The AM Forum

More experimenting...

I'm thinking about building a small, single-ended class D transmitter. I don't have values posted on the schematic below since this is just at the concept level.

My question is about the drain output network. I believe that in class D you need to have a tank circuit at the drain? So, say if I did a 1 turn / 2 turn ratio to bump up the impedance of the FET using a single FB-43-1020 core, I would need to resonate that one turn with a shunt capacitor?

I calculated the approximate inductance of a single turn, and it's around 4uh. I would need to shunt the drain with about a 423pf cap? But then there's some amount of capacitance inherent in the FET itself, so it would be a little less than that, I guess.

Jon

I've been using this design for a while now, simplicity itself!

It's kinda like class D/E as I get >500KHz of B/W.

You can swap the GaN for a SiC but they're way harder to drive and will need 12-15V and the driver gets a bit toasty!

JB.

I see the difference. Mine is transformer coupling without a choke, and yours is capacitor coupling with a choke. With what little research I’ve done, I think the difference is voltage mode vs current mode. At low powers I guess it doesn’t matter.

As long as it puts out RF! I’m on top of South Mountain in Phoenix right now trying to make a few contacts for the AM Rally with a cheap rice box. Not going too well...  :)

Rice box modulation with a mobile antenna is almost hopeless. Anyway, it’s worth a try!

Jon

Hi Jon, John here too!

Yes I used to use the transformer method for low power (10W carrier) with a bifilar wound toroid.

I've found for a 40W carrier this design is a simple as it gets.

2 devices in push pull for 80W and 4 devices for 160W ALL with a 48V supply.

I've not tried using higher Z O/P stage and more volts but with GaN or SiC there's plenty of headroom.

I've seen some of your designs and they look well engineered!

I'm into making stuff with SMT and somewhat smaller, the 40W version is only 3"x1.5" on a PCB.

Seems to cover the whole of 40m whilst keeping efficiency over 90%.

Cheers and good luck with your design.

JohnB.

Thanks John! Yeah, I like to putter building stuff, but my real passion is women.

I gave up calling CQ on the mountain with the rice box. Time to head down the hill.

Jon

Radio, women and wine in varying amounts and order!

J.