LECTURE 240 – SIMULATION AND MEASUREMENTS OF OP AMPS
Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
Page 240-1
LECTURE 240 ? SIMULATION AND MEASUREMENTS OF OP AMPS
(READING: AH ? 310-323)
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 ? Measurement must consider all nonidealities
ECE 6412 - Analog Integrated Circuit Design - II Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
? P.E. Allen - 2002 Page 240-2
Simulating or Measuring the Open-Loop Transfer Function of the Op Amp Circuit (Darkened op amp identifies the op amp under test):
vIN +VOS-
VDD vOUT
Simulation:
This circuit will give the voltage transfer function curve. This curve should identify:
CL RL
VSS
1.) The linear range of operation
Fig. 240-01
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 small-signal frequency response can be obtained
7.) From the open-loop 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.
ECE 6412 - Analog Integrated Circuit Design - II
? P.E. Allen - 2002
Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
Page 240-3
A More Robust Method of Measuring the Open-Loop Frequency Response Circuit:
VDD
vIN
vOUT
CR
CL RL
Resulting Closed-Loop Frequency Response:
dB Av(0)
VSS
Fig. 240-02
Op Amp Open Loop Frequency Response
0dB 1 RC
Av(0) RC
Make the RC product as large as possible.
log10(w)
Fig. 240-03
ECE 6412 - Analog Integrated Circuit Design - II Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
? P.E. Allen - 2002 Page 240-4
Simulation and Measurement of Open-Loop Frequency Response with Moderate Gain Op Amps
R
R vIN
+ vi
-
VDD vOUT
CL RL
VSS
Fig. 240-04
Make R as large and measure vout and vi to get the open loop gain.
ECE 6412 - Analog Integrated Circuit Design - II
? P.E. Allen - 2002
Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
Simulation or Measurement of the Input Offset Voltage of an Op Amp
Page 240-5
+
VOS
-
R
VDD vOUT=VOS
CL RL
VSS
Fig. 240-05
Types of offset voltages:
1.) Systematic offset - due to mismatches in current mirrors, exists even with ideally matched transistors.
2.) Mismatch offset - due to mismatches in transistors (normally not available in simulation except through Monte Carlo methods).
ECE 6412 - Analog Integrated Circuit Design - II Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
Simulation of the Common-Mode Voltage Gain
? P.E. Allen - 2002 Page 240-6
+
vcm
-
+VOSR
VDD vout
CL RL
VSS
Fig. 240-06
Make sure that the output voltage of the op amp is in the linear region.
ECE 6412 - Analog Integrated Circuit Design - II
? P.E. Allen - 2002
Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
Page 240-7
+ -
+ -
Measurement of CMRR and PSRR
Configuration:
Note
that
vI
vOS 1000
or vOS 1000vI
How Does this Circuit Work?
CMRR: 1.) Set
VDD' = VDD + 1V VSS' = VSS + 1V vOUT' = vOUT + 1V 2.) Measure vOS
called vOS1 3.) Set
VDD' = VDD - 1V VSS' = VSS - 1V vOUT' = vOUT - 1V 4.) Measure vOS
called vOS2 5.)
2000 CMRR=|vOS2-vOS1|
PSRR: 1.) Set
VDD' = VDD + 1V VSS' = VSS vOUT' = 0V 2.) Measure vOS called vOS3 3.) Set VDD' = VDD - 1V VSS' = VSS vOUT' = 0V 4.) Measure vOS
called vOS4 5.) PSRR+=|vOS240-0v0OS3|
ECE 6412 - Analog Integrated Circuit Design - II
vOS 10k
100k
100k
vSET
+
10 vI
-
vOUT
VDD
CL RL
VSS
Note:
Fig. 240-07
1.) PSRR- can be measured similar to
PSRR+ by changing only VSS. 2.) The ?1V perturbation can be
replaced by a sinusoid to measure
CMRR or PSRR as follows:
PSRR+
=
1000?vdd vos
,
PSRR-
=
1000?vss vos
and
CMRR
=
1000?vcm vos
? P.E. Allen - 2002
Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
Page 240-8
How Does the Previous Idea Work?
A circuit is shown which is used to measure the CMRR and PSRR of an op amp. Prove vos that the CMRR can be given as
100k 100k
CMRR
=
1000 vicm vos
10k
Solution
The definition of the common-mode rejection
ratio is
CMRR
=
Avd Acm
=
(vout/vid) (vout/vicm)
+
10 vi
-
vOUT CL RL
However, in the above circuit the value of vout is the same so that we get
CMRR
=
vicm vid
But vid = vi and vos 1000vi = 1000vid
vid
=
vos 1000
Substituting in the previous expression gives,
CMRR =
vicm vos
=
1000 vicm vos
1000
vicm
VDD vicm
VSS
Fig. 240-08
ECE 6412 - Analog Integrated Circuit Design - II
? P.E. Allen - 2002
Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
Simulation of CMRR of an Op Amp None of the above methods are really suitable for simulation of CMRR. Consider the following:
Vcm V2 V1 +
Vcm
VDD Vout
VSS
Vcm V2
Av(V1-V2)
V1
Vout
Vcm ?AcVcm
Fig. 240-09
Vout = Av(V1-V2) ?AcmV1+2V2 = -AvVout ? AcmVcm
Vout
=
?Acm 1+Av
Vcm
?Acm Av
Vcm
|CMRR|
=
Av Acm
=
Vcm Vout
(However, PSRR+ = PSRR-)
Page 240-9
ECE 6412 - Analog Integrated Circuit Design - II Lecture 240 ? Simulation and Measurement of Op Amps (2/25/02)
CMRR of Ex. 6.3-1 using the Above Method of Simulation
? P.E. Allen - 2002 Page 240-10
|CMRR| dB Arg[CMRR] Degrees
85
200
80
150
75
100
70
50
65
0
60
-50
55
-100
50
-150
45
-200
10 100 1000 104 105 106 107 108 Frequency (Hz)
10 100 1000 104 105 106 107 108
Frequency (Hz)
Fig. 240-10
ECE 6412 - Analog Integrated Circuit Design - II
? P.E. Allen - 2002
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