I see the common emitter configuration like the following frequently:
This is not a configuration I would normally use, but I would like to understand it better. It seems to appear frequently in
60s/70s era designs.To bias this, you take the ratio of R3 to R2, to arrive at a collector voltage (target usually about half of Vcc). This part is easy- in this example the ratio is 10:1. So the resulting collector voltage is approx (10 + 1) times Vbe, giving about 7 volts.
The link above is a LTspice simulation using 2N3904, with a DC Beta of 300.
QUESTIONS:
(I only care about audio frequencies here)
Since there is negative feedback involved:
- How do you compute the small signal gain? 2. Input impedance? 3. Output impedance?
LTspice output of a small AC wave form showed a gain of about 219 (9.9mV amplitude in, 2.165V amplitude out).
But with Beta at 300, I would have thought the gain to be near 300 minus the signal feedback (1/10th), which would be about 270. But the simulation showed about
219 instead.I'm guessing the input impedance to be approx R2||R3+R1 (nearly just R2). But with the negative fb, I suspect that the input impedance is raised considerably.
I'm also guessing the output impedance to be approx R1||R3+R2 (nearly just R1).
Thanks, Warren