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Old 7th Dec 2017, 1:59 am   #9
Argus25's Avatar
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 1,655
Default Re: Matched Transistors

Originally Posted by kalee20 View Post
The input impedance of the stage, whether emitter follower or common-emitter, is proportional to hfe. So, if the hfe's of the two transistors in a push-pull amplifier are widely different, there will be different loads on the driver stage on the two half-cycles.
I agree with this entirely and I think for transformer coupled push pull output stages its best to have matched transistors.

One issue is that the transistor's small signal current gain value alters depending on the static or DC bias conditions. So there is static and dynamic matching to consider.

Transistors may look matched for hfe on a tester but then they are sometimes not at the same emitter current in two identical bias circuits. So the best move is to set the transistors up in a similar bias condition as in use
and drive them both with the same signal from a 1k source impedance. Connect the two outputs into the + and minus inputs of a voltage differencing op amp to subtract the signals, when matched the output will be close to zero. Also under the same bias circuitry conditions the emitter currents can be compared.

Many years ago I built a jig like this for matching OC72's and OC74's for push pull amplifiers. I think it is worth using matched pairs, as Mullard once supplied them, but perhaps the subtleties of it might be wasted on domestic radio repairs , but for my own radios I think its worth it.

I wonder if there is anyone who knows what the matching protocol once was in Mullard's factory ?

Last edited by Argus25; 7th Dec 2017 at 2:11 am. Reason: Simplify test suggestion
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