Why am I getting no gain from this differential circuit?

Question

I am having difficulty understanding why I am getting unity gain from the circuit below. Before I added the common emitter stage, there was a gain of around 150, and I wanted to add another 10 X gain to that, but as soon as the CE part was connected that gain went to around -1.

Looking at the probes, it seems to me that everything is in order: 1/10th current at CE transistor base, mid point DC bias at output.. shouldn't the ratio of collector and emitter resistors give at least 8 X gain?

I've studied separate components of these amplifiers in textbook format, but getting stages to work together in a practical application is, unfortunately, beyond me for now.

Thanks for any help you can give.

Answer 1

Your bias network is being upset by the DC coupling to R3 — note that R3 and R7 are connected directly in parallel. You need a blocking capacitor there.

Or else arrange the biasing of the differential stage so that you don't need to bias the next stage separately.