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Sourav SarkarSIT, Sukna

Darjeeling

FEEDBACKin

AMPLIFIERS

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Feedback

When a fraction of the output of an amplifier is combined with the input, feedback exists; if the feedback opposes the original signal, it is negative feedback and if it increases the signal it is positive feedback

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Feedback

• Like most engineered systems, the op-amp uses feedback to realize its potential value.

• Feedback comes in two forms– Positive Feedback– Negative Feedback

• It seems like positive feedback might be best, but negative feedback makes the op-amp work

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Positive and Negative Feedback

• Connecting the output to the positive input is positive feedback

• Connecting the output to the negative input is negative feedback

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Figure 8.5 A transistor amplifier with shunt–series feedback. (Biasing not shown.)

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Figure 8.6 An example of the series–series feedback topology. (Biasing not shown.)

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Feedback of Amplifier Circuits

• Feedback is very useful in Control Theory and Systems and is well researched

• Amplifier circuit can have negative feedback and positive feedback. Negative feedback returns part of the output to oppose the input, whereas in positive feedback the feedback signal aids the input signal.

• Negative feedback can reduce the gain of the amplifier, but it has many advantages, such as stabilization of gain, reduction of nonlinear distortion and noise, control of input and output impedances, and extension of bandwidth

Graphs are from Prentice Hall

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Concept of amplifier feedback

gain loop:

tcoefficienfeedback :

amplifier theofgain loopopen the:

amplifier theofgain loopclosed the:

feedback negative then , if ,1

A

A

A

AAA

A

x

xA

f

fs

of

.0 so,1

,1 If

fsissf xxxxA

AxxA

./1then ,1 If fAA

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Figure 8.4 The four basic feedback topologies: (a) voltage-mixing voltage-sampling (series–shunt) topology; (b) current-mixing current-sampling (shunt–series) topology; (c) voltage-mixing current-sampling (series–series) topology; (d) current-mixing voltage-sampling (shunt–shunt) topology.

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Amplifier negative feedback types• If the feedback network samples the output voltage, it is voltage

feedback. If it samples the output current, it is current feedback.• The feedback signal can be connected in series or in parallel with the

signal source and the amplifier input terminals, so called series feedback and parallel feedback.

• So, there are four types of negative feedback in amplifier circuits: Series voltage feedback (corresponding to (a) in previous slide) Series current feedback (corresponding to (b) in previous slide) Parallel voltage feedback (corresponding to (c) in previous slide) Parallel current feedback (corresponding to (d) in previous slide)

In voltage feedback, the input terminals of the feedback network are in parallel with the load, and the output voltage appears at the input terminals of the feedback block.

Whereas in current feedback, the input terminals of the feedback network are in series with the load, and the load current flows through the input of the feedback block.

As a result, a simple test on the feedback type is to open-circuit or short-circuit the load. If the feedback signal vanishes for an open-circuit load, then it is current feedback. If the feedback signal vanishes for a short-circuit load, it is voltage feedback.

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Negative Feedback Properties

Negative feedback takes a sample of the output signal and applies it to the input to get several desirable properties. In amplifiers, negative feedback can be applied to get the following properties

Desensitized gain – gain less sensitive to circuit component variations

Reduce nonlinear distortion – output proportional to input (constant gain independent of signal level)

Reduce effect of noise

Control input and output impedances – by applying appropriate feedback topologies

Extend bandwidth of amplifier

These properties can be achieved by trading off gain

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Effect of negative feedback on gain

v

vvff A

AA

A

AA

1 so,

1,generalIn

m

mmff G

GG

A

AA

1 so amplifier, uctance transcondaby modeled is this,

1 generalIn

m

mmff R

RR

A

AA

1 so amplifier, stance transresiaby modeled is this,

1 generalIn

• In series voltage feedback, input signal is voltage and output voltage is sampled, so it is natural to model the amplifier as a voltage amplifier.

• Amplifier employing series current feedback is modeled as a transconductance amplifier.

• Amplifier employing parallel voltage feedback is modeled as a transresistance amplifier.

