Editing Operational amplifier (section)

=== Biasing circuits ===
Biasing circuits provide appropriate quiescent current for each stage of the op amp.

The resistor (39&nbsp;kΩ) connecting the (diode-connected) Q11 and Q12, and the given supply voltage (''V''<sub>''S''+</sub>&nbsp;−&nbsp;''V''<sub>''S''−</sub>), determine the current in the [[current mirror]]s, (matched pairs) Q10/Q11 and Q12/Q13. The collector current of Q11, ''i''<sub>11</sub> × 39&nbsp;kΩ = ''V''<sub>''S''+</sub> − ''V''<sub>''S''−</sub> − 2 ''V''<sub>BE</sub>. For the typical ''V''<sub>''S''</sub> = ±20&nbsp;V, the standing current in Q11 and Q12 (as well as in Q13) would be ~1&nbsp;mA. A supply current for a typical 741 of about 2&nbsp;mA agrees with the notion that these two bias currents dominate the quiescent supply current.<ref name="NS LM741" />

Transistors Q11 and Q10 form a [[Widlar current mirror]], with quiescent current in Q10 ''i''<sub>10</sub> such that ln(''i''<sub>11</sub>&nbsp;/ ''i''<sub>10</sub>)&nbsp;= ''i''<sub>10</sub>&nbsp;× 5&nbsp;kΩ&nbsp;/ 28&nbsp;mV, where 5&nbsp;kΩ represents the emitter resistor of Q10, and 28&nbsp;mV is ''V''<sub>T</sub>, the [[thermal voltage]] at room temperature. In this case ''i''<sub>10</sub>&nbsp;≈ 20&nbsp;μA.

==== 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.

==== Voltage amplifier ====
Quiescent currents in Q16 and Q19 are set by the current mirror Q12/Q13, which is running at ~1&nbsp;mA. The collector current in Q19 tracks that standing current.{{elucidate|reason=Through what means?|date=October 2023}}

==== Output amplifier ====
In the circuit involving Q16 (variously named [[rubber diode]] or ''V''<sub>BE</sub> multiplier), the 4.5&nbsp;kΩ resistor must be conducting about 100&nbsp;μA, with Q16 ''V''<sub>BE</sub> roughly 700&nbsp;mV. Then ''V''<sub>CB</sub> must be about 0.45&nbsp;V and ''V''<sub>CE</sub> at about 1.0&nbsp;V. Because the Q16 collector is driven by a current source and the Q16 emitter drives into the Q19 collector current sink, the Q16 transistor establishes a voltage difference between the Q14 base and the Q20 base of ~1&nbsp;V, regardless of the common-mode voltage of Q14/Q20 bases. The standing current in Q14/Q20 will be a factor [[diode modelling|exp(100&nbsp;mV&nbsp;mm/ ''V''<sub>T</sub>) ≈ 36]] smaller than the 1&nbsp;mA quiescent current in the class A portion of the op amp. This (small) standing current in the output transistors establishes the output stage in class AB operation and reduces the [[crossover distortion]] of this stage.