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Unit: II

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Chapter 5:Operational Amplifiers

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Basic Electronics

18EECF101

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Topic Learning Outcomes

At the end of the topic, students should be able to:

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1.Explain op-amp characteristics and its performance metrics.

2.Realize various signal operations such as adder, subtractor, integrator, differentiator using op-amp

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CO-4: Realize circuits to perform arithmetic operations such as addition, subtraction,

integration on signals using operational amplifiers.

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An operational amplifier or op-amp is simply a linear Integrated Circuit (IC) having multiple-terminals.

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The op-amp can be considered to be a voltage amplifying device that is designed to be used with external feedback components such as resistors and capacitors between its output and input terminals.

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It is a high-gain electronic voltage amplifier with a differential input and usually a single-ended output.

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Op-amps are among the most widely used electronic devices today as they are used in a vast array of consumer, industrial and scientific devices.

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In 1968, the μA741 was released, leading it to wide production.

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Introduction

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If a transistor amplifies voltage, why are op amps used instead in many circuits? What's the advantage of op amps?

Why Op-amps?

An operational amplifier or “op amp” is an integrated circuit containing a number of transistors and other components, and typically has the following characteristics:

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  • A very high voltage gain (commonly in excess of 100,000X)
  • A very high input impedance (high enough that the input current to the op amp itself can generally be ignored)
  • A very low output impedance (the op amp output, within it design limits, looks very much like an ideal voltage source).

These characteristics taken together make op-amps a good choice for a number of applications; it’s very easy, by applying different kinds of feedback and using different passive components “around” the op-amp, to make very stable and repeatable amplifiers, filters, mixers, etc..

They can also be used to perform certain “mathematical” operations (in the sense of analog computing) such as addition, subtraction, multiplication, integration, etc. (and this is where the term “operational amplifier” originally came from.

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Significance of Operational Amplifiers

Courtesy: https://images.app.goo.gl/yNVL4YD5tHjD7Y886

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Other applications include

  1. Audio amplification
  2. Signal filtering
  3. Signal generation and
  4. Several biomedical applications

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How do we represent Op-Amp?

Courtesy:https://images.app.goo.gl/82xvvcMSYAdoAvFLA

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Commercially, Op-Amp’s are available in the form of DIP or SMT IC

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Pin Diagram of Op-Amp

Courtesy:https://www.elprocus.com.ic-741

Features of µA 741

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  1. DIP packaged IC with 8 pins
  2. Power Supply: Requires a minimum voltage of 5V and can withstand upto 18V
  3. Input Impedance: About 2 megaohms
  4. Output impedance: About 75 ohms
  5. Voltage Gain: 200,000 for low frequencies
  6. Maximum Output Current: 20mA
  7. Recommended Output Load: Greater than 2 kilo-ohms
  8. Input Offset: Ranges between 2mV and 6mV
  9. Slew Rate: 0.5V/microsecond (It is the rate at which an Op-Amp can detect voltage changes)

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Ideal Characteristics of Op-amp

  1. Infinite Input Impedance: The ideal Op-amp does not draw any current from the voltage sources connected to its input terminals. This implies that the input impedance of an Op-amp is infinity.

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2. Zero Output Impedance: The voltage at the output terminal is independent of the current drawn from it i.e. output impedance is zero. Hence the Op-amp can drive an infinite number of devices.

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3. Infinite Bandwidth: This implies that the amplifier can amplify any frequency from zero to infinity without attenuation. In other words, the ideal Op-amp will amplify signals of any frequency with equal gain.

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4. Input offset voltage: The presence of the small output voltage though V1=V2=0 is called offset voltage. It is zero for an ideal op-amp

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5. Infinite Voltage Gain: The open-loop voltage gain of an ideal Op-amp is very large, i.e., infinity. 

6. Perfect Balance: The output voltage is zero when equal voltages are present at the two input terminals.

7. Infinite CMRR: This means that the output common-mode noise voltage is zero. 

8. Infinite Slew Rate: Slew rate indicates the rapidity with which the output of an Op-amp changes in response to the changes in input frequency. (how fast the output of op-amp is going to respond for any change in input)

9. Temperature :The characteristics do not change with temperature

10. Power Supply Rejection Ratio(PSRR): The PSRR is the ratio of the change in input offset voltage due to change in the supply voltage producing it, keeping other power supply voltage constant

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Ideal Characteristics of Op-amp

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Practical Op-amp

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Ideal Vs Practical Characteristics

Sl.No.

PROPERTY

SYMBOL

IDEAL CHARACTERISTICS

PRACTICAL CHARACTERISTICS

PRACTICAL VALUES OF

Ma 741 IC

1.

Voltage gain

A

∞

104 to 106

2 x 105

2.

Input Resistance

Ri

∞

MΩ

2MΩ

3.

Output Resistance

Ro

0

few ohms

75Ω

4.

Common Mode Rejection Ratio

CMRR

∞

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About 90 dB

90 dB

5.

Bandwidth

BW

∞

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1-100MHz

1 MHz

6.

Slew Rate

SR

∞

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V/µs

0.5V/µs

7.

Power supply Rejection Ratio

PSRR

0

µV/V

30µV/V

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Basics of differential amplifier

Ideal differential amplifier

The diff-amp amplifies the difference between two input voltage signals. Hence it called differential amplifier.

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Differential gain Ad

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Basics of differential amplifier

Common mode gain AC

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Performance Metrics

Common-Mode Rejection Ratio (CMRR)

The Common-Mode Rejection Ratio (CMRR) indicates the ability of a differential amplifier to suppress signals common to the two inputs.

Desired signals should appear on only one input or with opposite polarities on both inputs. 

These desired signals are amplified and appear on the outputs.

