Here’sfor QRP with low cost. This is a broadband one from 1.8 to 30 MHz, so it is “no-tune” and you need only to regulate the quiescent bias multiturn trimmer of each Mosfets couple. This means also low efficiency (for a lower IMD it is even low as around 20-30%) together with good heat sink + blower system.
The push-pull configuration permits to get a better attenuation of even harmonic frequencies (a good benefit since the second is the closer 2x fundamental freq.). The output stages have 2 separate transformers in each couples so the power supply current does not flow in to the output transformer to avoid saturation and the floating winding helps for balance for harmonic attenuation and stability.
We have emploied also in this low power unit the circuit found in higher power units to stabilise thermically and regulate the bias current around a trusted LM723 IC, the transistors are used only for PTT control. In the first stage Mosfet couple we have used a feedback circuit to optimize gain-frequency response. Without it the low frequency gain is very high, too high: at 3 MHz 50 dB ! and a lower 30 dB at 30 MHz, but with this feedback circuitthe overall gain variation is close to a +/-3 dB, a good result.
We tried a similar feedback in the output couple but results where a poorer IMD performance so it has been omitted. We selected Mosfets couples in a simplified way (while not so precise RF matching practice) measuring drain-source resistence: a better way could be to measure in a simple testcircuit current drain at a given gate voltage applied for all devices and choose the ones that showed closer current values.