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Why is it so hard to answer this question. (Which transistor to select).

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vk6zgo:

--- Quote from: Someone on October 29, 2023, 09:45:10 pm ---
--- Quote from: Zero999 on October 29, 2023, 06:19:06 pm ---That's true, but note that J-FETs can also be biased into the enhancement region. The gate impedance drops, but it's not an issue, so long as it doesn't load the circuit driving it and the gate current isn't exceeded.
--- End quote ---
At which point we also discuss "reverse active" region of bipolar transistors ?

--- End quote ---

Back in the dear, dead days beyond recall, when Philips & Mullard were virtually the "only game in town", such things as true "bipolar" transistors existed, which were electrically symmetrical.
They were used in various rather esoteric circuits.
That's why I prefer the term "BJT".

chinoy:
Ask me a question which you think is complex about the design of a two stroke engine maybe I could give you a simple answer in simple english. That even a newbie could understand. Ive spent the last 20 years of my life answering such questions for my forum members.

Lets narrow it down to TO92 BJT NPN transistors only.
I dont know why everybody feels they could answer the question better if they had the complete circuit. So here goes.
These are the two amps Ive built.
If you scroll down you will see the transistors he has listed as optional.
https://vaspelectronics.com/400w-hifi-mono-amplifier

And the other is this one.
http://sound-au.com/project3a.htm

Complete Schematics and transistor options listed.

What I have self learned from studying old amps that sound amazing is.
1. The more stages you have the better.
2. A low Pf value is critical.
3. Low gain low speed transistors seem to sound and work better.
I also scrounge junk yards for high end Amps just to pull parts that you can no longer find new and which work so much better than just using a regular BC547.
I have a cheap china component tester a 100 Mhz Scope, DatsV3, A USB DAC and a UMIK-1. On the s/w side I have ARTA and REW. To do spectrum analysis and measure THD + Noise.

JohanH:
That's narrowing down your question considerably. It sounds like you already know in general what works or not. Maybe ask the question in diyaudio.com forums.

Btw, I remember your forum from way back. I still have an RD350 and H1 500 in the shed that I haven't ridden in years. I used to read lots of 2-stroke literature back then (Jennings, Blair, Bell, Cameron, etc. books I have), even wrote a basic 2-stroke exhaust tuning program. I'm a little disappointed the 2-stroke advancements took so long to make it into production and now electric motors are taking over. But that's off topic...

tooki:

--- Quote from: chinoy on November 03, 2023, 08:20:11 am ---Lets narrow it down to TO92 BJT NPN transistors only.

--- End quote ---
The fact that it’s a TO-92 tells you one thing, and one thing only, that is of any practical consequence: that the expected heat dissipated in the transistor isn’t expected to exceed the thermal characteristics of the TO-92 package. (Assuming the engineer did power calculations…) That’s it. It doesn’t tell you anything else about the device. If you happen to already know that it’s an NPN BJT, you now know that it’s an NPN BJT with power dissipation under around 600mW.

I suppose you can also make some (completely pointless) assumptions about its maximum voltage (in that it must be something below a voltage that would arc between the leads, so it’s not going to be a 10kV part), and its maximum current (in that it’s not going to be designed to pass 50A through those thin legs). But it can’t tell you anything useful about its voltage and current. Nor will it tell you anything at all about its other characteristics.


--- Quote from: chinoy on November 03, 2023, 08:20:11 am ---I dont know why everybody feels they could answer the question better if they had the complete circuit.

--- End quote ---
:palm:
Because to select a transistor, you need to know what function it’s going to perform, and what its operating environment (by which I mean the circuit it’s operating in) is — what voltages and currents will it handle? What frequencies will it see? What kind of gain does it need? Is it operating in the linear region or in saturation? Etc etc etc. And you need to know what your design priorities are, because selecting for one characteristic may mean sacrificing another.

Tation:

--- Quote from: tooki on November 03, 2023, 11:06:10 am ---But it can’t tell you anything useful about its voltage and current. Nor will it tell you anything at all about its other characteristics.

--- End quote ---

It will give a clue about its Cbc being smaller than the Cbc of transistors with bigger packages. The Cbc in the 2nd stage has impact on overall amplifier distortion. So preferring TO-92 packaged transistors on such place, better than others with bigger packages, has some foundation. Suppose that the OP is aware about the books from Douglas Self on the matter.

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