Unit 8

Operational Amplifier

Objectives:• Ideal versus practical operational amplifier

• Performance parameters

• Some applications– Peak detector– Absolute value circuit– Comparator– Active filter– Non linear amplifier

…etcetera

Intro.:• Deferential amplifier: amplifier that amplifies

the difference of two inputs• VO = AV (V1 ~ V2)

• Op amp contains differential amplifier

AV

V1

V2

VO

Features / why op amp:

Ideal Op Amp. -

• Amplifies any frequency signal, 0 to ∞ Hz, i.e., also DC.

• Infinite bandwidth

• Infinite gain (AV)

• Infinite input impedance (resistance - Ri)

• Zero output impedance (resistance - Ro)

Block schematic of op amp:

Basic differential op amp:

16.3 Ideal v/s practical op amp:Thevenin’s equivalent circuit:

Thevenin’s models -ideal v/s practical:

Ideal op amp assumptions: (primary)

Ideal op amp properties:

16.4 performance parameters:

Band width:

Frequency

Hz

Voltage Gain Av in dB

Av (max)

f1 f2

Band width = f2 – f1

3 dB

Frequency Response Curve

Frequency response of IC 741:

Frequency response of AD829:

Slew rate:Defn.:

rate of change of output voltage with time -dV/dt

• Indication of how well the op amp output follows the rapidly changing waveform at the input.

• Slew rate puts a limit on high bandwidth at input

Open loop gain :• Open loop gain:

• Closed loop gain:

AOL

ACL

Vi Vo

Vi Vo

Loop gain is ratio of open loop gain to closed loop gain

Common Mode Rejection Ratio –CMRR:• CMRR is measure of ability of the op amp to

suppress common mode signals

• Defn Ad

CMRR = ---------- Ac

Where Ad = difference mode gain Ac = common mode gain

Power Supply Rejection Ratio – PSRR:

• PSRR is defined as ratio of change in power supply voltage to change in output voltage

• A measure of op amp’s ability to maintain the operating point stable.

• The power supply may change the operating point of the differential amplifier.

Input Impedance (resistance):• Input impedance is the impedance looking into

the input terminal measured in ohms.

Output Impedance:• Output impedance is defined as the

impedance measured between the output terminal and ground

Settling time:• It is expressed as the time taken by the op amp

output to settle within a specified percentage of the final value (0.1% or 0.01%) in response to a step input.

• Speed of operation is affected by large settling time

Step input to op amp

Offsets:• Ideal op amp should produce zero output for

zero differential input (VO=0 for Vd=V1-V2=0)

• We need to apply a DC differential voltage externally to get a zero output, known as input offset (range: 200µV – 5mV)

• or Offset null:

741

V1

V2

VO

Some Applications:1. Peak detector

2. Absolute value circuit

3. Comparator

4. Active filter

5. Phase shifter

6. Instrumentation amplifier

7. Non-linear amplifier

8. Relaxation oscillator

9. Current-to-voltage converter

10. Voltage-to-current converter

11. Sine wave oscillator

Revision: Inverting Amplifier

Revision: Non-inverting Amplifier

17.12 Peak detector:

First op amp is amplifying Vi, diode clipping –ve half cycle, capacitor getting charged to +ve peak and holding constant voltage, second op amp amplifying constant DC.

Resistor R to discharge in case Vi peak is decresed.

Absolute value circuit:

Comparator:

Active filters:

HF

LF

Phase shifter:

Non-linear amplifier:Working:

•Diode resistance decides gain

•For small i/p diode is open, diode resistance is high, gain high

•For large signal diode is closed, diode resistance is less, gain is less.

Balance detector:

•Output of bridge varies over a wide range (1 – 10000)

•Every time sensitivity of milli-ammeter has to be adjusted, if this non-linear amplifier is not used.

Relaxation oscillator:

Working:

1. R1, R2, op amp constitutes an amplifier with gain (1+ R2/R1)

2. Assume, initially C has not charged, Vo=high (logic 1)

3. Through R, C starts getting charged,

4. When it crossed above +Vsat, Vo=low( 0 volt) –(comparator action)

5. C starts dishrging, till crosses –Vsat

6. When it crosses –Vsat, Vo=high –(comparator action)

Current-to-voltage converter:

Voltage-to-current converter:

Sine wave oscillator:

Questions:

1. Compare ideal and practical op amps

2. Explain the op amp parameters

3. Explain Peak detector

4. Explain absolute value circuit

5. Explain comparator

6. Explain active filter

7. Explain phase shifter

8. Explain non-linear amplifier

9. Explain relaxation oscillator

10. Explain current-to-voltage & voltage-to-current converters

11. Explain sine wave oscillators

Ends

Unit 8 Operational Amplifier