EC2251 U1 Notes

8/10/2019 EC2251 U1 Notes

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Electronic circuits-II

EC2251

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8/10/2019 EC2251 U1 Notes


5

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Unit-1

Feedback amplifiers

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8/10/2019 EC2251 U1 Notes


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Feedback

Consists of returning part of the output of a system to the input

Negative Feedback: a portion of the output signal is returned to the input inopposition to the original input signal

Positive Feedback: the feedback signal aids the original input signal

Negative Feedback Effects:

– Reduces gain

–Stabilizes gain

– Reduces non linear distortion

– Reduces certain types of noise

– Controls input and output impedances

– Extends bandwidth

The disadvantage of reducing the gain can be overcome by adding few morestages of amplification

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8/10/2019 EC2251 U1 Notes


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Feedback amplifier. Note that the signals are denoted as x i , x f , x o, and so on.The signals can be either currents or voltages

A

A A f

1 Negative feedback(Af<A)

A

A

A f

1 Positive feedback (Af>A)

Af--- closedloop gain

A---O pen loop

gain

A----loop gain

Positive feedback provides an easy way to obtain large gain.

It leads to poor gain stability , a slight shift in power supply

Or temp can change the magnitude of loop gain to unity &

cause the Amplifier to break into oscillation .

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8/10/2019 EC2251 U1 Notes




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The four basic feedback topologies: (a) voltage-sampling series-mixing (series-shunt) topology; (b) current-

sampling shunt-mixing (shunt-series) topology; (c) current-sampling series-mixing (series-series) topology; (d)

voltage-sampling shunt-mixing (shunt-shunt) topology.

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8/10/2019 EC2251 U1 Notes


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Effects of various types of feedback on gain

A

A

x

x A

s

f

1

0

Av Av

vv A s

vf

1

0

Gm

Gm

x

xG

s

mf

1

0

Rm

Rm

x

x R

s

mf

1

0

Ai

Ai

x

x A

sif

1

0

Gain Stabilization

If we design the amplifier so that A >> 1, then theclosed loop gain Af is approximately 1/

Under this condition Af depends only on the stablepassive components (resistor or capacitors) used in thefeedback network, instead of depending on the open loopgain A which in turn depends on active device parameters

(gm) which tend to be highly variable with operating pointand temperature

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8/10/2019 EC2251 U1 Notes


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Summary (Effects on feedback)

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Analysis of feedback amplifiers

Steps

1.Identify the type of feedback

2.Redraw the amplifier circuit without the effect of feedback .

3.Use a thevenin’s source at the input for series mixing and use a Norton’s

source at the input for shunt mixing.

4.After drawing the amplifier circuit without feedback determine the ac

parameters of the circuit using the h parameter model.

5.Determine the feedback ratio = xf / xo from the original circuit.

6. Find he desensivity factor(D).

7.Knowing A,D,Ri,and Ro , Find Af, Rif , Rof.

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8/10/2019 EC2251 U1 Notes


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Nyquist criterion

Criterion Of Nyquist:

The amplifier is unstable if this

curve encloses the point –1+j0

and the amplifier is stable if

the curve does not enclose

this point

Re (A)

Im(A)

-1+j0

A

f2 f 1

fn

1+ A

Gain and

phase margins

These are a measure

of the stability of a

circuit

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EC2251 U1 Notes