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Your "expert" is wrong.From what an expert told me, a manufacturer installs resistors in the legs to the transistor, so as to produce a certain gain, or limit gain to a certain level. In this way they get predictable performance, by conservative utilitization of components.
hfe can be anything from 10 to 150, but as long as it is not widely toleranced.What hfe value? At what frequency?
The gain of the Q2 is a feedback loop parameter as you know. Therefore, if we are to accurately compute gain and phase margins for the SMPS, we need to know the gain of Q2, ie the hfe of Q2.Why do you think you need a tight tolerance for Q2's gain in the referenced diagram?
That circuit is quite tolerant of any gain variation.
I thought R25 provided emitter degeneration which got rid of most of the temperature variation of the hfe of Q2? (I am referring again to fig31, page 25 of the AN-47 document referred to in the top post)But whatever you choose, you get a large temperature dependent gain variation on top.
How? The amplifier stage is current in/current out. If R25 helps against temperature variations, it also would against initial tolerances.I thought R25 provided emitter degeneration which got rid of most of the temperature variation of the hfe of Q2?
Your "expert" is wrong.
The range of current gain is determined by the manufacturing process variability.
There is no adding of resistors to control this gain.
R25 is apparently just providing current limiting as a protection means.
Prevent RF oscillations, I presume. Sounds plausible.So what they mean by “stability” I do not know.
You are mixing apples and oranges.As you state, transistors all have different hfe. Then that is the gain when no resistor is in the emitter leg. However by putting in an emitter resistor, it reduces gain. Say we want gain =20. From what I understand there is a certain combination of resistors that gives us that. Then we can manufacture a thousand radios, all with gain=20, regardless of transistor hfe, whether 100 or 200 or 300.
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300 is typical hFE, not maximal. Maximal value isn't specified in the datasheet. I would expect a maximal value of 600 or 800 as with similar transistors.2N3904 that has an hFE from 100 to 300
Whose datasheet are you looking at?300 is typical hFE, not maximal. Maximal value isn't specified in the datasheet. I would expect a maximal value of 600 or 800 as with similar transistors.
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