Differential and Multistage
Amplifiers
1
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
DC level Shifting
Voltage gain
Diff-in
Diff-out
Diff-in
Single-ended
output
Output
Example 7.4
Connecting the two input terminals to ground.
(a) Assume >>1, |VBE|=0.7V, neglect Early effect
(b) The quiescent power
dissipation
(c) Input bias current for
=100
(d) Input common-mode
range
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
ie8 ie8 ib8 ic 7 ib 7 ic 5 ib 5 ic 2
      
ii ib8 ic 7 ib 7 ic 5 ib 5 ic 2 ii
Frequency Response
Feedback
10
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 some
form 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 a
reduction of gain. Gain-reduction factor –the
amount of feedback
General Feedback Structure
xo Axi
x f xo
A : Open-loop gain
: Feedback factor
xi xs x f
A: Loop gain
xo
A
Af  
1+A: Amount of feedback
xs 1 A Af : Closed-loop gain
A
xf 
xs
1 A
1
xi 
xs
1 A
Exercise 8.1
(c) What is the amount of feedback
In decibels?
(d) If Vs=1V, find Vo, Vf, and Vi
(a) Assume that the op amp has infinite Rin
and zero Rout, find 
(b) If the open-loop voltage gain A=104, find
R2/R1 to obtain closed-loop voltage gain Af=10
(e) If A decreases 20%, what is the
Corresponding 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
Gain Desensitivity
A
Af 
1 A
dA
dA f 
2

1 A
dA f
Af
1
dA

1 AA

1+A: desensitivity factor
Bandwidth Extension
A
A
s  M
1 s H
A
s
A f 
s 
1 A
s
A 
1 AM 
A f 
s  M
1 s H 
1 AM 
Hf H 
1 AM 
L
Lf 
1 AM 
Noise Reduction
S Vs

N Vn
Vo Vs
A1 A2
A1
Vn
1 A1 A2 
1 A1 A2 
S Vs
 A2
N Vn
Reduction in Nonlinear Distortion
Gain = 1000
Gain = 100
Gain = 0
100
Af 2 
50
1 100 0.01
1000
Af 1 
90.9
1 1000 0.01
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
Voltage-mixing Voltage-sampling
Series –Shunt feedback topology
Input signal: voltage
Output signal: voltage
Current Amplifiers
Current-mixing Current-sampling
Shunt –Series feedback topology
Input signal: current
Output signal: current
Transconductance Amplifiers
Voltage-mixing Current-sampling
Series –Series feedback topology
Input signal: voltage
Output signal: current
Transresistance Amplifiers
Current-mixing Voltage-sampling
Shunt –Shunt feedback topology
Input signal: current
Output signal: voltage
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 =
Z if 
s Z i 
s
1 A
s s 
The Ideal Situation (con’
t)
Rof =
Z o 
s
Z of 
s 
1 A
s s
The Practical Situation
Problems:
1) Feedback network is not an ideal
voltage-controlled voltage source
2) The source and load resistances
 A, Ri, and Ro will be affected!!
Given an amplifier, find the A circuit and the
 circuit as the ideal structure
Two-Port Network Parameters
Four variables: V1, I1, V2, I2
Two can be excitation, and the other two as response
Equivalent-Circuit Representation
1) y parameters: V1, V2 excitation, I1, I2 response
2) z parameters: I1, I2 excitation, V1, V2 response
3) h parameters: I1, V2 excitation, V1, I2 response
4) g parameters: V1, I2 excitation, I1, V2 response
h Parameters
The Practical Situation (con’
t)
Rin Rif Rs
1
Rout 
1

1
  
R

R
of
L


|h21|feedback << |h21|forward
The Practical Situation (con’
t)
|h12|forward << |h12|feedback
V1
h12 
V2
The loading effect of the feedback network on the basic amplifier is
represented by the components h11 and h22
I1 0
Summary
Example 8.1
Find A, , Vo/Vs, Rin, and Rout
= 104, Rid = 100k, ro = 1k, RL = 2k
R1 = 1k, R2 = 1M, Rs =10k
Op amp has f3dB = 1kHz, what is f3dB
of the closed-loop gain?
Example 8.1 (con’
t)