QRP Power Amplifier for BITX-style rigs (40 m) — V9.5

I’ve just finished a new PA board optimized for simple homebrew rigs like the Bitx40.

The bottom layer is now a nearly solid ground plane for lower RF impedance, cleaner layout, and better stability. 

What’s inside

Final: IRF510 on heatsink, adjustable bias (RV1)

Driver chain: 2N3904 → 2N2219A

RF chokes & transformers: FT37-43 cores, link-coupled stages for 50 Ω I/O

Low-pass filter (7 MHz): T30-6 toroids + film caps for clean spectrum

Supply: Works from 12–18 Vdc, onboard regulation and PTT control headers

Connectors: Antenna out, RF in, PTT (+Rx/+Tx), and bias test points

Expected performance (40 m)

Drive: ~100–300 mW SSB/CW from your exciter

Output: ~4–8 W with good efficiency (IRF510 + 13.8–18 Vdc)

Stable on long coax thanks to the improved ground plane and layout

Quick tune-up (first power-on)

No RF drive. Set bias so the IRF510 idles ~50–100 mA.

Feed a small RF drive and into a 50 Ω dummy load.

Increase drive while watching current & output; aim for ~0.5–1 A at 13.8 V.

Peak the inter-stage trimmers/links for max power with clean waveform.

Verify the LPF: harmonics should be >35–40 dB down at 14 MHz+.

On-air: keep the heatsink happy and don’t overdrive—clean RF first!

Why this revision?

Ground plane first: the bottom copper is mostly GND for lower noise and fewer parasitics.

Shorter returns & thicker traces: less RF feedback, easier to tame on 40 m.

Serviceable: labeled test points and headers make biasing and PTT wiring simple.

note:

https://www.facebook.com/groups/QrpCuba