Unit-I (Feedback amplifiers)Features of feedback amplifiers

Presentation by:

S.Karthie, Lecturer/ECE

SSN College of Engineering

OBJECTIVES

To make the students understand the effect of negative feedback on the following amplifier characteristics:

• Gain

• Distortion

• Noise

• i/p & o/p impedance

• Frequency response & Bandwidth

Feedback of Amplifier Circuits

• Feedback is to return part of the output to the input for a circuit/system

• 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.

• Both negative feedback and positive feedback are used in amplifier circuits

• 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

General structure of the feedback amplifier

This is a signal-flow diagram, and the quantities x represent either voltage or current signals

gain loop:

tcoefficienfeedback :

amplifier theofgain loopopen the:

amplifier theofgain loopclosed the:

feedback negative then , if ,1

β

β

β

A

A

A

AAA

A

x

xA

f

f

s

of

−

−

<+

==

• Thus, the closed-loop gain would be much more stable and is nearly independent of changes of open-loop gain

• Thus, in a negative feedback amplifier, the output takes the value to drive the amplifier input to almost 0 (this is summing point constraints).

./1then ,1 If ββ ≈>> fAA

.0 so,1

,1 If ≈−=≈+

=>> fsissf xxxxA

AxxA

β

ββ

Gain formula

Amplifier negative feedback:To reduce nonlinear distortion

1>>βA

010-for 98.9

100for 99.9

<<=

<<=

of

of

xA

xA

• If a pre-amplifier with gain 1000 is placed before the nonlinear one so that the whole amplifier is used with negative feedback,

and the gain for whole amplifier becomes:

which greatly reduce the nonlinear distortion.

• This is achieved through compensatory distortion of the input signal

Amplifier negative feedback:noise reduction

2

22

2

21

1

21

21

11

221

2

)()(

)(

1)(

1)()(

)()(

)()()(

)()()(

Ax

xSNR

AA

Atx

AA

AAtxtx

txAtx

txtxAtx

txtxtx

noise

s

noiseso

o

noise

os

=

++

+=

=

+=

−=

ββ

β2

2

11

)(

)(

)()()(

noise

s

noiseso

x

xSNR

AtxAtxtx

=

+=

• If an amplifier (assumed to be noise free or very low noise) is placed before the noisy amplifier, then the Signal-to-Noise (SNR) ratio is greatly enhanced (by a factor equal to the preceding amplifier gain)

Features of negative feedback amplifier

Stability:

Due to temperature, aging and replacement effects, the amplifier gain becomes

unstable. The fractional change in amplification with feedback divided by the

fractional change without feedback is called the sensitivity of the transfer gain.

A

dA

AA

dA

f

f

β+=

1

1

sensitivity

If |βA| >> 1, then Af ~ 1/β Gain depends entirely on the feedback network

The main reason for the instability is due to the active devices involved, if the feedback

network contains only stable passive elements, the improvement in stability is

expected.

Feedback topologies

As mentioned above, the four feedback topologies can be defined as:

(connection of feedback network to input terminal) –(connection of feedback network to output terminal)• i.e,

– Series-shunt (input to F: Voltage, output to F: Voltage)

– Series-series (input to F: Voltage, output to F: Current)

– Shunt-series (input to F: Current, output to F: Current)

– Shunt-shunt (input to F: Current, output to F: Voltage)

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.

Effect of negative feedback on gain

ββ v

v

vffA

AA

A

AA

+=

+=

1 so,

1,generalIn

ββ m

m

mffG

GG

A

AA

+=

+=

1 so amplifier, uctance transcondaby modeled is this,

1 generalIn

ββ m

m

mffR

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 avoltage 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.

ββ i

i

iffA

AA

A

AA

+=

+=

1 so amplifier,current aby modeled is this,

1 generalIn

Negative feedback on input impedance

)1( βARR iif +=iR

)1/( βARR 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 be

so, parallel feedback decrease the input impedance

feedback negativefor 1 notice ),1( >>+= AβARR iif β

Effect of feedback on the input resistance of amplifier

Feedback mixing

series shunt

Input resistanceincreases decreases

In feedback amplifiers, series mixing tends to increase the input resistance and shunt mixing tends to decrease the input resistance.

remember

Negative feedback on output impedance

)1/( βARR 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

)1( βARR oof +=

Feedback sampling

voltage current

output resistance increasesdecreases

remember

How?

Let us think of voltage sampling. If RL increases so that Vo increases, the effectof feeding this voltage back to the input in negative manner will cause Vo toincrease less than if there were no feedback. Hence, the output voltage remainsconstant as RL changes, which means Rof << RL.

For current sampling. In negative feedback which samples the output currentwill hold this current constant. Hence, an output current source is created,which means Rof >> RL.

Effect of feedback on the output resistance of amplifier

Effect of negative feedback on gain and bandwidth

Bandwidth B

i

o

Vv

vA =

ff1f2

Avo

0.707 Avo

Bandwidth Bf

Avf

f1f f2f

v

v

VfA

AA

β+=

1

ββ 11 =⇒>> vfv AA

But gain drops at higher and lower frequencies, therefore βA >> 1 no longer holds.

Gain × bandwidth is same for a feedback and basic amplifier.

It is important to note that even β is constant gain A depends on frequency. Let us

Consider a single pole amplifier.

)1(;1

)/(1

)/(1)/(1/1

)/(1

)/(1

0

oHHf

o

oof

Hf

of

f

Ho

o

H

Hof

H

o

AffA

AA

ffj

AA

ffjA

A

ffjA

ffjAA

ffj

AA

ββ

ββ

+=+

=

+=⇒

++=

++

+=

+=

o

LLf

HoHfof

A

ff

fAfA

β+=

=

1

Where Ao is midband gain without feedback and fH is the

high 3-dB frequency.

Af is gain with feedback.

fL is low 3-dB frequency.

Summary

• As a summary, negative feedback tends to stabilize and linearizegain, 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.