unit 3 op amp

8/4/2019 UNIT 3 OP AMP

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TOPIC 3OP AMP

OPERATIONAL AMPLIFIER


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OPERATIONAL AMPLIFIERS (OP AMP)

Operational amplifier (op amp) is an integratedcircuit that amplifies the difference between twoinput voltages and produces a single output.

Have high gain amplifier and able to amplify signal

with frequency ranging from 0 to 1MHz.

To perform mathematical operations likesummation, subtraction, multiplication, differentialand integration etc in analogue computer.

It has 2 input terminal :

a) Inverting input terminal

b) Non inverting input terminal


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Continue.

A complete amplifier electronic circuit may containstransistor, diode, resistor, capacitorand otherscomponents and constructed on a single silicon ship.

Op-amp application: As scalar orlinear (e.g. small signal) constant gain

amplifierboth inverting and no-inverting

As unity follower

Adder or summer

Subtractor Integr ator

Differentiator

comparator


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Symbol of operational amplifier

NON INVERTING INPUT

INVERTING INPUT


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Inverting

Noninverting

Internal Block Diagram

Op Amp

Differential

Amp.

Highimpedance

High Gain

Voltage

Amplifier

Low impedance

Output

Output


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First Stage

Input terminal

Has 2 input:-

+ non inverting input

- inverting input

high input impedance


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Second Stage

Gain Voltage amplifier

Is the ability of an electronic circuit toincrease level of a signal.

High voltage gain

High drive current gain to trigger output and

not burden to input.


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Third Stage

Output terminal

has 1 output terminal

low output impedance


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Differential Amplifier block diagram


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Push pull amplifier block diagram


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DIFFERENTIALAMPLIFIER

Connect 1 voltage signalonto 1 input terminal andanother voltage signalonto other input terminal.

The output voltage will beproportional to

the "Difference" betweenthe two

input voltage signals of V1and V2.

Vout = R3 (V2-V1)

R1


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Input Bias Current

Ideally, no currentflows into the inputterminals of an opamp.

In practice, there arealways two input biascurrents, IB+ and IB-(see figure beside).

It is an average

current flowing bothinputs.

Ideally, two input biascurrents are equal.

Diagram of inputbias current


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Continued

A very variable parameter!

IB can vary from 60 A to many A,

depending on the device.

Some structures have well-matched IB ,

others do not. Some structures' IB varies little with

temperature, but a FET op amp's IBdoubles with every 10C rise in temp.

Some structures have IB which may flowin either direction.


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Input Offset Current

Input offset current, IOS= Ib+- Ib-.

Offset current arises from incidental

imbalances in the internal component of theamplifiers.

This is the difference of the two input bias

currents when the output is zero.The offset current value is usually smaller

than bias current.


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INPUT OFFSET VOLTAGE

The voltage source Voffin series with the input terminals iscalled an offset voltage, it is caused by internal circuitimbalances.

This is the voltage that must be applied to one of the inputpins to give a zero output voltage.

Foran ideal op amp, output offset voltage is zero.


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Common Mode Gain

Gain whenboth input terminal have same signal Should be when Vid =0, Vo= 0

Pr actically, when Vo have value Acm will have smallvalue.

Knows as CMRR, Common Mode Rejection RatioCMRR = Closed loop gain, Av

Common mode gain, Acm

NormallyAcm


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A measure of the ability of the op-ampto reject signals that are simultaneously

present at both inputs is called the

Common Mode Rejection Ratio orCMRR.

It is the ratio of the common-mode input

voltage to the generated output voltage.

Expressed in decibels (dB)

Common Mode Rejection Ratio(CMRR).


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More stages of gain amplifier

When a number of stages are connectedin series, the overall gain is the product of

the individual stage gain.

Figurebelow shown a connection of threestage.

More stages of gain amplifier


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The first stage is connected to provide

non inverting gain as given by equation

belowA = 1 + Rf

R1


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The overall circuit gain is then

noninverting and calculate by

A = A1A2A3Where

A1 = 1+Rf , A2 = -Rf and A3 = -Rf

R1 R2 R3


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PUSH PULL AMPLIFIER

When we get amplifier output current for 1800 ofinput it's called B class amplifier.

In a push pull class B amplifier, one of the 2 powertransistors or other amplifying elements handles the

positive half of the waveform and the other elementhandles the negative half of the waveform as shownin figure below.

Figure: Block representation of push pull operation.


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Continued

Two amplifiers are used to accomplishclass B power amplifier.

One is used to push the current and the

other one is used to pull the current.

These two amplifiers are almost same butone is connector supplied and the other

one is emitter supplied.

This "push-pull" amplifier is used where

high power output and good fidelity areneeded: Example: receiver output stages,

public address amplifiers, and AM

modulators.


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Operation circuit..

R1 provides the proper bias for Q1 and Q2.

The tapped secondary ofT1 develops thetwo input signals for the bases of Q1 and Q2.

Half of the original input signal will beamplified by Q-1, the other half by Q-2.

T2 combines (couples) the amplified output

signal to the speaker and providesimpedance matching

Figure: Push Pull circuit


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Definition of IDEAL OP AMP

An idealised op-amp has the

following characteristics:

infinite input impedance, infinite open-loop gain,

zero output impedance,

infinite bandwidth,

zero noise


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Parameters Characteristics of Ideal OP AmpVoltage gain

(to increase the output voltage)Input impedance

(when Rin = ,zero current flowsfrom V+ to V- )

Output impedance 0 (i.e., Rout = 0, so that output voltage

does not vary with output current).

Input offset voltage 0 (i.e., when the input terminals are

shorted so that , the output is a virtual

ground or Vout = 0).

Bandwidth (i.e., the frequency magnituderesponse is considered to be flat

everywhere with zero phase shift)

IDEAL OP AMP


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An offset voltage

means that there exists a voltage vd

when both inputs are grounded. Thisoffset is called an input offset

because the voltage vd is offset from

its ideal value of zero volts. The input

offset voltage is multiplied by the

open loop gain to create an output

offset voltage.


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unit 3 op amp

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