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Neutrality 26th July 2012 10:19 AM

Bias and stability
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In this circuit biasing is done with the four TO-126 transistors using the B-E diode stretch.

Bias stability is helped by the 10 Ohm Emitter resistors.

However, one could mount anywhere from 0 to 4 of the bias diodes on the same heatsinks as the output Q to get even better bias stability.

I usually like to work with Lateral FETs where bias stability is a non issue so any advice on bias stability of BJT's is much appreciated.

EDIT : Hmm, seems like this is similar to

EDIT : It looks like he does not have any of the 2 diodes he uses placed on the heatsink so I guess it is not really needed.

nattawa 27th July 2012 06:46 PM

3 diodes and you're over compensating, and 4 is way over what you need. You may want to leave the 4th off the heat sink to be sure of staying in class A and avoiding under biasing at higher temperature conditions.

I would not load the op amp up with a low-ish 750-ohm resistance and degrade the performance for nothing. A constant current source should be a good replacement for the R2 and R7 for me.

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