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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