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Old 7th Dec 2017, 5:54 am   #10
Argus25's Avatar
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 1,578
Default Re: Matched Transistors

Originally Posted by mhennessy View Post
Any distortion caused by hFE mismatch is likely to be swamped by other non-linearities, such as from the transformers and the limitations of the basic topology of such a circuit. Some of these might produce 10% THD even when in the best of health!
Some output and driver transformers in transistor radios unfortunately were not bifilar wound, as they should have been and there is a not insignificant DC resistance difference from the center tap to either winding end which is not ideal. This puts asymmetry into the output, no matter how well the transistors are matched. Also the distortion and frequency response at the low frequency end is dominated by the core size and often the transformers, for weight & space considerations, were smaller than they could have been.
Interesting though the transformers were often a substantial size in pre 1960's early transistor radios such as the Sony TR72 or the NZ made Pacemaker brand radio that used 3 watt sized cores.

I also agree that the kind of transformer-less audio stages Hacker were fond of do not require any special selection or matching of transistors most of the time, it is really only any advantage to match transistors for the prior generation audio circuits which are transformer coupled class AB push pull amplifiers.

For the most part if the amplifier's performance is checked into a dummy load with the scope and a 1kHz test signal, its pretty easy to see if one of the output transistors is sick, or if there are other issues like crossover distortion.
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