I finally got a few days of decent sleep (decent meaning more than 4 hours), so I had a little energy to work on the simple DC transceiver. A few days ago, I got the remainder of the audio chain working. The emitter follower on one of the outputs of the differential mixer was yanked, and I connected a class-A audio amp directly to the mixer. Then I stuck the emitter follower on the output of the class-A amp to enable the receiver to drive low-impedance headphones. No, it’s not extremely efficient, but it is simple and it works. Best of all, no transformers are needed. As an afterthought, I added a simple shunt-to-ground mute circuit with a 2N7000. That might have to be tweaked a bit later
The transmitter is also a simple design. The second output of the differential mixer is tapped with an emitter follower that will have its VCC line keyed to control transmit. Directly following this is a 2N7000 class-C PA. After a bit of work tweaking the impedance matches to get the right amount of drive to the PA, I can easily get 2 watts out of the amp (before low-pass filtering). What’s neat is that the emitter follower puts out about +10 dBm, and it gets amplified up to +33 dBm in one stage. A very compact design that can generate a decent amount of power.
So in order to make this a true transceiver, I have a few things left to do. First thing, of course, is to get a low-pass filter on the transmitter. I’ll also need to provide a keying circuit and T/R switching. I’m still not sure what I’m going to do about a sidetone. I also think that I’ll put an RIT circuit in there and not worry about a fixed transmit offset (that would be very hard to get right in such a simple transceiver). Keep watching for another update, hopefully soon.