Biasing a Class AB with a transistor

Thread Starter

bennyq

Joined Sep 13, 2014
6
Hi, Im trying to design a Class AB power gain stage with the following circuit. Im having trouble with achieving a load of 8ohms(5vpp).. I would like the transistor to bias the current gain stage as I would eventually like to place a darlington pair package that has a VBE on of 1V.. Can I achieve that with this circuit? Or should I consider trying to use diodes..
 

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GopherT

Joined Nov 23, 2012
8,009
Hi, Im trying to design a Class AB power gain stage with the following circuit. Im having trouble with achieving a load of 8ohms(5vpp).. I would like the transistor to bias the current gain stage as I would eventually like to place a darlington pair package that has a VBE on of 1V.. Can I achieve that with this circuit? Or should I consider trying to use diodes..
That circuit is a mess.

That is not how a transistor bias works.

Google a transistor bias and post a reasonable result. We can work from there. Your circuit is not a reasonable starting point for a discussion.

Start near the bottom of this one
http://en.m.wikipedia.org/wiki/Bipolar_transistor_biasing
 

Thread Starter

bennyq

Joined Sep 13, 2014
6
Well this is my initial design, I'd want 1v preferably at the bases for turn on so voltage, 0.7V is a bit low. Is this design better?
 

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JDT

Joined Feb 12, 2009
657
Your second circuit is a simpler way (and not so good) of controlling the quiescent current in the output transistors. There are a few other things wrong as well but I will leave that for now!

Take your drive capacitor to the junction of D2 and base of Q2. Use another 100uF capacitor to bypass the 2 series diodes. Will work but won't produce full output swing as the transistors run out of drive current as the voltage across the 1K resistors gets low.
 
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