[HomeBrew] Help with QRP Driver/PA

[Andrew Roos]

Hi All

I have designed and built an "almost QRP" dual-band CW transceiver. The
desired output power is 10W on the 40m band and 5W on the 20m band.

My local oscillator (a DDS synthesizer) produces a signal of about 4 V p/p
on 40m and 2.5 V p/p on 20 m. This is buffered by an emitter follower
(2N3904, 30mA collector current, 100 ohm emitter resistor), and then
amplified by a common emitter stage to get the voltage swing of about 6 V
p/p that I need to drive the IRF 510 MOSFET final. The common emitter stage
drives a push-pull emitter follower stage (2N3904/3906) which drives the
IRF510.

The problem I have is that the CE stage is not providing any gain on 20m.
The output at the collector is about the same as the input signal. The stage
consists of a 2N2369A high-speed switching transistor running at 25 mA
collector current, with a 180 ohm collector resistor and 100 ohm emitter
resistor. A 100 pF capacitor in parallel with the emitter resistor provide
some bypassing to increase gain at high frequencies.

Last night I tried adding a 1K5 resistor in series with the base, with a 22
pF capacitor in parallel with the resistor, in an attempt to alleviate any
charge storage problems (component values were just guesses/what I had on
hand). This provided a slight improvement. At present the power output is
8.9W at 7 MHz (I can live with that) and 3.4 W at 14 MHz (I would like at
least 5W). Drive on the gate of the final is only about 1.3 v P/P on 14 MHz,
which is much too low, and results in the device running hot because the
drive is insufficient to completely turn off the MOSFET, which is biased for
5-6V drive.

I'm thinking that perhaps I need a resistor between the collector of the CE
stage and the bases of the push-pull driver pair, to prevent overdriving the
push-pull stage and reduce the load on the CE stage (at the moment, the
collector of the CE stage is directly connected to the bases of the
push-pull pair). However I would like to hear other peoples' opinions before
I start cutting tracks on the PC board!

I'm very new to RF design - this is my first RF project - so any suggestions
would be most gratefully accepted.

73
Andrew ZS1AN