View Single Post
Old 7th Dec 2017, 9:56 am   #13
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 4,517
Default Re: Matched Transistors

Quote:
Originally Posted by mhennessy View Post
I think the final paragraph of my first post covered it really... I've repaired hundreds - perhaps thousands - of transistor radios in the last decade, and I take the time to measure each one carefully (including distortion). It's that experience - plus my experience designing "proper" amplifiers - which gets me here... Such precautions are never necessary in a well designed audio amplifier though...
I think Mark sums this up well!

I stand by my earlier post, that matching is good, to present equal loads on the driver for each half-cycle. But a good driver will be tolerant of different loads on it, hopefully giving a only few percent difference in outputs for widely different loads. That would be a well-designed driver stage.

Some of the cheaper transistor radios would have cut things to the bone at the design stage, though, on the basis that it might be cheaper to spend a few moments matching transistors than designing the thing better (doubtless using more components). Made in volume, with thousands of output transistors to choose from, sorting them into hfe 'groups' would be simple.

Wrapping negative-feedback around the whole thing as a cure-all-ills solution largely works, of course, though some decry it as it increases higher-order harmonics.

Whether necessary or not, it won't do any harm to match the transistors. But if it's a cheap Far East radio, it's not going to be worth buying more than 3 transistors and using the 2 which match most closely.
kalee20 is offline