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?
The current source is voltage dependent. This can be improved by replacing R1 with 2 silicon diodes connected in series, so that the current will depend mainly on the value of R3.
As already mentioned in the comments, the gain (as well as the bandwidth) depends on Rload and the negative feedback Rsource/R4.
Here are the results (Fig1) with some changed values and the resulting current source of about 1mA, LTSpice simulating :
At 1kHz frequency, Vin=1mV sine
1M - gain 2500;
10k - 350;
1k - 40;