Course Outline

1. Chapter 1: Signals and Amplifiers

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2. Chapter 3: Semiconductors

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3. Chapter 4: Diodes

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4. Chapter 5: MOS Field Effect Transistors (MOSFET)

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5. Chapter 6: Bipolar Junction Transistors (BJT)

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6. Chapter 2 (optional): Operational Amplifiers

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Chapter 4: �Diodes�Part II

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4.5 Rectifier Circuits

Figure 4.20: Block diagram of a dc power supply

• The primary application of diode is the rectifier –
• Electrical device which converts alternating current (AC) to direct current (DC)
• One important application of rectifier is dc power supply.

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step #1: increase / decrease rms magnitude of AC wave via power transformer

step #2: convert full-wave AC to half-wave DC (still time-varying and periodic)

step #3: employ low-pass filter to reduce wave amplitude by > 90%

step #4: employ voltage regulator to eliminate ripple

step #5: supply dc load .

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4.5.1 The Half-Wave Rectifier

• Half-wave rectifier – utilizes only alternate half-cycles of the input sinusoid
• Constant voltage drop model is employed.

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4.5.1 The Half-Wave Rectifier

• Small inputs? Regardless of the model employed, one should note that the rectifier will not operate properly when input voltage is small (< 1V)

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• Those cases require a precision rectifier (diode with op amps).

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4.5.2 Full-Wave Rectifier

Center-tapping of the transformer, allowing “reversal” of certain currents…

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When instantaneous source voltage is positive, D₁ conducts while D₂ blocks…

4.5.2. Full-Wave Rectifier

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when instantaneous source voltage is negative, D₂ conducts while D₁ blocks

4.5.2 Full-Wave Rectifier

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Center-tapped Transformer

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4.5.3 Bridge Rectifier

• An alternative implementation of the full-wave rectifier is bridge rectifier
• Does not require center-tapped transformer
• Four diodes instead of two

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4.5.3 Bridge Rectifier

when instantaneous source voltage is positive, D₁ and D₂ conduct while D₃ and D₄ block

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4.5.3 Bridge Rectifier

when instantaneous source voltage is negative, D₃ and D₄ conduct while D₁ and D₂ block

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4.5.4. The Rectifier with a Filter Capacitor

• Why is this example unrealistic?
• Because for any practical application,

the converter would supply a load

(which in turn provides a path for capacitor discharging)

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4.5.4. The Rectifier with a Filter Capacitor

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4.5.4. The Rectifier with a Filter Capacitor

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4.5.4. The Rectifier with a Filter Capacitor

• Precision rectifier – is a device which facilitates rectification of low-voltage input waveforms
• How?

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4.6: Limiting and Clamping Circuits

• Q: What is a limiter or clamping circuit?
• A: One which limits voltage output.

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single limiters employ one diode

double limiters employ two diodes of opposite polarity

linear range may be controlled via string of diodes and dc sources

zener diodes may be used to implement soft limiting

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4.6: Soft vs Hard limiter

• soft vs. hard limiter

• Q: How are limiter circuits applied?
• A: Signal processing, used to prevent breakdown of transistors within various devices.

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• Why use soft?

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4.6.2 The Clamped Capacitor or DC Restorer

• Q: What is a DC restorer?
• A: Circuit which provides the dc component of an AC without DC value.
• Q: Why is this ability important?
• A:

1) Average value of the output is effective way to measure duty cycle

2) Duty cycle is modulated to carry digital data (PWM): use DC restorer followed by RC low pass filter

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4.6.3 The Voltage Doubler

• Q: What is a voltage doubler?
• A: One which multiplies the amplitude of a wave or signal by two.
• How?

dc restorer

peak rectifier

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4.7 Special Diodes

Schottky-Barrier Diode or Schottky Diode

• metal and moderately doped semiconductor junction
• current flows from metal to semiconductor
• current is conducted by majority carriers: can switch it on and off faster than p-n junction
• forward voltage drop is lower than p-n junction

(0.3-0.5 V)

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4.7 Special Diodes

Varactors

• reverse-biased p-n junction
• junction capacitance is a function of reverse bias voltage
• how?
• voltage variable capacitor
• tuning of receivers, Phase locked loops

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Anode

Cathode

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4.7 Special Diodes

Photodiodes

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• reverse-biased p-n junction
• expose to light: covalent bonds break, electron-hole pairs are created
• free electrons sweep to n side and holes to p side
• reverse current is created
• photocurrent is proportational to intensity of incident light
• convert light into electric signal
• applications: CD-ROM, fiber-optic
• what happens when you don't reverse bias the photodiode and expose it to light?

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Anode

Cathode

P_o...P₂= light levels

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4.7 Special Diodes

LEDs

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• convert forward current into light
• forward bias region: when minority carriers diffuse into p and n sides, they recombine with majority carriers, e.g. electrons with holes.
• recombination: emission of light
• special semiconductor material: direct band-gap
• known spectra of light when electrons leave orbit
• emitted light is proportional to number of recombinations which is proportional to the forward current

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Anode

Cathode

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