Hi ive just built a closed loop uhf mosfet modulator.

[G1BFK]

ive tested it with some surplus russian devices get about 50w on 70cms. i used 3 x 19v 4 Amp laptop supplys avaliable cheap on ebay. one -s a bit of a over kill because its only used for the -19v for the opamp. I used 3mm brazing rod for the anode line. mounted obout a cm obove the ground plane. Heres a link for the circuit. .Regard Roger G1BFK

[G8GKQ]

Hi Roger

A novel approach! Any chance of a photo of the construction and a plot of the output spectrum?

Dave
G8GKQ

[G1BFK]

Hi Dave, Could probably do with a harmonic trap. Only used it on dummy load so far.The modulater drives a loud speaker well,all the advantages of dc coupling and all that just up the feedback resister a bit.

[G1BFK]

Hi there a mistake on the drawing the 10k negative bias resistor on the IRF10's should go to ground not the source of the devices. you cant see much on my construction because i solder copper clad board round it for screening i am doing another build with a bigger heatsink because it gets hot very quickly i will do some intermediate picys. DONT BE TEMPTED TO TWEAK THE THE TRIMER CAPS (THEY ARE HOT!) TRIM THEM USIND A GRID DIP METER. Regards and good constructing Roger G1BFK.

[G1BFK]

Ther are alot of fast op amps avaliable but they probably wont work. The LM715 has a cascade input which compensates for propagation delays in the loop. Regards Roger G1BFK