Completely homebrew cheap MOSFET amplifier 300W

[X-Rated]

I don't know what the inductance is on the primary winding, but it seems like there is an attenuator on the input with the pi circuit of resistors. Maybe that attenuation could be decreased if the amplitude drops on the lower bands with the 200pF taken out, but it should not roll off at the higher freq bands. If you need the cap in there, it might be best to switch out the cap for the higher bands and use matching caps to tune to the proper band between the inductance of the primary and the reactance of the cap. Ideally, the XL and XC will match and that is impossible between 1.8MHz and 30MHz.

[mw0uzo]

Remarkably simple device, Dan. Do you think it's a possibility that the phasing issues Jerry mentioned could be what's limiting it to below 18MHz?

I don't think so, I would have thought any troublesome phase shifting would have occurred at higher frequency. But that's just a hunch, some hidden inductance or capacitance could cause it. (hmm quite likely!!!)

I don't know what the inductance is on the primary winding, but it seems like there is an attenuator on the input with the pi circuit of resistors. Maybe that attenuation could be decreased if the amplitude drops on the lower bands with the 200pF taken out, but it should not roll off at the higher freq bands. If you need the cap in there, it might be best to switch out the cap for the higher bands and use matching caps to tune to the proper band between the inductance of the primary and the reactance of the cap. Ideally, the XL and XC will match and that is impossible between 1.8MHz and 30MHz.

Yeah the gain of the amplifier is a lot at low frequency, plus the input SWR is all over the place, so attenuator is necessary to reduce gain and provide decent input SWR.

The cap was put in to increase gain at 14Mhz, as it was rolling off quickly. With the arrangement shown, the output power is fairly level across 160-20m, with the switch for 20m.

The IRF MOSFETS were intended for use in switching applications like motor control or power supplies, where operating frequencies are low. Also, it was important to keep the channel resistance low when on to minimize waste heat, which limits lifetime and requires expensive heat sinking or a larger package design.

The high-current capability required a fairly wide channel, even for VMOS, TMOS, etc. designs. Put a metal gate across that, and you have a nice capacitor. That capacitance forms an RC circuit with any gate series resistance which limits frequency response. A related parameter, gate charge, provides a useful guide. The bigger it is, the harder it is to move it back and forth to vary channel conductance. The larger the gate charge, the higher the gate current (i.e., drive) required to modulate the channel conductance.

Yes. Reason for choice is low cost. IRF520N's were the cheapest, lowest input capacitance devices I could find, before getting to the point of headbutting the wall in frustration.

There is clearly a lot of capacitance somewhere in my design as the gain rolls off very early and more drive is required. It is probably what we expect, the mosfets, and tucked away in the bias circuit is a 33k/2n2 RC network which rolls off at 2.2Mhz. I did reduce R significantly, but then encountered some stability problems and lost a mosfet, so it was returned to its previous value.

I also experimented with the gate resistance, removing them gave a good rise in gain. Unfortunately, I also experimented with the output transformer ratio at the same time... and lost some more mosfets, and because of this silliness could not isolate the cause of failure. I now know the failure to have been down to the output transformer ratio and excessive current peaks through the mosfets (I was trying to get as much power as I could at 14V).