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Unwanted signals (noise) appearing with the same polarity on both input lines are ideally cancelled by the differential amplifier as these amplifiers are used as a means of suppressing common-mode signals.

Such noise signals can arise from the following sources:

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(1) radiated signals coupled equally to both lines,

(2) offset from signal common created in the driver circuit, or

(3) ground differential between the transmitting and receiving locations.

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Common-Mode Rejection Ratio (CMRR)

The measure of an amplifier’s ability to reject noise is the CMRR.

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The ideal differential amplifier provides a very high gain for desired signals (single-ended or differential) and zero gain for common-mode signals.

The higher the differential gain compared to the common-mode gain, the better the performance of the differential amplifier in terms of rejecting common-mode signals.

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Common-Mode Rejection Ratio (CMRR)

A good measure of the diff-amp’s performance in rejecting undesirable common-mode signals is the ratio of the differential voltage gain (Av(d)) to the common-mode gain (Acm).

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This ratio is the CMRR.

A very high value of CMRR means that the differential gain Av(d) is high and the common-mode gain Acm is low. Thus the higher the CMRR, the better

A well-designed differential amplifier typically has a high differential gain and low common mode gain, resulting in a high CMRR. The CMRR is often expressed in decibels (dB) as

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Slew rate

The slew rate is defined as the maximum rate of output voltage change per unit time.

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It is denoted by the letter S.

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The slew rate helps us to identify the amplitude and maximum input frequency suitable to an operational amplifier (OP amp) such that the output is not significantly distorted.

The slew rate should be as high as possible to ensure the maximum undistorted output voltage swing.

The Slew rate of the op-amp can limit the performance of a circuit and it can distort the output waveform if its limit is exceeded.

The Slew rate should be ideally infinite and practically as high as possible.

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Slew rate

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Example

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Example

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Saturable property of op-amp

As the open-loop gain of op-amp is very large, of the order of 105 or more, even a very small difference input voltage(v2-v1) produces an extremely high output voltage, whose polarity depends on whether the (v2-v1) is positive or negative.

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However, the maximum output voltage of the op-amp is limited by the supply voltages.

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As a rule of thumb, the maximum output voltages may be taken as 1.5 V less than the supply voltages.

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For typical supply voltages of ±15V, the output voltage is limited to a maximum of ±13.5V.

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Once the output reaches this limit, it does not increase further even if the magnitudes of the input voltages are increased. Under this condition the op-amp is said to be clipped or saturated.

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Saturable property of op-amp

Because of the saturable property, the open loop configuration of the op-amp is of little use in practical applications!!!!!!

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Closed loop configuration of op-amp

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Realistic assumptions

1. Zero input current

2. Virtual ground concept

This means the differential input voltage Vd between the non-inverting and inverting input terminals is essentially zero.

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This is obvious because even if output voltage is few volts, due to large open loop gain of op-amp, the difference voltage Vd at the input terminals is almost zero.

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Thus we can say that under linear range of operation, there is virtually short circuit between the two inputs, in the sense that their voltages are same.

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No current flows from the input terminals to ground.

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Now if the non-inverting terminal is grounded, by the concept of virtual short, the inverting terminal is also at ground potential. This is the principal of virtual ground.

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Linear applications of opamp

  1. Inverting amplifier
  2. Non-inverting amplifier
  3. Voltage follower
  4. Summer
  5. Subtractor
  6. Integrator
  7. Differentiator
  1. Inverting amplifier

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

2. Non-Inverting amplifier

An amplifier which amplifies the input without producing any phase shift between input and output is called non-inverting amplifier.

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

3. Voltage Follower

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Linear applications of opamp

4. Summer

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

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Linear applications of opamp

5. Subtractor

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Linear applications of opamp

6. Integrator

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Linear applications of opamp

Input and Output waveforms

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Linear applications of opamp

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Linear applications of opamp

7. Differentiator

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Linear applications of opamp

Input and Output waveforms

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Linear applications of opamp

3. Sine wave input signal

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Non- Linear applications of opamp

Zero crossing detector(ZCD)

The zero crossing detector circuit is an important application of the op-amp comparator circuit.

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It can also be called as the sine to square wave converter.

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Non- Linear applications of opamp

Comparator

 

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Non- Linear applications of opamp

Example

 

  • Consider two friends A & B with heights hA & hB.
  • Design a circuit using a suitable device to indicate the relative height of A & B with all possibilities.

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Non- Linear applications of opamp

 

Solution:

  • Write all Possibilities of inputs & expected outputs
  • Inputs Expected output
  • 1) hA > hB output should show +Vsat
  • 2) hA < hB output should show -Vsat
  • 3) hA = hB output should show 0V

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Non- Linear applications of opamp

 

Design of comparator circuit

  • Take voltage corresponding to Height of B as VhB.Connect it to inverting terminal
  • Make voltage at Non-inverting terminal as voltage source corresponding to height of A i.e.VhA.
  • Now Output reaches +Vsat whenever hA> hB
  • Output reaches -Vsat whenever hA< hB
  • Output reaches 0 whenever hA= hB

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Non- Linear applications of opamp

 

Circuit diagram

Waveforms with VhB as refernce

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Non- Linear applications of opamp

 

Problems on comparator

  • Design a Circuit to switch on AC when the room temperature

exceeds 25oC. Assume that the sensor output at 25oC is 1V.

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  • Design a circuit to switch on a bulb during dark hours & switch off during day light. Assume that sensor output goes below -1V during day light & above -1V during night.

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At the end of the topic, students should be able to:

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1.Explain op-amp characteristics and its performance metrics.

2.Realize various signal operations such as adder, subtractor, integrator, differentiator using op-amp

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CO-4: Realize circuits to perform arithmetic operations such as addition, subtraction,

integration on signals using operational amplifiers.

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Thank you