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Old 6th Sep 2010, 12:27 pm   #10
mhennessy
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Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,674
Default Re: Transistor Choice in radio project

Um, what has hfe got to do with the gain of the amplifier?

Transistors are transconductance amplifiers - voltage in, current out. This is proven by the Ebers-Moll model which describes the relationship of VBE to IC, and from which one can derive the simplified rules gain=gm.RL, where gm=35.Ic (or 40.Ic, depending on the operating temperature). You only need to consider hfe when establishing the biasing circuits.

Incidentally, AGC circuits work by changing the bias which changes collector current, which in turn changes gm.

Given how "shifty" hfe is (temperature, device sample, collector current, planetary alignment), it's a damn good job it's a pretty incidental parameter. When you do consider it, you always find the worst-case published value, then assume something well less again. Examples might be emitter-followers in audio output stages or pass transistors in power supplies.

In your case, if you get significantly different results from different transistors, assuming that the DC conditions remain identical (important!), then I would offer the following possible explanations:

1. Germanium transistors have quite high leakage, and this can be modelled by an internal resistance between collector-emitter. All transistors have this as a predictable part of their operation (it's called hoe), but the leakage appears in parallel with it. As these are in parallel with RL, the gain comes down... In extreme cases, the collector-emitter junction might be a few hundred ohms and there might be no transistor action at all - and depending on the surrounding circuit, the DC conditions might actually be OK (I last saw this on a Roberts R707 where TR4 (BC158) had failed. Actually, I've found the Lockfit transistors to be pretty unreliable for silicon...)

2. The frequency matters, and at audio, pretty much all transistors are the same, but at RF or even IF frequencies, you have to consider the inter-electrode capacitance. These vary between different transistor types, although are reasonably consistent between different samples of the same transistor...

Hope this helps,

Mark
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