Simple DCR (Direct Conversion Receiver) – Part 2: Common Emitter Amplifier

RE: Simple DCR (Direct Conversion Receiver) – Part 2: Common Emitter Amplifier

shabaz — Fri, 24 Apr 2026 15:33:06 GMT

Here's the complete intercom circuit proposal.. The simulation of the sub-circuits seems OK, but needs constructing!

I scrapped the 30 kHz alert detect circuit, I didn't have a good feeling about it. Instead, now there's DC applied when the user wants to alert the remote end. The wires need to be connected with polarity for that though (future circuit could toggle positive and negative on the line slowly to get around that), but it seemed a minor limitation.

RE: Simple DCR (Direct Conversion Receiver) – Part 2: Common Emitter Amplifier

michaelkellett — Fri, 27 Mar 2026 16:13:31 GMT

That's a really weird pre-amp.

R16 - R19 set up the DC biasing of Q5 with 3.3 mA of collector current.

The AC impedance at the emitter is the impedance of C13//R19, the LF breakpoint is at 0.72Hz - why !!!!!!

At higher frequencies the emitter load impedance tends to zero.

The AC impedance at the collector is more complex at low frequencies 1k//(180R + ZC12).

The LF breakpoint is 19Hz.

The input impedance will be very low (because Q5 emitter is effectively grounded for AC).

The actual gain will depend on the emitter resistance of Q5 .

Since the intention is to drive about 100mV into a 1k load a lower Q5 collector current would be fine.

I tried this (in simulation)

It has good PSU rejection (18dB worst case at 30 Hz and 90dB at 100kHz.

About 2mA current drain.

Much smaller capacitors.

It's not the best way to use two transistors but not bad. It still uses the first transistor open loop so the gain at 1kHz is lower that you would expect from the ratio of R3 to R4 because of emitter resistance.

It manages 28.7 dB of gain (you would expect 33.4) and can drive 1V pk-pk into the 1K load.

RE: Simple DCR (Direct Conversion Receiver) – Part 2: Common Emitter Amplifier

DAB — Tue, 24 Mar 2026 19:40:26 GMT

Very good post.

Excellent walk through the circuit and the changes you made.

Well done.

RE: Simple DCR (Direct Conversion Receiver) – Part 2: Common Emitter Amplifier

jc2048 — Tue, 24 Mar 2026 13:11:03 GMT

Good fun.

This is for SSB and morse?

I'm not much of a transistor circuit designer, but I've noticed that one thing they sometimes used to do in the old days was to flip the polarity of stages. As you've got it, the transistor Q3 develops its output through R10 (ie relative to the rail), but the input to Q4 is relative to GND (the BE junction). If you flip the entire 2nd stage, so that Q3 becomes PNP, then the output will be relative to GND and that then isolates somewhat from the supply noise. The input to that flipped second stage is then relative to the power rail and can come from the pot going to the rail rather than ground, which matches what the output of the first stage is doing anyway. The input of the first stage remains relative to ground, which suits the mixer output. The only potentially poor PSRR is then the output stage, but I think that one works ok because of the transformer coupling to the loudspeaker. You'd have to be careful around the pot, but you need to take care there anyway with so much gain following it and a middling impedance.

Whilst it might improve the situation, you'd probably still need to work on the power side of things.

Another approach is to transformer-couple between all the stages (which is essentially how it works going through an IF strip).