I've been self-studying electronics and am currently trying to design a high-gain single-stage BJT amplifier. Here is what I have so far:
Q1 serves as a current source "active load" for the common emitter amplifier. R4 and R5 form a biasing voltage divider supplied from Q2's collector. C2 bypasses R6 to increase the gain at signal frequencies.
I've simulated this circuit in MacSpice and found a gain of about 250. This is comparable to a collector resistor of about 12.5 kΩ (assuming total emitter resistance around 50 Ω at signal frequencies). I expected the current source "active load" in Q2's collector to present a higher impedance and result in a higher gain.
On the other hand, since I'm using Q2's collector to supply the voltage for the biasing voltage divider (a form of negative feedback) I might have expected that the gain would roughly be (R4 + R5)/R5 ≈ 10. Specifically, the open loop gain is 250, I'm returning 10% of the output signal to the base through feedback, and the gain equation gives 250/(1+250*0.1) = 9.6 expected gain. What is faulty about this logic?
How would you generally improve this circuit for higher gain, sticking to a single stage using only BJT transistors?