Lecture 250 – Measurement and Simulation of Op amps (3/28/10)
Page 2501
CMOS Analog Circuit Design
© P.E. Allen  2010
LECTURE 250 – SIMULATION AND MEASUREMENT OF OP
AMPS
LECTURE ORGANIZATION
Outline
• Introduction
• Open Loop Gain
•
CMRR
and
PSRR
• A general method of measuring
A
vd
,
CMRR
, and
PSRR
• Other op amp measurements
• Simulation of a TwoStage Op Amp
• Op amp macromodels
• Summary
CMOS Analog Circuit Design,
2
nd
Edition Reference
Pages 310341
Lecture 250 – Measurement and Simulation of Op amps (3/28/10)
Page 2502
CMOS Analog Circuit Design
© P.E. Allen  2010
INTRODUCTION
Simulation and Measurement Considerations
Objectives:
•
The objective of simulation is to verify and optimize the design.
•
The objective of measurement is to experimentally confirm the specifications.
Similarity between Simulation and Measurement:
•
Same goals
•
Same approach or technique
Differences between Simulation and Measurement:
•
Simulation can idealize a circuit
 All transistor electrical parameters are ideally matched
 Ideal stimuli
•
Measurement must consider all nonidealities
 Physical and electrical parameter mismatches
 Nonideal stimuli
 Parasistics
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Lecture 250 – Measurement and Simulation of Op amps (3/28/10)
Page 2503
CMOS Analog Circuit Design
© P.E. Allen  2010
OPEN LOOP GAIN
Simulating or Measuring the OpenLoop Transfer Function of the Op Amp
Circuit (Darkened op amp identifies the op amp under test):
Simulation:
This circuit will give the voltage transfer
function curve.
This curve should identify:
1.)
The linear range of operation
2.)
The gain in the linear range
3.)
The output limits
4.)
The systematic input offset voltage
5.)
DC operating conditions, power dissipation
6.)
When biased in the linear range, the smallsignal frequency response can be
obtained
7.)
From the openloop frequency response, the phase margin can be obtained (
F
= 1)
Measurement:
This circuit probably will not work unless the op amp gain is very low.
Fig. 24001
+

V
OS
v
IN
v
OUT
V
DD
V
SS
R
L
C
L
Lecture 250 – Measurement and Simulation of Op amps (3/28/10)
Page 2504
CMOS Analog Circuit Design
© P.E. Allen  2010
A More Robust Method of Measuring the OpenLoop Frequency Response
Circuit:
v
IN
v
OUT
V
DD
V
SS
R
L
C
L
R
C
Fig. 24002
Resulting ClosedLoop Frequency Response:
dB
log
10
(
w
)
A
v
(0)
1
RC
RC
A
v
(0)
Op Amp
Open Loop
Frequency
Response
Fig. 24003
0dB
Make the
RC
product as large as possible.
Lecture 250 – Measurement and Simulation of Op amps (3/28/10)
Page 2505
CMOS Analog Circuit Design
© P.E. Allen  2010
CMRR
AND
PSRR
Simulation of the CommonMode Voltage Gain
V
OS
v
out
V
DD
V
SS
R
L
C
L
+

v
cm
+

Fig. 6.65
Make sure that the output voltage of the op amp is in the linear region.
Divide (subtract dB) the result into the openloop gain to get
CMRR
.
Lecture 250 – Measurement and Simulation of Op amps (3/28/10)
Page 2506
CMOS Analog Circuit Design
© P.E. Allen  2010
Simulation of CMRR of an Op Amp
None of the above methods are really suitable for simulation of
CMRR
.
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 '10
 PHILLIPALLEN
 Operational Amplifier, Vdd, Vout, Frequency Independent Op Amp Macromodel

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