoidal oscillator

(b) Relaxation oscillator

2. According to the fundamental mechanisms involved:

(a) Negative resistance oscillators

(b) Feedback oscillators

3. According to the frequency generated:

(a) Audio frequency oscillator (AFO): up to 20 kHz

(b) Radio frequency oscillator (RFO): 20 kHz to 30 MHz

(c) Very high frequency (VHF) oscillator: 30 MHz to 300 MHz

(d) Ultra high frequency (UHF) oscillator: 300 MHz to 3 GHz

(e) Microwave frequency oscillator: above 3 GHz

4. According to the type of circuit used, sine-wave oscillators may be classified as

(a) LC tuned oscillator

(b) RC phase shift oscillator.

Condition for oscillation (Barkhausen Criterion)

• The oscillator circuit is set into oscillations by a random variation caused in the base current due to noise component or a small variation in the d.c. power supply.
• Even when no external signal is applied, the ever-present noise will cause some small signal at the output of the amplifier. When the amplifier is tuned at a particular frequency fo, the output signal caused by noise signals will be predominantly at fo.
• If a small fraction ( β ) of the output signal is fed back to the input with proper phase relation, then this feedback signal will be amplified by the amplifier.
• If the amplifier has a gain of more than 1/ β, then the output increases and thereby the feed back signal becomes larger.
• This process continues and the output goes on increasing. But as the signal level increases, the gain of the amplifier decreases and at a particular value of output, the gain of the amplifier is reduced exactly equal to 1/ β. Then the output voltage remains constant at frequency fo, called frequency of oscillation.
• The essential conditions for maintaining oscillations are:

1. | A β | = 1, i.e. the magnitude of loop gain must be unity.

2. The total phase shift around the closed loop is zero or 360 degrees.

General Form of an Oscillator

Hartley Oscillator

• Z1 and Z2 are inductors and Z3 is a capacitor.
• Resistors R1, R2 and RE provide the necessary d.c. bias to the transistor.
• CE is a bypass capacitor. CC1 and CC2 are coupling capacitors.
• The feedback network consisting of inductors L1 and L2, and capacitor C determines the frequency of the oscillator.
• When the supply voltage +VCC is switched ON, a transient current is produced in the tank circuit and consequently, damped harmonic oscillations are set up in the circuit.
• The oscillatory current in the tank circuit produces a.c. voltages across L1 and L2. As terminal 3 is earthed, it is at zero potential If terminal 1 is at a positive potential with respect to 3 at any instant, terminal 2 will be at a negative potential with respect to 3 at the same instant.
• Thus the phase difference between the terminals 1 and 2 is always 180°. In the CE mode, the transistor provides the phase difference of 180° between the input and output. Therefore, the total phase shift is 360°

COLPITS OSCILLATOR

• Z1 and Z2 are capacitors and Z3 is an inductor. The resistors
• R1, R2 and RE provide the necessary d.c. bias to the transistor.
• CE is a bypass capacitor. CC1 and CC2 are coupling capacitors.
• The feedback network consisting of capacitors C1 and C2 and an inductor L determines the frequency of the oscillator.
• When the supply voltage +VCC is switched ON, a transient current is produced in the tank circuit and consequently, damped harmonic oscillations are set up in the circuit.
• The oscillatory current in the tank circuit produces a.c. voltages across C1 and C2. As terminal 3 is earthed, it will be at zero potential.
• If terminal 1 is at a positive potential with respect to 3 at any instant, terminal 2 will be at a negative potential with respect to 3 at the same instant.
• Thus the phase difference between the terminals 1 and 2 is always 180°. In the CE mode, the transistor provides the phase difference of 180° between the input and output. Therefore, the total phase shift is 360

RC oscillators

• All the oscillators using tuned LC circuits operate well at high frequencies. At low frequencies, as the inductors and capacitors required for the time circuit would be very bulky,
• RC oscillators are found to be more suitable. Two important RC oscillators are
• (i) RC Phase shift oscillator and
• (ii) Wien Bridge oscillator.

RC Phase shift oscillator

• In a BJT based RC phase shift oscillator using phase lead RC network BJT is used as an active element of the amplifier. Here, the output of the feedback network is loaded appreciably by the relatively low resistance of the transistor. Thus the resistance R of the feedback network is in parallel with the low input resistance hie of the transistor, which reduces the effective value of R in the last section of feedback network
• The feedback signal is coupled through the feedback resistor R3 in series with the amplifier stage input resistor., R3 is chosen as R3 = R – Ri where Ri is the input impedance of the circuit.
• The BJT amplifier provides a phase shift of 180° and the feedback RC network provides the remaining 180° phase shift to obtain a total phase shift of 360° around the loop. Hence, each RC section is designed so as to provide a phase shift of 60° at the desired frequency of oscillation.

Wien Bridge Oscillator

• The circuit consists of a two-stage RC coupled amplifier which provides a phase shift of 360° or 0°.
• A balanced bridge is used as the feedback network which has no need to provide any additional phase shift.
• The feedback network consists of a lead-lag network (R1 – C1 and R2 – C2) and a voltage divider (R3 – R4).
• The lead-lag network provides a positive feedback to the input of the first stage and the voltage divider provides a negative feedback to the emitter of Q1.

Crystal oscillators

• it is a Colpitts crystal oscillator in which the inductor is replaced by the crystal.
• In this type, a piezo-electric crystal, usually quartz, is used as a resonant circuit replacing an LC circuit.
• The crystal is a thin slice of piezo-electric material, such as quartz, tourmaline and rochelle salt, which exhibit a property called Piezo-electric effect. T
• he piezo-electric effect represents the characteristics that the crystal reacts to any mechanical stress by producing an electric charge; in the reverse effect, an electric field results in mechanical strain.