What value R2 may have in that schema? Anyway it will be loaded by pn-junction of Q5 and will have fixed voltage drop near 0.65V.

What value R2 must have if the current source is used instead R1? The current in that situation will be fixed and R2 will not influence to constant current through it.

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  • \$\begingroup\$ You have a (long tailed pair) diff-amp and a VAS, without the final output stage. It's not unlike the LM380, except of course that the LM380 has an output stage, darlingtons at the input, and some current mirrors to make it more rigorous. Your circuit also includes some components that would not be found in an IC (a few caps, for example.) So this is more of a discrete thing, except that the 2N3906 should probably be a BCV62 or BCM62 pair, instead. Start here: electronics.stackexchange.com/questions/273867/… \$\endgroup\$ – jonk Oct 5 '17 at 5:09
  • \$\begingroup\$ If you are still interested in a specific discussion of this circuit, I may be able to help. But I need to know that you are still alive. \$\endgroup\$ – jonk Oct 5 '17 at 5:10

R6/(R4+R6) set a trip point about 4volts. Thus 3.5ma through R1 of 1Kohm. You can keep R2 at 1Kohm.

Problem is base of Q2: always up near +9v. So Q2 is always off, unless C8 brings in a Large voltage swing.

Does this circuit work?

Can the 9volt battery even provide enough current thru R8(100 ohm) to swing rail-rail? Consider making R8 also be 1Kohm.

What is purpose of circuit?

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    \$\begingroup\$ The circuit works. It's a sample from power amp without output current stage. I check with virtual voltmeter - the base of Q2 is under near of 4.5V. And the oscilloscope shows amplified input signal from generator. I'm interested in how you find R1 and R2 value? What formulas? \$\endgroup\$ – MaxMil Sep 28 '17 at 16:57

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