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Here is a simple audio amplifier with an npn transistor. What should be the value of voltage source V2? How can I calculte it?

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You first need to tell us what is driving the amplifier and what you plan to drive with the amplifier. So if it's a speaker, we'll need its power rating and impedance. For the input we'll need to know if it's a standard line-level input or some other voltage range. – AndrejaKo May 26 '12 at 19:54
Yes output connects to a speaker and input can be pc or mp3 player. – Marilyn Robinson May 26 '12 at 20:00
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I'm analyzing the amplifier and I don't like the way it works. At 1 kHz inpuf frequency and standard PC output voltage, it can barely provide 180 mV output voltage, which is less than no amplification. – AndrejaKo May 26 '12 at 20:07
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This is no amplifier at all since the output comes from the input (base) of the transistor. This circuit would make more sense with R2 and R3 connected. – Olin Lathrop May 26 '12 at 23:21
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@Olin - like this? Top right. I use the bottom two. – stevenvh May 27 '12 at 11:56
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Answer: 20V

Simple (very crude) approach, say when collector resistor is given (not asking why is it 10k): Transistor power limit -> reduce to 1/10 for safety -> choose collector current limit -> divide power by current to validate to 1/4 of max collector voltage -> multiply by resistor value -> double it to keep output in the middle -> get voltage.

The datasheet says: ~625mW is a limit, then 62.5 mW is nominal. Then choose sensible collector's current. Looking at datasheet it is between 1..10mA. Looks like for ~50mW the CE should be around 5..50V. Validating the CE limit from datasheet 40V/4=10V. Deriving for 10K resistor and voltage of 10V getting 1 mA current. Doubling it -> 20V.

No matter how safe it all looks, the number looks too high and overall the cascade feels a bit fragile. I'd rather go with no more than 9-10V and reconsider output impedance and optimize on something like bandwidth or noise figure.

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20V, or 9-10V ? – stevenvh May 27 '12 at 17:06
15 then! :) Now I realize why it is some sort of sweet spot with all analog schematics demanding dual 15V supply. – Rocket Surgeon May 27 '12 at 17:13

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