More on Biasing Class AB Output Stage

Thread Starter

crazyengineer

Joined Dec 29, 2010
156
Okay so I here's what I learned about class AB output stage thus far

1) Need a bias voltage between the transistors to eliminate if not reduce the dead band for the output stage
2) One transistor turns on while the other stays off.
3) Under DC biasing, each one can be treated as if you're biasing a current mirror
4) To reduce thermal runaway, diodes are used as the bias voltage between the transistors pins (have to be switching) and to have emitter resistors as well.


My main questions is if I matched the output impedance of the transistor to the impedance of my load, will DC current from the emitter be the same when it reaches my load or is it solely based on the current gain of the transistor I'm working with?
 

Audioguru

Joined Dec 20, 2007
11,251
A modern audio amplifier has a lot of open-loop gain and a lot of negative feedback that reduces distortion to almost nothing and makes its output impedance extremely low (0.04 ohms or less). An audio amplifier with an extremely low output impedance has an excellent "damping factor" that damps the resonances of speakers for very good "tight" sound.

Old fashioned vacuum tube amplifiers had an output impedance that was almost the same as the speaker impedance so the speakers were designed with internal damping that did not work well (sloppy boom boom sound or no bass sound).
 

bountyhunter

Joined Sep 7, 2009
2,512
4) To reduce thermal runaway, diodes are used as the bias voltage between the transistors pins (have to be switching) and to have emitter resistors as well.?
If you actually want them to track thermally, you use diode-connected-transistors to match against the amplifier transistors so they actually do match. Throwing a small diode in to track a transistor is a weak approximation at best.
 

Audioguru

Joined Dec 20, 2007
11,251
If you actually want them to track thermally, you use diode-connected-transistors to match against the amplifier transistors so they actually do match. Throwing a small diode in to track a transistor is a weak approximation at best.
Many little audio amplifiers have diodes squeezed against the output transistors for pretty good thermal tracking.
But I agree that a transistor VBE multiplier bolted to the heatsink of the output transistors is much better.

Today people use an audio amplifier IC instead of many separate parts.
The transistors in an IC are perfectly thermally matched.
 

Audioguru

Joined Dec 20, 2007
11,251
I think that teachers do not understand electronics well enough to explain how an extremely good LM3886 amplifier IC works.
 

bountyhunter

Joined Sep 7, 2009
2,512
I think that teachers do not understand electronics well enough to explain how an extremely good LM3886 amplifier IC works.
I used a similar product I procured from national semiconductors audio group when I built my power amp for driving my stereo TV speakers. I built the speakers too, and incorporated the power supply inside one speaker with the amp screwed to the back. Been working great for 17 years. Sound is really good.
 
Top