Lecture 270 – High Speed Op Amps (3/28/10)
Page 2701
CMOS Analog Circuit Design
© P.E. Allen  2010
LECTURE 270 – HIGH SPEED OP AMPS
LECTURE ORGANIZATION
Outline
•
Extending the GB of conventional op amps
•
Cascade Amplifiers
 Voltage amplifiers
 Voltage amplifiers using current feedback
• Summary
CMOS Analog Circuit Design,
2
nd
Edition Reference
Pages 368384
Lecture 270 – High Speed Op Amps (3/28/10)
Page 2702
CMOS Analog Circuit Design
© P.E. Allen  2010
INCREASING THE GB OF OP AMPS
What is the Influence of
GB
on the Frequency Response?
The unitygainbandwidth represents a limit in the tradeoff between closed loop voltage
gain and the closedloop 3dB frequency.
Example of a gain of 10 voltage amplifier:
0dB
20dB

A
vd
(0) dB
Magnitude
log
10
(
ω
)
GB
ω
A
ω
3dB
Op amp frequency response
Amplifier with a gain of 10
Fig. 7.21
What defines the
GB
?
We know that
GB
=
g
m
C
where
g
m
is the transconductance that converts the input voltage to current and
C
is the
capacitor that causes the dominant pole.
This relationship assumes that all higherorder poles are greater than
GB
.
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