The Transmit Pre-Driver Stage


One of the most critical elements following the IF Block, in any transceiver is the Transmit Pre-Driver stage as it must present a 50 Ohm Load in/out and must be stable. W7ZOI often said it is better to have less gain with several cascaded stages versus one single really high gain stage that is on the brink of oscillation. At times I have even added a 50 Ohm termination resistor just to assure that there is a constant load.

It is also no secret that I often use and reuse standard modules in my projects and one of the best is the one zi often refer to as the EMRFD Module which I first spotted in of course EMRFD. It is Figure 2.93 and is good for a Pout of about 500 MW up through 6 Meters. This stage is nothing more than a 2N3904 driving an 2N3866. The 2N3866 transistors today are a bit "spendy" and I have had good success with the 2N2219 as well as the 2N3053. At times I have even popped in the circuit a BD139. These substitutes might require some adjustment to the bias and emitter resistors but that is your work to do.

This is the circuit as it appears in EMRFD.

NOTE: The 15uH RFC is 5 Turns of #20 Enamel on a FT-37-43 core and the 3.9 Ohm resistor is a 4.7 Ohm 1/4 W in Parallel with a 22 Ohm 1/4 watt resistor. OK a math lesson ... Resistors in parallel is the product over the sum so we 4.7 * 22 = 103.4 Ohms and the sum is 4.7 + 22 = 26.7 Ohms. Thusly, 103.4 / 26.7 = 3.872659176 which we will call 3.9 Ohms!

Cautionary note -- a heatsink is mandatory --especially if you like to rag chew!