Electronics > Beginners
Why is this simple Class B amplifier not working?
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alsetalokin4017:
Ah, am I confused yet? Isn't Vout effectively looking at the voltage drop across the 10 ohm resistor?
rstofer:
Look at Q1:  For it to conduct, the base has to get 0.7V above the OUTPUT.  It can only get to 6V so the output can only get to 6 - Vbe or around 5.3V
schmitt trigger:
You have not mentioned what the output load will be.
One thing is to drive a 1000 ohm load, another is to drive a 4 ohm load @ +/-48 volts.
The output architecture increases significantly.

Audioguru again:
Like many amplifiers, the power supply for these two transistors has a single polarity. Therefore the input must be DC biased at half the supply voltage so that the input and output of the transistors can swing up and down.
A speaker is damaged by the resulting continuous DC on it so an output coupling capacitor blocks the DC but passes the AC audio.

The transistors are simple "emitter followers" with a gain of 1 but minus two base emitter voltage drops. Lookup the voltage gain of an Emitter Follower in Google.
colejohnson66:
Thanks for the suggestion @magic! I searched the interwebs and found AN-1593 ("Extending Op Amp Operating Range via Bootstrapping"). I recreated the schematic in LTspice using my -5~5V source and my -45~45V rails (48V from the PSU, but I'll lose some from inverting it (from what I've read)), and it seems to work! The Vcc and Vee rails are "flying."

For parts, I just chose a random op amp from the list and set it to a gain of 9 (45/5=9), and two random P-channel and N-channel MOSFETs.

I only have one more problem now: the "flying rails" are not going all the way to the -45V and 45V rails. Instead, they only go to roughly -37~37V. This clips the output of the op amp to roughly -35~35V. Now, clipping at the rails is fine, if they were at the PSU rails, but they're not. How can I fix this?
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