1. At first glance this looks like a cascode input stage, but Q3 and Q4 are actually pnp rather than npn like the input transistors. Why is that?
2. What are the 25 and 50 ohm resistors doing on the AB output?

Thanks

 

Hello,

The 25 and 50 Ohm resistors are part of the short circuit protection.
Your posted schematic misses a transistor.
Have a look at the following page:
http://www.righto.com/2015/10/inside-ubiquitous-741-op-amp-circuits.html

Bertus

 

That is a fascinating read! Thanks.

But it doesn't answer either of my questions.

 

Hello,

Did you read this about the resistors?

The 25 and 50 Ohm resistors are part of the short circuit protection.

In your posted schematic the 25 Ohms resistor will limit the current on the output using Q17.

Bertus

 

At first glance this looks like a cascode input stage, but Q3 and Q4 are actually pnp rather than npn like the input transistors. Why is that?

The PNP's are mainly to provide input reverse overvoltage protection.
The PNP's have a much higher reverse base-emitter breakdown than the NPNs ((comparable to the collector-base breakdown of the NPNs) so they prevent reverse voltage breakdown for high reverse input voltages.

 

I see thank you. So is this considered a 3 stage op amp? I thought those were to be avoided due to compensation difficulties. But a non cascoded input stage seems like it won't provide much gain, neither will an AB output stage.

 

I see thank you. So is this considered a 3 stage op amp? I thought those were to be avoided due to compensation difficulties. But a non cascoded input stage seems like it won't provide much gain, neither will an AB output stage.

No, it's still two stage since the PNPs do not add any voltage gain.
It's just a kind of cascade stage.

 

Here you will find the answer
https://cdn.evilmadscientist.com/KitInstrux/741/741_principles_RevA104.pdf
Or this for more advance users.
http://www.ti.com/lit/an/snoa737/snoa737.pdf