Editing Operational amplifier (section)

==== Small-signal common mode gain ====
The ideal op amp has infinite [[common-mode rejection ratio]], or zero common-mode gain.

In the present circuit, if the input voltages change in the same direction, the negative feedback makes Q3/Q4 base voltage follow (with 2 ''V''<sub>BE</sub> below) the input voltage variations. Now the output part (Q10) of Q10-Q11 current mirror keeps up the common current through Q9/Q8 constant in spite of varying voltage. Q3/Q4 collector currents, and accordingly the output current at the base of Q15, remain unchanged.

In the typical 741 op amp, the common-mode rejection ratio is 90&nbsp;dB,<ref name="NS LM741" /> implying an open-loop common-mode voltage gain of about 6.<!-- 200,000 / 10^^(4.5) -->