• Amplifier employing parallel current feedback is modeled as a current amplifier.

i

iiff A

AA

A

AA

1 so amplifier,current aby modeled is this,

1 generalIn

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Negative feedback on input impedance

)1( ARR iif iR

)1/( ARR iif

• For series feedback, the following model can be used for analysis of input impedance (the output x could be either voltage or current)

If the input impedance of the open-loop amplifier is Ri, then the closed-loop impedance is

so, series feedback (either current or voltage) increase the input impedance

• Similarly, the effect of parallel feedback on input impedance can be analyzed using a similar model, the closed-loop input impedance would then beso, parallel feedback decrease the input impedance

feedback negativefor 1 notice ),1( AβARR iif

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Negative feedback on output impedance

)1/( ARR oof

• For voltage feedback, (it could be either series or parallel feedback), the closed-loop impedance is

so, voltage feedback decrease the output impedance• Similarly, for current feedback (either series or parallel feedback), the

closed-loop impedance is

so, current feedback increase the output impedance

• As a summary, negative feedback tends to stabilize and linearize gain, which are desired effects.

• For a certain type of amplifier, negative feedback tends to produce an ideal amplifier of that type.

• For example, series voltage feedback increases input impedance, reduces output impedance, which gets closer to an ideal voltage amplifier.

• So, negative feedback should be used in amplifiers circuits.

)1( ARR oof

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Gain Desensitivity

Feedback can be used to desensitize the closed-loop gain to variations in the basic amplifier. Let’s see how.

Assume beta is constant. Taking differentials of the closed-loop gain equation gives…

Divide by Af

This result shows the effects of variations in A on Af is mitigated by the feedback amount. 1+Abeta is also called the desensitivity amount

We will see through examples that feedback also affects the input and resistance of the amplifier (increases Ri and decreases Ro by 1+Abeta factor)

21 AdA

dAf

A

dA

AA

A

A

dA

A

dA

f

f

1

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1 2

AA

Af

1

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Some Properties of Negative Feedback

Gain Desensitivity

AfA

1 A

deriving

dAfdA

1 A ( )2

dividing by AfA

1 A

dAf

Af

1

1 A ( )

dA

A

The percentage change in Af (due to variations in some circuit parameter) is smaller than the pecentage cahnge in A by the amount of feedback. For this reason the amount of feedback

1 A

is also known as the desensitivity factor.

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Positive feedback

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An electronic device used for the purpose of generating a signal. An amplifier with positive feedback. The signal regenerate and sustain itself.Gain for amplifier with positive feedback

A

B

Vs+

-

VoOutput Signal

InputSignal

Feedback Signal

Vf = βVo

Vi = Vs - Vf

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Derive

AB

A

V

VABA

V

BVVA

V

V

BVVA

VVAV

ABVVBVV

VVV

AVV

s

o

s

os

s

o

os

fso

isos

fsi

io

1

)(

)(

)(

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Derive

s

of V

V

A

AA

1

where

Af = gain with feedback

A = open-loop gain

β = feedback factoro

f

V

V

(3.1)

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Positive Feedback AmplifierOscillator.

• A transistor amplifier with proper positive feedback can act as an oscillator. You must remember that a positive feedback amplifier is the one that produces a feedback voltage, Vf that is in phase with the original input signal.

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Requirement for Oscillation

Loop Gain, Loop Gain,

|A|Aββ| ≥ 1| ≥ 1

Net Phase Shift = 0Net Phase Shift = 0

Phase APhase Aββ = 0 = 0

Barkhausen Criteria

If a negative-feedback circuit has a loop gain that satisfies two conditions:

Note that for the circuit to oscillate at one frequency the oscillation criterion should be satisfied at one frequency only; otherwise the resulting waveform will not be a simple sinusoid.

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Important Important

Important for second internal : OPAMP Important for second internal : OPAMP (inverting , adder etc) ,Read about (inverting , adder etc) ,Read about SCR, CROSCR, CRO

DO mathematical problems from DO mathematical problems from chapters JFET and Feedbackchapters JFET and Feedback

Talk to me if you have any problem Talk to me if you have any problem with any part of the syllabuswith any part of the syllabus

Prepare for main practical exams also.Prepare for main practical exams also.

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ALL the BEST for your SEMESTER ALL the BEST for your SEMESTER EXAMSEXAMS