Differential Amp & OPAMP

8/13/2019 Differential Amp & OPAMP

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DIFFERENTIAL AMPLIFIER

AND OP-AMP

1

Gyan Ranjan Biswal

PhD, IIT Roorkee

Assistant Professor

Department of Electrical Engineering

School of Engineering

Shiv Nadar University, Noida


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Topics covered in this module

The basic operation amplifier

The differential amplifier

Offset error voltage and current

Basic operation amplifier applications

Analog integration and differentiation


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Basic Op-Amp

Operational amplifier or op-amp, is a very high gain

differential amplifier with a high input impedance (typically a

few meg-Ohms) and low output impedance (less than 100 W).

Note the op-amp has two inputs and one output.


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The function of a differential amplifier4 DIFF AMP is to amplify the

difference between two signals. It is the basic stage of integrated operational

amplifier.

The output depends

upon the difference.

vd is the differential-

mode signal and the

common-mode signal is

vc

where;

The Differential Amplifier


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We should like to haveAdlarge, Acshould equal zero.

Common mode rejection ratio

The Common Mode Rejection Ratio

Op-amp CMRR is a measure of the ability to cancel out common-mode

signals.

Because the op-amp has two inputs that are opposite in phase (inverting

input and the non-inverting input) any signal that is common to both

inputs will be cancelled.


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Equating current in R and R and using R

= R1and R = R1


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Vo= Vo1Vo2

= - Vd/ 2 = + Vd/ 2


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For perfect symmetry and identical transistor;

Re= Re1ll Re2Where;

Re1= Re2, Rc1= Rc2,

VCC and VEEacross Q1 and Q2 transistor are the same.

This method of obtaining O/P w.r.t. GND is known as Floating O/P.

Therefore,Vo= Vo1Vo2

As, IEQ1 = IEQ2 = IEQ; and therefore, current flowing through Re

is 2xIEQ.

CASE 1: Set VS2=VS1= Vd/ 2 = Vdm;

As both the source are 180oout of phase from same Q point.


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Differential transistor at Differential mode ;

As change in value of Q point is zero, so change across Re is

zero (A.C. voltage drop across Reis zero).


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Therefore, in case of common mode gain; the A.C. equivalent circuit and

approximated small signal circuit will be;


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The common mode

rejection ratio increases

with Re

However the emitter

supply VEE must become

larger as

Reis increased


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An op-amp is a wide-bandwidth amplifier. Thefollowing affect the bandwidth of the op-amp:

Gain

Slew rate

Frequency Parameters


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Op-Amps have a very high gain. They can be connected

open-loop or closed-loop.

Open-loop refers to a configuration where there is no

feedback from output back to the input. In the open-

loop configuration the gain can exceed 10,000.

Closed-loopconfiguration reduces the gain. In order to

control the gain of an op-amp it must have feedback.

This feedback is a negative feedback. A negative

feedback reduces the gain and improves many

characteristics of the op-amp.

Op-Amp Gain


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Gain and Bandwidth

The op-amps high

frequency response is limited

by internal circuitry. The plot

shown is for an open loop gain

(AOL or AVD). This means that

the op-amp is operating at the

highest possible gain with no

feedback resistor.

In the open loop, the op-

amp has a narrow bandwidth.

The bandwidth widens in

closed-loop operation, butthen the gain is lower.


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Maximum Signal Frequency

The slew rate determines the highest frequency of the op-amp without

distortion.

Where, VPis the peak voltage.

pV2

SRf


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Slew Rate (SR)

Slew rate (SR) is the

maximum rate at which an op-

amp can change output without

distortion.

s)V/(in

t

VSR o

Note: The SR rating is given in the specification sheets as V/s rating.


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Op-Amp SpecificationsDC Offset Parameters

Input offset voltage

Input offset current

Input offset voltage andinput offset current

Input bias current

Even when the input voltage is zero, there can be an output

offset. The following can cause this offset:


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Input Offset Voltage (VIO)

The specification sheet for an op-amp indicate an input offset voltage (VIO).

The effect of this input offset voltage on the output can be calculated with

1

f1

IOo(offset) R

RR

VV


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Output Offset Voltage Due to Input Offset Current (IIO)

The input offset Current (IIO) is specified in the specifications for

the op-amp.

The effect on the output can be calculated using:

IOo(offset) o(offset due to I )V V

If there is a difference between the dc bias currents for the same

applied input, then this also causes an output offset voltage:

l ff d


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Total Offset Due to VIOand IIO

Op-amps may have an output offset voltage due to

both factors VIOand IIO. The total output offset voltage willbe the sum of the effects of both:

)Itodue(offsetV)Vtodue(offsetV(offset)V IOoIOoo


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Input Bias Current (IIB)

A parameter that is related to input offset current (IIO) is called input

bias current (IIB)

The separate input bias currents are:

The total input bias current is the average:

2

III IOIBIB

2

III IOIBIB

2

III IBIBIB


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I ti O A


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The signal input is applied to the inverting () input

The non-inverting input (+)is grounded

The resistor Rf is the feedback resistor. It is connectedfrom the output to the negative (inverting) input. This is

negative feedback.

Inverting Op-Amp


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1

1 1 1

1

.

