Editing Operational amplifier (section)

=== Small-signal differential mode ===
A small differential input voltage signal gives rise, through multiple stages of current amplification, to a much larger voltage signal on output.

==== Input impedance ====
The input stage with Q1 and Q3 is similar to an emitter-coupled pair (long-tailed pair), with Q2 and Q4 adding some degenerating impedance. The input impedance is relatively high because of the small current through Q1-Q4. A typical 741 op amp has a differential input impedance of about 2 MΩ.<ref name="NS LM741">National Semiconductor LM741 data sheet</ref> The common mode input impedance is even higher, as the input stage works at an essentially constant current.

==== Differential amplifier ====
A differential voltage ''V''<sub>in</sub> at the op amp inputs (pins 3 and 2, respectively) gives rise to a small differential current in the bases of Q1 and Q2 ''i''<sub>in</sub>&nbsp;≈ ''V''<sub>in</sub>&nbsp;/ (2''h''<sub>ie</sub>''h''<sub>fe</sub>). This differential base current causes a change in the differential collector current in each leg by ''i''<sub>in</sub>''h''<sub>fe</sub>. Introducing the transconductance of Q1, ''g''<sub>''m''</sub>&nbsp;= ''h''<sub>fe</sub>&nbsp;/ ''h''<sub>ie</sub>, the (small-signal) current at the base of Q15 (the input of the voltage gain stage) is ''V''<sub>in</sub>''g''<sub>''m''</sub>&nbsp;/ 2.

This portion of the op amp cleverly changes a differential signal at the op amp inputs to a single-ended signal at the base of Q15, and in a way that avoids wastefully discarding the signal in either leg. To see how, notice that a small negative change in voltage at the inverting input (Q2 base) drives it out of conduction, and this incremental decrease in current passes directly from Q4 collector to its emitter, resulting in a decrease in base drive for Q15. On the other hand, a small positive change in voltage at the non-inverting input (Q1 base) drives this transistor into conduction, reflected in an increase in current at the collector of Q3. This current drives Q7 further into conduction, which turns on current mirror Q5/Q6. Thus, the increase in Q3 emitter current is mirrored in an increase in Q6 collector current; the increased collector currents shunts more from the collector node and results in a decrease in base drive current for Q15. Besides avoiding wasting 3&nbsp;dB of gain here, this technique decreases common-mode gain and feedthrough of power supply noise.

==== Voltage amplifier ====
A current signal ''i'' at Q15's base gives rise to a current in Q19 of order ''i''β<sup>2</sup> (the product of the ''h''<sub>fe</sub> of each of Q15 and Q19, which are connected in a [[Darlington pair]]). This current signal develops a voltage at the bases of output transistors Q14 and Q20 proportional to the ''h''<sub>ie</sub> of the respective transistor.

==== Output amplifier ====
Output transistors Q14 and Q20 are each configured as an emitter follower, so no voltage gain occurs there; instead, this stage provides current gain, equal to the ''h''<sub>fe</sub> of Q14 and Q20.

The current gain lowers the output impedance and although the output impedance is not zero, as it would be in an ideal op amp, with negative feedback it approaches zero at low frequencies.