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Old 7th Dec 2017, 2:55 pm   #16
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,368
Default Re: Matched Transistors

We all know the theory, but what this boils down to is context.

10 years ago, before I started messing about seriously with old radios, I would have instinctively agreed about matching the hFE to 20% or better. And with a "quality" hi-fi amplifier using this sort of topology - such as the Rogers Ravensbourne, to pick a random example - then yes, attention to detail can make a difference. But that is because such an amplifier was designed to give as good a performance as possible, given the constraints of the topology.

But in the context of a transistor radio, even a primitive one using a pair of same-polarity transistors sat between a pair of transformers - and perhaps even with no overall NFB to help - hFE matching is a non-issue in my experience, providing both transistors are healthy.

And quite apart from the amplifier, how about the speakers found in these sorts of radios? Again, quite different to hi-fi speakers. Some radio loudspeakers have terribly high levels of harmonic distortion, along with a very uneven frequency response - again, I've measured lots of these in recent years (and put some of the results on this forum). The speaker dominates the overall performance, and as such, there's simply no point making a transistor radio amplifier with 0.01% distortion. In practice, you're lucky to get below a few percent with ancient transformer-coupled designs IME.

And remember, most of these sorts of sets are AM only - ever measured the THD of a typical AM receiver? If anything, some "nice" even-order harmonics might usefully brighten up the sound of an AM transmission

Of course, transistors were relatively new back then, and manufacturers lacked experience and knowledge, so it is possible that some believed that hFE matching was important, even in a radio. But of course, people used to believe that transistors are current operated devices

Anyway, sorry to have waffled on. I hope some of it is of interest
mhennessy is offline