.o

f o ff

VI V Y

R

VI V Y

R

1

f

CL

RA

R

V-= V+

As, V+= 0; therefore, V-= 0

I ti O A G i


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Gain can be determined from external resistors: Rfand R1

Unity gainvoltage gain is 1

The negative sign denotes

a 180phase shift between

input and output.

o fv

1 1

V RA

V R

1R

RA

RR

1

fv

1f

Constant GainRfis a multiple of R1

Inverting Op-Amp Gain

Vi t l G d


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Virtual Ground

An understanding of the

concept of virtual ground

provides a betterunderstanding of how an

op-amp operates.

The non-inverting input

pin is at ground. The

inverting input pin is also at

0 V for an AC signal.

The op-amp has such high

input impedance that even with

a high gain there is no currentfrom inverting input pin,

therefore there is no voltage

from inverting pin to groundall

of the current is through Rf.


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Fig. 15.2a represents voltage-shunt feedback the voltage gainAVf

with feedback is given by


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Practical Op Amp Circuits


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Practical Op-Amp Circuits

Inverting amplifier

Non-inverting amplifier

Unity follower

Summing amplifier

Integrator

Differentiator

Inverting / Non inverting Op Amps


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Inverting / Non-inverting Op-Amps

11

fo VRRV

Inverting Amplifier Non-inverting Amplifier

11

fo V)RR1(V


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Unity Follower


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Unity Follower

1o VV


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Z andZ are equal in magnitude but differ in angle. Any phase shift,

from 0 to 360 (or 180) may be obtained.

R1=R2=R3=.=Rn=R1

Summing Amplifier


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Summing Amplifier

Because the op-amp hasa high input impedance,

the multiple inputs are

treated as separate inputs.

33f

22

f11

fo V

RRV

RRV

RRV

Integrator


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Integrator

The output is the integral

of the input. Integration isthe operation of summing

the area under a waveform

or curve over a period of

time. This circuit is useful

in low-pass filter circuits

and sensor conditioning

circuits.

(t)dtvRC1(t)v 1o


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Differentiator


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Differentiator

The differentiator

takes the derivative of

the input. This circuit is

useful in high-pass filter

circuits.

dt

(t)dv

RC(t)v 1

o


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Op-Amp Applications


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Op Amp Applications

Constant-gain multiplier

Voltage summing

Voltage buffer

Controlled sources

Instrumentation circuits

Active filters

Constant-Gain Amplifier


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Inverting Version



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Constant Gain Amplifier

Noninverting Version

Multiple-Stage Gains


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Multiple Stage Gains

1 2 3

f f f

1

A A A A

R R RA 1 R R2 R3

The total gain (3-stages) is given by:

or


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Voltage Buffer


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Voltage Buffer

Realistically these

circuits are designed

using equal resistors

(R1=Rf) to avoid

problems with offset

voltages.

Any amplifier with no gain or loss is called a unity gain amplifier.

The advantages of using a unity gain amplifier:

Very high input impedance

Very low output impedance

Controlled Sources


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Voltage-controlled voltage source

Voltage-controlled current source

Current-controlled voltage source

Current-controlled current source

Voltage-Controlled Voltage Source


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g g

The output voltage is thegain times the input voltage.

What makes an op-amp

different from other

amplifiers is its impedance

characteristics and gain

calculations that depend

solely on external resistors.

Non-inverting Amplifier Version

more

Voltage-Controlled Voltage Source


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g g

The output voltage is the

gain times the input voltage.

What makes an op-amp

different from other

amplifiers is its impedance

characteristics and gain

calculations that dependsolely on external resistors.

Inverting Amplifier Version

Voltage-Controlled Current Source


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g

The output current is:

1

1

1o kV

R

VI

Current-Controlled Voltage Source


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Current Controlled Voltage Source

This is simply another way

of applying the op-ampoperation. Whether the input

is a current determined by

Vin/R1or as I1:

or in1

fout VR

R

V

L1out RIV

Current-Controlled Current Source


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This circuit may appear more

complicated than the others but it isreally the same thing.

in

in

f

out

21

in

f

out

inin

fout

R

V

R

V

R||R

V

R

V

V

R

RV

kIRR1II

R

RR

R

VI

RR

RRVI

R||R

VI

2

1o

2

21

1

ino

21

21ino

21

ino

Instrumentation Circuits


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Some examples of instrumentation circuits using op-amps:

Display driver

Instrumentation amplifier

Display Driver


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p y

Instrumentation Amplifier


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For all Rsat the same value (except Rp):

2121P

o VVkVVR

2R1V

Active Filters


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Adding capacitors to op-amp circuits provides external control of the

cutoff frequencies. The op-amp active filter provides controllable cutoff

frequencies and controllable gain.

Low-pass filter

High-pass filter

Band-pass filter

Low-Pass FilterFirst-Order


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11OH

CR21f

1

fvRR1A

The upper cutoff frequency

and voltage gain are given by:

High-Pass Filter


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11OL CR2

1f

The cutoff frequency is determined by:

Bandpass Filter


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There are two cutoff

frequencies: upper and

lower. They can be

calculated using the same

low-pass cutoff and high-

pass cutoff frequency

formulas in the appropriate

sections.


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An op amp proportional-mode controller.


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An op amp integral-mode controller.


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An op amp derivative-mode controller.


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Differential Amp & OPAMP