741 Op Amp Schematic

Thread Starter


Joined May 2, 2007
I'm looking at the Wikipedia schematic of a 741 Op Amp, trying to figure out how this thing works:


Though I have a lot of questions, this is the current one that's bugging me. I see that the output from the differential amplifier (the blue box) is attched to the base of Q15 and the collector of Q22. Let's say that Vs+ is 15V and Vs- is -15V. What is the approximate range of this output signal under normal operating conditions?

In other words, will the output of the differential amplifier stage hover around Vs- (say, -13V or so), or can it swing from positive to negative (say, 13V to -13V)? This question arises because I'm having trouble seeing how the magenta and green boxes (class A and output bias stages) can function if the differential output voltage is a substantial amount above -15V.

I mean, if you put 12V to the Base of Q15, you'll wind up with approximately 11 volts at the 50 ohm resistor, which would pull 540mA from the output of the Q12/Q13 current mirror, which is limited to 738uA given the assumed voltages at the rails. But even if that is wrong, if Q22 is in the active region (~0.6V), that alone would cause a 12mA current across the 50 ohm resistor, which again exceeds the output of the Q12/Q13 current mirror. I could see (maybe) how this whole thing could work if the Q15's base voltage is a maximum of -14 volts or so.

I hope this question makes sense. Perhaps I just don't understand how the differential stage works. Or, perhaps my understanding of BJTs is poor. Whatever the case, I'd appreciate somebody's help.

Ron H

Joined Apr 14, 2005
I think it's like this: Under normal operation, Q22 never turns on, because Q13 is not supplying enough current to forward bias the b-e junction of Q22. Under normal operation, the base of Q15 sets at about 2 diode drops above the negative rail (plus the drop across the 50 ohm resistor), and only varies by probably tens of millivolts, depending on output current swing, because the gain of Q15/16/13 is very high.
I think Q22 and the 50 ohm resistor constitute overload protection. If the output is heavily loaded, and tries to go low, Q22 limits Q19 current to 12mA, as you pointed out. Q22 steals base drive from Q15 in order to do this. This prevents Q19 from burning up due to excessive power dissipation.


Joined Jan 22, 2004
yes i concur to what "ron h" says about Q22. as can be gleaned the output of the gain stage is connected to the base of Q22 (parallel pnp), operating as an an emitter follower hence providing a a very high input resistance. in effect the gain stage composed of Q16 & Q17 does not suffer from significant loading effects due to the output stage.

Q13 is actually 2 transistor connected in parallel with common base and emitter terminals. Q13b provides bias current for Q17 and at the same time acts an active load to come up with a high voltage gain. Q17 is configured as a common emitter so the voltage at the collector of Q17 is the input signal to the output stage. hence the signal undergoes another dc level shift as it passes thru the gain stage. Q13a provides bias current to Q22 as well as Q18 & 19. Q18 & 19 are used to establish a quiescent bias current in the output transistors. Q15 & 21 are short circuit protection devices. they are normally off and it conducts only if the output is accidentally connected to gnd

some of the transistors i have mentioned are not depicted in your reference material which just a basic representation of a 741.