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Old 6th Dec 2017, 2:19 pm   #8
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,115
Default Re: Matched Transistors

I think the final paragraph of my first post covered it really. If the amplifier gives good results, then that's all that matters.

I've had failure modes where the positive half-cycle was clearly a different amplitude than the negative half-cycle - in each case this was caused by a faulty output transistor that had a hFE of around 10 or 20 when measured rather than the expected 100+. Clearly very unwell, and the low hFE would have been but one symptom of its illness.

Yes, in this case, it would have been loading down the preceding VAS transistor because it would have presented a low input impedance. But lower by an order of magnitude, not a few percent, or the 20% quoted in the marketing material in post #5, or even the 50% that I suggested.

By way of example, take the amplifier used by Hacker (schematic available on this page. T4 is the VAS, and has a standing current of around 9mA - this is the most base current that can be supplied to the PNP output transistor on the negative peaks, thanks to bootstrapping (more can be supplied to T5 by T4 on the positive peaks). The 15 ohm loudspeaker requires around 440mA peak (1.5W), so T6 (an AC128) must have a minimum hFE of around 50. However, providing this is met, it really doesn't matter what the hFE is of both output transistors. This circuit uses negative feedback to ensure the output voltage is a multiple of the input voltage, and the only times I've had distortion problems with this circuit is - as I've described above - an output transistor has a hFE of around 10 or 20 rather than the usual 100+ (I've seen over 150 with good AC128s).

OK, compared to what the OP asked about, this is a more sophisticated circuit with lots of negative feedback. If you take a simple circuit with a pair of transformers with two transistors in the middle, then yes, you'd perhaps expect the hFE matching to be more critical. But I can only say that this is not my experience. Providing they are both healthy (100+), it really doesn't matter. 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!

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. But as always with "rules of thumb", there will always be "outliers", so I wouldn't be surprised if someone comes back with an example of where 50% was not good enough (if so, full data required, please - I'd like to measure it for myself )

Differential amplifiers were mentioned earlier. With those, if anything ought to be matched, it's Vbe rather than hFE (although it is possible to design poor circuits which rely on it, naturally). And of course, Vbe varies with temperature (-2.1mV/C), so that's why it's good to thermally bond them in critical circuits. A pair sharing the same silicon will have inherently good matching and thermal tracking. Such precautions are never necessary in a well designed audio amplifier though...


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