Editing Operational amplifier (section)

=== Architecture ===
A small-scale [[integrated circuit]], the 741 op amp shares with most op amps an internal structure consisting of three gain stages:<ref>{{cite web |url=http://www.righto.com/2015/10/inside-ubiquitous-741-op-amp-circuits.html |title=Understanding silicon circuits: inside the ubiquitous 741 op amp |website=www.righto.com |access-date=28 April 2018 |url-status=live |archive-url=https://web.archive.org/web/20171009072728/http://www.righto.com/2015/10/inside-ubiquitous-741-op-amp-circuits.html |archive-date=9 October 2017 }}</ref>
# [[Differential amplifier]] (outlined dark blue) — provides high differential amplification (gain), with rejection of [[common-mode signal]], low noise, high [[input impedance]], and drives a
# Voltage amplifier (outlined [[magenta]]) — provides high voltage gain, a single-pole frequency [[roll-off]], and in turn drives the
# Output amplifier (outlined [[cyan]] and [[green]]) — provides high current gain (low [[output impedance]]), along with output current limiting, and output short-circuit protection.
Additionally, it contains [[current mirror]] (outlined red) bias circuitry and [[Frequency compensation|compensation]] capacitor (30&nbsp;pF).

==== Differential amplifier ====
The input stage consists of a cascaded [[differential amplifier]] (outlined in dark blue) followed by a current-mirror [[active load]]. This constitutes a [[transconductance amplifier]], turning a differential voltage signal at the bases of Q1, Q2 into a current signal into the base of Q15.

It entails two cascaded transistor pairs, satisfying conflicting requirements. The first stage consists of the matched NPN [[emitter follower]] pair Q1, Q2 that provide high input impedance. The second is the matched PNP [[common-base]] pair Q3, Q4 that eliminates the undesirable [[Miller effect]]; it drives an [[active load]] Q7 plus matched pair Q5, Q6.

That active load is implemented as a modified [[Wilson current mirror]]; its role is to convert the (differential) input current signal to a single-ended signal without the attendant 50% losses (increasing the op amp's open-loop gain by 3&nbsp;dB).<ref group="nb">Widlar used this same trick in μA702 and μA709</ref> Thus, a small-signal differential current in Q3 versus Q4 appears summed (doubled) at the base of Q15, the input of the voltage gain stage.

==== Voltage amplifier ====
The (class-A) voltage gain stage (outlined in [[magenta]]) consists of the two NPN transistors Q15 and Q19 connected in a [[Darlington transistor|Darlington configuration]] and uses the output side of current mirror formed by Q12 and Q13 as its collector (dynamic) load to achieve its high voltage gain. The output sink transistor Q20 receives its base drive from the common collectors of Q15 and Q19; the level-shifter Q16 provides base drive for the output source transistor Q14.<!-- Note the similarity between the transistors Q15 and Q7 -- just accidentally a similar position in the plan, not a matched pair--> The transistor Q22 prevents this stage from delivering excessive current to Q20 and thus limits the output sink current.

==== Output amplifier ====
The output stage (Q14, Q20, outlined in cyan) is a [[Class AB]] amplifier. It provides an output drive with impedance of ~50{{nbsp}}Ω, in essence, current gain. Transistor Q16 (outlined in green) provides the quiescent current for the output transistors and Q17 limits output source current.