I am trying to analyze the building blocks of an operational amplifier and for that purpose I have chosen, probably one of the most famous and earliest op-amps built, ua741.

I have figured out most of the building blocks, such as differential pairs, push-pulls, current sources, class AB op amps etc, however there are still some parts that I don't have any explains yet. One of which the highlighted line below for the class AB amp which I was expecting to be connected to Vcc-.

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What is the function of the yellow line?

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    \$\begingroup\$ righto.com/2015/10/inside-ubiquitous-741-op-amp-circuits.html \$\endgroup\$ – G36 Jun 9 '17 at 15:45
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    \$\begingroup\$ Everything you need to know, right here \$\endgroup\$ – Mtk59 Jun 9 '17 at 21:44
  • \$\begingroup\$ Two fantastic, fantastic recommendations, however, if you have noticed already, the path that I have highlighted does not exist on any of the earlier 741 schematics and the path feeds back to the Gain Stage rather than the Differential Amplifier block, going in particular from Q17 to Q22 and not to Q5 in Figure 2: Detailed schematic diagram of the @Micah 's recommendation. :) \$\endgroup\$ – Mehrad Jun 12 '17 at 23:37

The PNP who's collector node you have highlighted - let's call it Q1 (as well as the NPN directly above it on the schematic) are for current limiting at the output. The output current develops a voltage over the two resistors connected to OUT - IOUT*RSENSE. When this voltage exceeds a diode voltage, Q1 starts to turn on. The positive and negative current limits work differently, but the negative current limit which you have asked about specifically operates by sending a current back to the input and reducing the differential current generated by the input pair!


From its position monitoring current from the -VDD rail, the effect will be current limiting or perhaps foldback limiting to restrict power dissipation.


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