chapter 9 frequency response - weber state university · 2017-01-04 · chapter 9 frequency...
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ECE 3120 Microelectronics II Dr. Suketu Naik
Chapter 9
Frequency Response
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ECE 3120 Microelectronics II Dr. Suketu Naik
Operational Amplifier Circuit Components
1. Ch 7: Current Mirrors and Biasing
2. Ch 9: Frequency Response
3. Ch 8: Active-Loaded Differential Pair
4. Ch 10: Feedback
5. Ch 11: Output Stages
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ECE 3120 Microelectronics II Dr. Suketu Naik
Op Amp Circuit Components
Two Stage
Op Amp
(MOSFET)
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ECE 3120 Microelectronics II Dr. Suketu Naik
Learning Objectives
1) How coupling and bypass capacitors cause the gain of discrete circuit amplifiers to fall off at low frequencies, and how to obtain an estimate of the frequency fL at which the gain decreases by 3dB below its value at midband.
2) The internal capacitive effects present in the MOSFET and the BJT and how to model these effects by adding capacitances to the hybrid-p model of each of the two transistor types.
3) The high-frequency limitation on the gain of the CS and CE amplifiers and how the gain falloff and the upper 3-dBfrequency fH are mostly determined by the small capacitances between the drain and gate (collector and base).
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ECE 3120 Microelectronics II Dr. Suketu Naik
Learning Objectives
3) Methods for the analysis of the high-frequency response of amplifier circuits of varying complexity.
4) How the cascode amplifier studied in Ch 7 can be designed to obtain wider bandwidth than is possible with CS and CE amplifiers.
5) The high-frequency performance of the source and emitter followers.
6) The high-frequency performance of differential amplifiers (Note: this topic will be covered after Ch8: Differential Pair)
7) Circuit configurations for obtaining wideband amplification.
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ECE 3120 Microelectronics II Dr. Suketu Naik
PART A:
Low Frequency Response
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ECE 3120 Microelectronics II Dr. Suketu Naik
Middle-frequency band (midband) is the range of frequencies over
which amplifier gain is constant
Frequency Response of an Amplifier
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ECE 3120 Microelectronics II Dr. Suketu Naik
Low Frequency Response of CS Amplifier
Q: What is the effect of
the coupling capacitors
CC1 and CC2 and the
bypass capacitor CS on the
overall gain (Vo/Vsig) at
low frequencies?
A: At low frequencies,
their reactances (1/jωC)
are high and gain is low.
Common Source Amplifier
Note: IC amplifiers don't have
coupling and by pass caps: midband
extends to DC
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ECE 3120 Microelectronics II Dr. Suketu Naik
9.1.1 The CS Amplifier
Determining Vo/Vsig
1) Circuit with dc sources eliminated
2) Small-signal analysis
3) Ignore ro as it has negligible effect at low frequencies
Goal: find the lower -3dB frequency or fL
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ECE 3120 Microelectronics II Dr. Suketu Naik
9.1.1 The CS Amplifier
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ECE 3120 Microelectronics II Dr. Suketu Naik
2
3
2
1 2
(9.5) output voltage:
(9.6) break frequency #3:
(9.9) midband gain:
(9.7) transfer func
1
1
||
tion:
D Lo o L d
D L
C D L
P
C D L
GM m D L
G sig
oM
sig P P
R R sV I R I
R R sC R R
C R R
RA g R R
R R
V s sA
V s s
3P
s
s
9.1.1 The CS Amplifier
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ECE 3120 Microelectronics II Dr. Suketu Naik
Low Frequency Response of CS Amplifier
fp2 to fp3: for every factor of 10 in the frequency, the gain drops (increases) by 20 dB
fp3 to fp1: for every factor of 10 in the frequency, the gain drops (increases) by 40 dB
fp1 to DC: for every factor of 10 in the frequency, the gain drops (increases) by 60 dB
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ECE 3120 Microelectronics II Dr. Suketu Naik
Determining the Pole Frequencies by Inspection
Reduce VSig to zero
Consider each capacitor separately
Find the total resistance seen between terminals of each
capacitor Req
time constant τ = ReqC
fL is the highest of the three
pole frequencies
9.1.1 The CS Amplifier
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ECE 3120 Microelectronics II Dr. Suketu Naik
Place Lower frequency fL at a specified value and minimize capacitor values
Cs should be selected at its pole frequency: fL=fp2
Place fp3 and fp1 between fp2/5 and fp2/10
Lower fp1, fp3 as much as possible while
choosing Cc1 and Cc2 so that they are
not so big
Design for Low Frequency
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ECE 3120 Microelectronics II Dr. Suketu Naik
Example 9.1
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ECE 3120 Microelectronics II Dr. Suketu Naik
Low Frequency Response of CE Amplifier
Q: What is the effect of the
coupling capacitors CC1 and
CC2 and the bypass capacitor
CE on the overall gain
(Vo/Vsig) at low frequencies?
1) Complicated analysis
due to finite β making
effects of CC1 and CE
inseperable
2) Determine the amplifier
gain and the transfer function
Common Emitter Amplifier
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ECE 3120 Microelectronics II Dr. Suketu Naik
9.1.2 The CE Amplifier
1
1
1
2
|| ||
||
1/
(9.12) midband gain:
(9.10) voltage gain:
(9.11) break frequency #1:
(9.14) break frequency #2
||
1
||
:
B
M m C L
B sig
oM
sig C B sig
P
C B sig
P
R rA g R R
R r R
VA s s
V C R r R
C R r R
1
||
1
B sig
E e
R RC r
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ECE 3120 Microelectronics II Dr. Suketu Naik
2
3
2
||||
||
1
(9.15) low frequency gain:
(9.16) break freque
ncy #3:
/
1
o Bm C L
sig B sig
C C L
P
C C L
V R rg R R
V R r R
s sC R R
C R R
1 2 3
1 1 2 2
(9.17) transfer function:
(9.18) fr
eq1 1 1 1
uency: 2
oM
sig P P P
L
C C E E C C
V s s sA
V s s s
fC R C R C R
9.1.2 The CE Amplifier
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ECE 3120 Microelectronics II Dr. Suketu Naik
9.1.2 The CE Amplifier
Low-frequency response of
the CE amplifier:
(a) the effect of CC1 is
determined with CE and CC2
assumed to be acting as
perfect short circuits
(b) the effect of CE is
determined with CC1 and CC2
assumed to be acting as
perfect short circuits
(c) the effect of CC2 is
determined with CC1 and CE
assumed to be acting as
perfect short circuits
(a)
(b)
(c)
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ECE 3120 Microelectronics II Dr. Suketu Naik
Assumption: CC1, CE, and CC2 do not interact and that their pole
frequencies are widely separated
Low Frequency Response of CE Amplifier
fp2 to fp1: for every factor of 2 in the frequency, the gain drops (increases) by 6 dB
fp1 to fp3: for every factor of 2 in the frequency, the gain drops (increases) by 12 dB
fp3 to DC: for every factor of 2 in the frequency, the gain drops (increases) by 18 dB
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ECE 3120 Microelectronics II Dr. Suketu Naik
Example 9.2
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ECE 3120 Microelectronics II Dr. Suketu Naik
Problem 9.4
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ECE 3120 Microelectronics II Dr. Suketu Naik
Problem 9.9
Use one significant digit