?

?

The input current to the base will be equal to the input voltage divided by the equivalent input impedance of the stage. ?

?

The collector current will be equal to the HFE of the transistor times the input current.

?

The output voltage will be equal to the collector resistor times the collector ?current.

?

So the output voltage is equal to

Rc times hfe times vin / Rin

?

Gain of the stage is equal to output voltage divided by input ?voltage and therefore is equal to

?

Collector resistor times HFE, divided by input resistance.

?

And in general it’s going to be pretty high! ?I don’t know the input impedance but you could guess maybe 1000 ohms ?or actually measure it by putting a 1K resistor in series and seeing how much the output voltage drops and doing a little math.

?

It’s a simple biasing circuit, if the collector voltage rises a bit then more bass current will be injected to bring the collector voltage lower. ? It’s not as fancy as a voltage resistor divider, but it works if you are using very very small signals and therefore of the biasing point of the collector can be off a volt or ?two. and not matter

?

Here are some links to educational systems I created to teach this kind of stuff to our local ARES group:

?

Gordon

?

?

?

?

That's a cool idea - that would give me a way of determining the existing and desired amount of gain, which takes me back to the desire to reconfigure Q70 stage gain...

I suspect Q70 and it's two biasing resistors (one in C-B path and one in C-Vdd path) have gain controlled by the two resistors (when I changed to a higher-gain transistor type, the circuit gain barely changed), but I've been searching and so far haven't found a model/analysis of this sort of amplifier stage, even though it's really only 3 parts (!)

Dave