Basic Amp Design

[ehque]

Hi guys,

I'm teaching myself transistors. While looking at wikipedia's "simple electronic amplifier", this picture pops up:



Q1 seems to be a transistor run in class A
R1 and R2 bias the input, C1 and C2 block DC at input and output
R7 and R8 seem to form a voltage divider for the negative feedback, which uses Q2 to vary the emitter voltage at Q1, controlling gain

I think Q4 and Q5 are matched pair push-pull transistors, and D1 and D2 bias them into class AB

What I don't understand, is what Q3 does.

Also, I don't understand how you can stack diodes D1 and D2 like that. Is it supposed to bias Q4 and Q5 to exactly ±0.7 volts?

Couldn't this design be massively simplified by only using 2 transistors, Q4 and Q5, in push-pull configuration, then adding the appropriate biasing elements?

[Passinwind]

There's a comprehensive explanation of that circuit here.

[ehque]

Thanks! Must have missed that page out. I'll look through it and get back here if i have any questions.

[ehque]

It appears that my analysis of the circuit was mostly correct. There are probably other methods than using another transistor in Q3's position, i guess?

A 2 stage (high input impedance / voltage gain -> lower input impedance / current gain) would have been my off-the-hip design.