Comparison of Amplifier Configurations

1
Comparison of Amplifier Configurations
Midband Characteristics

• These are approximate expressions
  neglecting the effects of the biasing
  resistors R1 and R2
• and the source resistance RS.

J. Millman and A. Grabel, Microelectronics, 2nd
Ed., McGraw Hill, NY (1987), p. 420.
2
Characteristics of Amplifier Configurations
Current gain is large (? ß) for CE and
EF, but lt 1 for CB.
Voltage gain is large for CE and CB, but
lt 1 for EF.

• Input resistance is
• Very small (few Os) for CB,
• Medium (few KOs) for CE, but
• Very large ( 10s of KOs) for EF.

• Output resistance is
• Very small (few Os) for EF,
• Very large ( 100s of KOs) for
• CE and CB.

3
Numerical Comparison of Amplifier
Configurationsfor the Same Transistor and
DC Biasing

• These are approximate expressions
  neglecting the effects of the biasing
  resistors R1 and R2
• and the source resistance RS.

4
Comparison of CB to CE Amplifier (with same
Rs 5 O)
CE (with RS 5 O) CB (with RS 5O)
Midband Gain Low Frequency Poles and
Zeros High Frequency Poles and Zeroes
5
Comparison of EF to CE Amplifier (For RS
5O )
CE EF
Midband Gain Low Frequency Poles and
Zeros High Frequency Poles and Zeroes
6
Comparison of Amplifier Configurations
Midband Gain and High and Low Frequency
Performance
CE CB EF
Midband Voltage Gain -191 V/V 102 V/V 0.987
V/V 45.6dB 40.2dB - 0.1dB Low 3dB
Frequency 1.7x104 rad/s 5.0x104 rad/s 2.6x102
rad/s High 3dB Frequency 5.0x107 rad/s 7.1x108
rad/s 1.0x1010 rad/s
RS 5 O

• Results for all three amplifiers with
  the smaller (5O) source resistance RS.

7
Cascade Amplifier
CE
EF

• Emitter Follower Common Emitter (EFCE)
• Voltage gain from CE stage, gain of one
  for EF.
• Low output resistance from EF provides a
  low source resistance for CE amplifier so
  good matching of output of EF to input of
  CE amplifier
• High frequency response (3dB frequency) for
  Cascade Amplifier is improved over CE
  amplifier.

8
Cascade Amplifier - DC analysis
IB1
IB2
IE1
IRE1
Small Signal Parameters
9
Cascade Amplifier - Midband Gain Analysis
Note rx1 rx2 0 so equivalent circuit is
simplified.
Ip1


Vp1
_

Vi
Vp2
_
_
Ri
Note Voltage gain is nearly equal to
that of the CE stage, e.g. 68 !
10
Cascade Amplifier - Low Frequency Poles and
Zeroes

• Use Gray-Searle (Short Circuit) Technique to find
  the poles.
• Three low frequency poles
• Equivalent resistance may depend on rp for both
  transistors.
• Find three low frequency zeroes.

11
Cascade Amplifier - Analysis of Low Frequency
Poles Gray-Searle (Short Circuit) Technique
Input coupling capacitor CC1 1 µF
rp1
Vp1

rp2
RE1
Vp2
_
IX
Ip1
Ri
Vi
RE1
rp2
12
Cascade Amplifier - Analysis of Low Frequency
Poles Gray-Searle (Short Circuit) Technique
CC2
rX2
Vo
gm2Vp2
Vp2
RL
RC
rp2
RE2
CE

• Output coupling capacitor CC2 1 µF

VX
Vo
RC
RL
13
Cascade Amplifier - Analysis of Low Frequency
Poles Gray-Searle (Short Circuit) Technique
Emitter bypass capacitor CE 47 µF
Ip1
gm1Vp1
r p1
Vp1
VE1
Ie1
Ip2
gm2Vp2
Vp2
re1
rp2
RE1
VE2
Ix
Ie2
RE2
re2
VX
IE2
Low 3 dB Frequency
The pole for CE is the largest and therefore
the most important in determining the low 3 dB
frequency.
14
Cascade Amplifier - Low Frequency Zeros

• What are the zeros for the Cascade amplifier?
• For CC1 and CC2 , we get zeros at ? 0 since ZC
  1 / j?C and these capacitors are in the signal
  line, i.e. ZC ?? at ? 0 so Vo ? 0.
• Consider RE in parallel with CE
• Impedance given by
• When ZE ? ?, Ip ? 0, so gmVp ? 0, so Vo ? 0
• ZE ? ? when s - 1 / RE2CE so pole for CE
  is at

15
Cascade Amplifier - High Frequency Poles and
Zeroes

• Use Gray-Searle (Open Circuit) Technique to find
  the poles.
• Four high frequency poles
• Equivalent resistance may depend on rp for both
  transistors.
• Find four high frequency zeroes.

