Editing Operational amplifier (section)

==== Differential amplifier ====
The biasing circuit of this stage is set by a feedback loop that forces the collector currents of Q10 and Q9 to (nearly) match. Any small difference in these currents provides drive for the common base of Q3 and Q4.<ref group="nb">The base drive for input transistors Q1/Q2 is the input bias current and must be sourced externally.</ref> The summed quiescent currents through Q1 and Q3 plus Q2 and Q4 is mirrored from Q8 into Q9, where it is summed with the collector current in Q10, the result being applied to the bases of Q3 and Q4.

The quiescent currents through Q1 and Q3 (also Q2 and Q4) ''i''<sub>1</sub> will thus be half of ''i''<sub>10</sub>, of order ~10&nbsp;μA. Input bias current for the base of Q1 (also Q2) will amount to ''i''<sub>1</sub>&nbsp;/ β; typically ~50&nbsp;nA,<ref name="NS LM741" /> implying a current gain ''h''<sub>fe</sub>&nbsp;≈ 200 for Q1 (also Q2).

This feedback circuit tends to draw the common base node of Q3/Q4 to a voltage ''V''<sub>com</sub>&nbsp;− 2 ''V''<sub>BE</sub>, where ''V''<sub>com</sub> is the input common-mode voltage. At the same time, the magnitude of the quiescent current is relatively insensitive to the characteristics of the components Q1–Q4, such as ''h''<sub>fe</sub>, that would otherwise cause temperature dependence or part-to-part variations.

Transistor Q7 drives Q5 and Q6 into conduction until their (equal) collector currents match that of Q1/Q3 and Q2/Q4. The quiescent current in Q7 is ''V''<sub>BE</sub>&nbsp;/ 50&nbsp;kΩ, about 35&nbsp;μA, as is the quiescent current in Q15, with its matching operating point. Thus, the quiescent currents are pairwise matched in Q1/Q2, Q3/Q4, Q5/Q6, and Q7/Q15.