Operational Amplifier Circuits

18

OPERATIONAL AMPLIFIER CIRCUITS: ANALYSIS

AND

DESIGN

and

A

fl

= (AJ2+A

l

J. (1.38)

Add (1.37) and (1.38):

A

a+

A

/2

= 2A,

m

. (1.39)

Subtract (1.37) from (1.38):

Afl-A^A^ (1.40)

Hence, from Equation (1.33), the common mode rejection ratio is given by

CMRR = 20 log

10

^-AI) '

(L41)

This value is computed by the computer program Common Mode Rejection

Ratio (CMRR) Determination (Appendix A.3), which accepts gains as pure

ratios and determines the CMRR in decibels.

This program clearly illustrates how closely the two gains must be

matched in order to obtain really high common mode rejection ratios. For

example, values of 99 and 100 for A

n

and A

i2

(a differenc ...