High Frequency Equivalent Circuit
16
Cascade Amplifier - High Frequency Poles
Pole for Capacitor Cp1 13.9 pF
Ix
_
VX
Ix- Ip1
Ip1
Ie1
17
Cascade Amplifier - High Frequency Poles
Pole for Capacitor Cµ1 2 pF
_
Ix
Ip1
Ix- Ip1
VX
Ie1
18
Cascade Amplifier - High Frequency Poles and
Zeroes
Simplified Equivalent Circuit
Using Millers Theorem, replace Cµ2 by two
capacitors.
19
Cascade Amplifier - High Frequency Poles
Pole for Capacitor CT 152 pF
_
Ip1
Vp1
Ve1
Ix
Ie1
re1
VX
Pole for Output Capacitor Cµ2 2 pF

VX
gm2Vp2
_
20
Cascade Amplifier - High Frequency Zeroes
Ie1

• When does Vo 0?
• When ? ? 8, ZCµ1? 0, so signal shorted to
  ground. ?ZH1 8.
• When ? ? 8, ZCp2? 0, so rp2 shorted, so Vp2 0.
  ?ZH2 8.
• For Cp1 , we get a zero when Ie1 0.

Ie1
21
Cascade Amplifier - High Frequency Zeroes
I Cµ2
IRL 0

• When does Cµ2 produce a zero, i.e. make Vo 0?
• For Cµ2 , we get a zero when IRL 0, or ICµ2
  gm2Vp2 , i.e. the output load resistance RL
  is starved of any current.

Zero for Output Capacitor Cµ2 2 pF
22
Cascade Amplifier - High Frequency Poles and
Zeroes
23
Comparison of Cascade to CE Amplifier
CE Cascade (EFCE)
2 X improvement in voltage gain !
Midband Gain Low Frequency Poles and
Zeros High Frequency Poles and Zeroes
25 X improvement in bandwidth !
CE stage with same transistor, biasing
resistors, source resistance and load as cascade.
24
Comparison of Cascade to CE Amplifier

• Why the better voltage gain for the cascade?
• Emitter follower gives no voltage gain!
• Cascade has better matching with source than CE.
  
• Cascade amplifier has an input resistance that is
  higher due to EF first stage.
• Versus Ri2 rp2 2.5 K for CE
• So less loss in voltage divider term (Vi / Vs )
  with the source resistance.
• 0.91 for cascade vs 0.37 for CE.
• Why better bandwidth?
• Low output resistance re1 of EF stage gives
  smaller effective source resistance for CE stage
  and higher frequency for dominant pole due to CT
  (including Cµ2)

Ri1
Ri2
Pole for Capacitor CT 152 pF
re1
25
Another Useful Amplifier Cascode (CECB)
Amplifier

• Common Emitter Common Base (CE
  CB) configuration
• Voltage gain from both stages
• Low input resistance from second CB stage
  provides first stage CE with low load
  resistance so Miller Effect multiplication
  of Cµ1 is much smaller.
• High frequency response dramatically improved
  (3 dB frequency increased).
• Bandwidth is much improved (130 X).

Large Miller Effect
Small Miller Effect
Bandwidth is improved by a factor of 130X
over that for the CE amplifier !
26
Example of Cascode (CE CB) Amplifier
http//www.freescale.com ECTW Conf. Proceedings
2003.
27
Other Examples of Multistage Amplifiers
CE CE
EF EF
Darlington Pair
28
Other Examples of Multistage Amplifiers
Push Pull Amplifier
Amplifier with Npn and Pnp Transistors
Amplifier with FETs and Bipolar Transistors
29
Differential Amplifier

• Similar to CE amplifier, but two CEs operated in
  parallel
• Signal applied between two equivalent inputs
  instead of between one input and ground
• Common emitter resistor or current source used
• Current shared or switched between two
  transistors (they compete)
• Analyze using equivalent half-circuit
• 1/2 of signal at input
• 1/2 of signal at output
• 1/2 of source resistance
• Gain and frequency response similar to CE
  amplifier for high frequencies
• Advantage
• Rejects common noise pickup on input
• No coupling capacitors so can operate down to
  zero frequency.

_

Vo
30
Differential Amplifier Analysis
Midband Gain
Vo
Vo /2
Low Frequency Poles and Zeros
Direct coupled so no coupling capacitors
and no emitter bypass capacitor No low
frequency poles and zeros Flat (frequency
independent) gain down to zero frequency
High Frequency Poles and Zeros
Vo /2
Dominant pole using Millers Thoerem
High frequency performance is very similar
to CE amplifier.
31
Summary

• In this chapter we have shown how to analyze the
  high and low frequency dependence of the gain for
  an amplifier.
• Analyzed the effects of the coupling capacitors
  on the low frequency response
• Found the expressions for the corresponding poles
  and zeros.
• Demonstrated Bode plots of magnitude and phase.
• Analyzed the effects of the capacitances within
  the transistor on the high frequency response.
• Found the expressions for the corresponding poles
  and zeros.
• Demonstrated Bode plots of the magnitude and
  phase.
• Analyzed the high and low frequency performance
  of the three bipolar transistor amplifiers
  common emitter, common base and emitter follower.
• Found the expressions for the corresponding poles
  and zeros.
• Demonstrated Bode plots of the magnitude and
  phase.
• Demonstrated how to find the expressions for the
  gain and the high and low frequency poles and
  zeros for multistage amplifiers.