I'm looking at the Wikipedia schematic of a 741 Op Amp, trying to figure out how this thing works:
http://en.wikipedia.org/wiki/Operational_amplifier
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.
http://en.wikipedia.org/wiki/Operational_amplifier
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.