The Series-Shunt Feedback Amplifier

1

Differential and MultistageAmplifiers

Outline

•The MOS Differential Pair•Small-Signal Operation of the MOS Differential Pair•The BJT Differential Pair•Other Nonideal Characteristics of the Differential

Amplifier•The Differential Amplifier with Active Load•Frequency Response of the Differential Amplifier•Multistage Amplifiers

A Bipolar Op Amp

Diff-inDiff-out

Diff-inSingle-endedoutput

DC level ShiftingVoltage gain

Output

Example 7.4Connecting the two input terminals to ground.(a) Assume >>1, |VBE|=0.7V, neglect Early effect

(b) The quiescent powerdissipation

(c) Input bias current for=100

(d) Input common-moderange

Example 7.5(a) Input resistance

(b) Output resistance

Example 7.5 (con’t)(c) Voltage gain

Input stage Second stage

Example 7.5 (con’t)(c) Voltage gain

Third stage Output stage

Analysis Using Current Gains

i

c

c

b

b

c

c

b

b

c

c

b

b

e

i

e

ii

ii

ii

ii

ii

ii

ii

ii 2

2

5

5

5

5

7

7

7

7

8

8

88

Frequency Response

10

Feedback

Outline

•The General Feedback Structure•Some properties of Negative Feedback•The Four Basic Feedback Topologies•The Series-Shunt Feedback Amplifier•The Series-Series Feedback Amplifier•The Shunt-Shunt and Shunt-Series Feedback Amplifier•Determining the Loop Gain•The Stability Problem•Effect of Feedback on the Amplifier Poles•Stability Study Using Bode Plots•Frequency Compensation

Feedback

•Most physical systems incorporate someform of feedback–Negative –Degenerative–Positive –Regenerative

•Negative feedback in amplifier design–Desensitize the gain–Reduce nonlinear distortion–Reduce the effect of noise–Control the input and output impedances–Extend the bandwidth of the amplifier

•Everything good is at the expense of areduction of gain. Gain-reduction factor –theamount of feedback

General Feedback Structure

fsi xxx

sf xA

Ax

1

si xA

x

1

1

io Axx A : Open-loop gain

of xx : Feedback factor

AA

xx

As

of

1

A: Loop gain1+A: Amount of feedbackAf : Closed-loop gain

Exercise 8.1

(a) Assume that the op amp has infinite Rinand zero Rout, find

(b) If the open-loop voltage gain A=104, findR2/R1 to obtain closed-loop voltage gain Af=10

(c) What is the amount of feedbackIn decibels?

(d) If Vs=1V, find Vo, Vf, and Vi

(e) If A decreases 20%, what is theCorresponding decrease in Af?

Outline

•The General Feedback Structure•Some properties of Negative Feedback•The Four Basic Feedback Topologies•The Series-Shunt Feedback Amplifier•The Series-Series Feedback Amplifier•The Shunt-Shunt and Shunt-Series Feedback Amplifier•Determining the Loop Gain•The Stability Problem•Effect of Feedback on the Amplifier Poles•Stability Study Using Bode Plots•Frequency Compensation

Gain and Bandwidth

AA

Af

1

21 AdA

dAf

AdA

AA

dA

f

f

11

Gain Desensitivity

H

M

sA

sA

1

sA

sAsAf

1

MH

MMf As

AAsA

11

1

MHHf A 1

M

LLf A

1

1+A: desensitivity factor

Bandwidth Extension

Noise Reduction

2AVV

NS

n

s

n

s

VV

NS

21

1

21

21

11 AAA

VAA

AAVV nso

Reduction in Nonlinear Distortion

Gain = 0

Gain = 100

Gain = 1000

9.9001.010001

1000

5001.01001

100

1

2

f

f

A

A

What does negative feedback do to amplifier saturation?

Outline

•The General Feedback Structure•Some properties of Negative Feedback•The Four Basic Feedback Topologies•The Series-Shunt Feedback Amplifier•The Series-Series Feedback Amplifier•The Shunt-Shunt and Shunt-Series Feedback Amplifier•Determining the Loop Gain•The Stability Problem•Effect of Feedback on the Amplifier Poles•Stability Study Using Bode Plots•Frequency Compensation

Voltage Amplifiers

Input signal: voltageOutput signal: voltage

Voltage-mixing Voltage-sampling

Series –Shunt feedback topology

Current Amplifiers

Input signal: currentOutput signal: current

Current-mixing Current-sampling

Shunt –Series feedback topology

Transconductance Amplifiers

Input signal: voltageOutput signal: current

Voltage-mixing Current-sampling

Series –Series feedback topology

Transresistance Amplifiers

Input signal: currentOutput signal: voltage

Current-mixing Voltage-sampling

Shunt –Shunt feedback topology

Outline

•The General Feedback Structure•Some properties of Negative Feedback•The Four Basic Feedback Topologies•The Series-Shunt Feedback Amplifier•The Series-Series Feedback Amplifier•The Shunt-Shunt and Shunt-Series Feedback Amplifier•Determining the Loop Gain•The Stability Problem•Effect of Feedback on the Amplifier Poles•Stability Study Using Bode Plots•Frequency Compensation

The Ideal Situation

Af =

Rif =

ssAsZsZ iif 1

The Ideal Situation (con’t)

Rof =

ssA

sZsZ o

of

1

The Practical Situation

Problems:

1) Feedback network is not an idealvoltage-controlled voltage source

2) The source and load resistances A, Ri, and Ro will be affected!!

Given an amplifier, find the A circuit and thecircuit as the ideal structure

Two-Port Network Parameters

Four variables: V1, I1, V2, I2Two can be excitation, and the other two as response

3) h parameters: I1, V2 excitation, V1, I2 response

4) g parameters: V1, I2 excitation, I1, V2 response

1) y parameters: V1, V2 excitation, I1, I2 response

2) z parameters: I1, I2 excitation, V1, V2 response

Equivalent-Circuit Representation

h Parameters

The Practical Situation (con’t)

|h21|feedback << |h21|forward

Lof

out

sifin

RR

R

RRR

11

1

The Practical Situation (con’t)|h12|forward << |h12|feedback

The loading effect of the feedback network on the basic amplifier isrepresented by the components h11 and h22

02

112

1

I

VV

h

Summary

Example 8.1Find A, , Vo/Vs, Rin, and Rout

= 104, Rid = 100k, ro = 1k, RL = 2kR1 = 1k, R2 = 1M, Rs =10k

Op amp has f3dB = 1kHz, what is f3dBof the closed-loop gain?

Example 8.1 (con’t)