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FT 817 Power Amplifier



This very simple 2 Fet power amplifier easily achieves 250W output with an FT817 5W drive. The key design details as follows: 3:1 broadband input transformer matches the 5.5 ohm gate load resistor (4 x 22 ohms in parallel) to the 50 ohms required by the FT817 . The 4:1 output broadband transformer presents 3 ohms (16:1 impedance ratio) to the balanced HEXFET pair each mounted on a 3mm copper heat spreader which is insulated from the 2 1w/degC heatsinks. These are blown cool by a fan underneath.

The power supply required is 28v at 30 amps. The amp is around 50% efficient with a standing 750mAmp temperature compensated bias. An IC 703, with 10watts output will drive the output to around 400 watts. The output filter shown is a 5 pole topband filter with T130-2 torroids and 400v silver mica caps. Peak output voltage on 160 metres with 5 watts drive is 160v or 320v p-p in 50 ohms equating to 250watts. This is slightly higher than the reading on the 3kw MFJ power meter. The inline FET fuse (orange lead to the output transformer is ok at 30 amps.

Comments

Karel OK2ZI said…
What Mosfets did you use? Thanks Karel OK2ZI
15 November 2011 at 07:23
Unknown said…
Would you be so kind as to provide a schematic of your circuit?
I have a general idea of how the circuit works, but I've never built an RF amplifier.
- Mike
13 December 2014 at 17:50
Unknown said…
Should have added my call sign to my post. Sorry about that!
Call sign is ae5lv.
Hope to hear from you soon.
k
73
ae5lv
13 December 2014 at 17:57
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