Quote:
Originally Posted by kalee20
Quote:
Originally Posted by Argus25
Quote:
Originally Posted by kalee20
Argus's method is good in that the transistor's operation is stabilised, but it does need some means of determining the actual (AC) base current to work out hfe.
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There is no requirement at all to identify the actual value of the hfe and it has no value for the application. The circuit was intended to simply illuminate differences in the behavior of two transistor specimens...The purpose is not to measure hfe, but to see if two different transistors have near identical static & large signal dynamic electrical properties..or not. |
Good point. I'd lost focus of the overall aim!  |
But it really depends on the circuit.
If you build the circuit above, but incorporate a variable resistance in the base so you can adjust for 6V at the collector for every transistor you test, you'll discover two things:
1. A measurement of Vb and Vc will show that the voltage gain of the transistor itself doesn't change. Even if you switch between silicon and (healthy!) germanium.
2. The overall gain goes change, thanks to the changes in hie as a result of changes in hfe.
Neither of these things are a surprise. Of course, a designer will expect them, so will design the circuit around this. For one, you might want to stabilise the bias so the collector current remains fairly constant with hFE. But in practice, it's relatively unusual to expect to get this much gain from a single transistor stage as the results aren't exactly linear and low distortion, so today a designer would use more active devices and negative feedback. Obviously things were different back in the early days of transistors...
Really, the point of all this was to see how transistor hFE affects the performance of the 2-transformer push-pull circuit the OP originally enquired about. My original reply - which goes against the accepted wisdom, but comes from years of experience - perhaps raised a few eyebrows but I'm not in the habit of making claims without good reason. Measurements of a single CE amplifier aren't really sufficient, as we don't know the source impedance "seen" by each transistor in a PP amplifier as yet. But I've remembered that up in the attic I have a scrap Roberts R300 chassis - the audio stages are OK, so if I can remember where that might be, I'll brave the cold later on today or (more likely) next week.
In the meantime, I'm looking at a Hacker amplifier - not the same one I linked to earlier, but a similar one from the earlier RP18. This gives a wholly respectable distortion of <0.5% when no load is connected to the output, but as soon as you connect the 30 ohm load, it jumps to 10%. On the 'scope, the positive-going parts of the sine wave are about 25% lower than the negative-going parts. The hFE of the upper output transistor is fine, so the fault is perhaps in the preceding VAS transistor, or somewhere else entirely - I'll get back to it in a moment. But suffice to say, it actually sounds quite reasonable on speech and music - the asymmetry causes mostly "nice" second-order distortion rather than unmusical 3rd-harmonic.
That set was not brought to me with a complaint about distortion - I'm only investigating it out of completeness and principle (and interest). Which certainly shows that in practice, it's quite possible that even a gross mismatch of output transistors in a transformer-coupled PP amp might go unnoticed by the listener
All the best,
Mark