Bi-Directional Amplifier Stage


My interest in developing a stage that could be used both on transmit and receive was of course in large measure about keeping things simple keeping costs down and reducing the labor required to build the project.

A little "noodling" (my term for giving thought about a problem) turned up that with two SPDT relays it would be possible to have such a circuit where a single device was the Receiver RF Amplifier Stage whilst also serving as the Transmitter Pre-Driver .

In looking at the IF Module, the ADE-1 and the Band Pass Filter these elements are inherently bilateral. The next piece of the puzzle was that the BPF in Receive is presented with an amplified signal from the preceding RF amplifier stage. Yet this same port was the output (after passing through the BPF) from the ADE-1 when used as the Transmit Mixer.

If I could steer the Input and Output terminals of this amplifier when connected to the BPF our problem would be solved. This is the solution.


Now this stage initailly was two J310 MOSFETs that were connected to simulate a Dual Gate MOSFET. The one desirable feature was that a trim pot could be inserted into the Biasing of Gate 2 so that you could control stage gain. I found once it has been set I never touched it. BUT this trim pot could be a critcal piece of an AGC or ALC Ciruit. Since the Simple SSB neither has AGC or ALC, I thought about a simplifaction of this stage.

The revised stage uses less parts, works well and we got a "TWOFER". With the simple change of one emitter resistor the Bi-Directional stage can also be used as the Transmit Driver Stage. The Driver stage has a lower value of Emitter resistance (4.7 Ohms versus 22 Ohms in the Bi-Directional stage).

The 2N2219A runs HOT and for the Bi-Directional Amp is ON all of the time. So with the very hot running device using 22 Ohms makes it run somewhat cooler. But the Drivers is only on for short periods of time and the extra gain nicely drives the IRF510 to 5 Watts. Thus the 4.7 Ohm is used in the Driver.