Page 37 - Op Amps Design, Applications, and Troubleshooting

20 BASIC CONCEPTS OF THE INTEGRATED OPERATIONAL AMPLIFIER

FIGURE 1.18 Measurement of the
common-mode voltage gain for an op
amp.

Manufacturers usually provide this data by contrasting the differential volt-
age gain and the common-mode voltage gain. This parameter, called common-mode
rejection ratio (CMRR), is computed as follows:

where A D and A CM are the differential and common-mode gains respectively. On a
specification sheet this is usually written in the decibel form. To convert from the
decibel value given in the data sheet to the form shown, the following conversion
formula is used:

where dB is the value of the common-mode rejection ratio expressed in decibels.
Now let us refer to Appendix 1 and determine the common-mode voltage gain for
a 741 op amp. The minimum value is listed as 70 dB with 90 dB being cited as typ-
ical. Converting the typical value to the standard CMRR ratio form requires appli-
cation of Equation (1.10).

To determine the actual common-mode voltage gain, we simply divide the differ-
ential voltage gain by the CMRR value [transposed version of Equation (1.9)].

Recall that a typical differential voltage gain for the 741 is 200,000. Thus a
typical common-mode voltage gain can be shown to be