RF Linear Amplifier using cheap switching MOSFETs — It Works!

Hello you!

I’ve been busy with the amp, and have a ton of updates to make — but, in a nutshell, it works! It will max out at around 1600W, and is linear to beyond 1.2kW. I know all you really want is the money-shot (!), so here is the first draft of the circuit diagram. Coil winding data will follow, and I hope to get time to document the control and protection circuitry, which is very simple. There is nothing more complicated anywhere than quad op-amps and comparators, though. (UPDATE: some of the simple auxilary circuitry for the linear amp is here)

As usual, click on the thumbnail. The image is actually quite large! Pay close attention to the notes on the schematic, unless you collect knackered FETs…


Switching MOSFET RF linear amplifier Circuit diagram

cheap mosfet linear amplifier circuit
cheap mosfet linear amplifier circuit

If you want to save a copy locally on your machine, then right-click the thumbnail and save picture as…

Once downloaded, you can view it in your picture viewer program, zooming in and out as required to read the component values and notes. Though I claim intellectual copyright for the design above, you can use it as you choose, provided its for any non-commercial use, specifically Amateur Radio, at your own risk. Anything else, please contact me first!

Here’s a copy of the amp – a cheap MOSFET rf linear amplifier project as a pdf, if you’d rather download that.

Some pictures of the Mark Two Amp: some during and some after construction

Power supply: transformer, 70Amp rectifier, capacitor bank and a small aux transformer for 12-0-12 for the opamp circuits:


A clearer view of the PSU, including the 1kVA transformer:


Power supply circuit diagram is included in this page.

RF deck section, mostly obscured by the fans. You can see how I “made up” the high power drive and negative feedback resistors, though. You can also see how straightforward the input binocular transformer is — its made from 2 EMC ferrites from CPC side by side. In fact the output transformer is made from 4 of these ferrites — side by side.


Here’s a view of the actual linear amplifier development board during early construction — there’s less clutter, and you can see things more clearly. Especially the output transformer, which may be of interest:


Schematic of the main MOSFET HF amplifier is here

Here I show the PCB I knocked up for the output filters (currently fitted: 160m, 80m and 40m Low-pass filters). You could do a much nicer job (and smaller) job if, unlike me, you bought some ferrites to wind the coils onto! Also note that disc ceramics are generally a poor choice in a high-current application like this. Luckily the huge ones I used from the junk-box seem to get barely warm @ over 1kW PEP. I got lucky. You may not — don’t be a skinflint, be advised that you should use silver micas. Yes, they are over a quid each. A fucking quid! Each! Jesus!

Output Filters

Schematics for the low pass filters are here

Amplifier working with carrier set to around 275-300W:

275-Watt carrier_2275W-carrier

300W carrier wholeAnd a view of the whole amp, running just under a 300 Watt carrier. (Dummy load, if you please!)

300W carrier whole

And the whole amp in standby, ready to rock ‘n’ roll!


It performs well, though the efficiency, especially at low drive levels where the thing mostly works in Class A, is low, as it is in all linear amplifiers. A quick measure up proved that harmonics are -40dBc or better, and IMD is on a par with modern ‘rice-box’ HF rigs: around -32dB for IM3 and IM5. Not brilliant, but probably limited by the exciter I use, an ICOM 756-PRO. Certainly a lot “cleaner” than many of the signals I see and hear on the bands on a daily basis. (Italians, Ukrainians, Russians: I’m not really looking at you!)

Ok, thats it for now, use the contact form on the website if you need to know anything else!