2 W Bipolar Power Amplifier, 7 MHz (MJE181)

What it does

The final amplifier for a 40 m QRP CW transmitter. About 100 mW in gives about 2.25 W out into 50 Ω (EMRFD p. 1.19).

How it works

Q5 runs class C with its base returned to ground through 33 Ω. T2, 5 bifilar turns on an FB43-2401 wired as a 4:1 autotransformer, steps the 50 Ω drive down to about 12.5 Ω at the base. The collector is fed through a 2.7 µH RFC. A pi network (1000 pF, L2, 750 pF) transforms the 50 Ω load to the collector load line and filters harmonics. D1, a 33 V zener, clamps collector peaks.

The simulation gives 4.1 W out for 100 mW available drive. That's well above the book's 2.25 W, because the MJE181 model and the ideal parts are optimistic. Treat it as simulated only.

Building it

• Q5: MJE181, per the EMRFD errata. The figure's 2N5321 is discontinued. Mount it with an insulating spacer; a small heat sink helps.
• T2: 5 bifilar turns of #22 on an FB43-2401, connected as a 4:1 step-down.
• L2: 12 turns of #22 on a T50-6, spread over half the core.
• Use mica for the 1000 pF and 750 pF output capacitors.
• Always follow it with a low-pass filter such as the 40 m 7th-order Chebyshev LPF.

Tuning and testing

Use a 50 Ω dummy load and a power meter. Raise the drive slowly to about 100 mW and watch the collector current and the heat sink temperature. Squeeze or spread L2 for the best output.