Completely homebrew cheap MOSFET amplifier 300W

[mw0uzo]

Got another amplifier project on the go to learn more about them, this time made completely from scratch.It uses 8 x IRF520N mosfets and works up to 20m and will deliver a significant amount of power. Its designed for 250W, but I have had over 400W out of it!!

View of completed module


Drilled heatsink, nicked from an old audio amplifier. And thermal pads.


Underside of amplifier


Start of development of amplifier controller


The amp has an RX/TX circuit on there so is usable at the moment with a suitable LPF for the band required. Developing a controller with ADCs etc for monitoring SWR and current and TX sequencing. Once tested and working with this smaller amp, it will be used to control the big EB104.

Output transformer is made from ferrites that were clipped to power leads in a dead switch mode audio amp.

It runs off any voltage up to 30V, 24V has been chosen for high power mobile use at 300W. At 13.8V, over 100W of power is available up to 20m.

Total cost very low, £50?

[mw0uzo]

PM me if interested in the circuit diagrams :)

[KA9MOT]

Very cool!

[X-Rated]

With so many transistors, are there issues with phasing them optimally?

[mw0uzo]

With so many transistors, are there issues with phasing them optimally?

Just make sure they're all from the same batch. I have not seen any signs of one device conducting harder than the rest.

For reliable operation up to 30V, use an output transformer secondary of 3 turns. For low voltage e.g. 14v, high power operation use 4 turns then you can get 200w+ out at 13.8V.

The voltage limit is 30V due to use of a 5V regulator (35V max) in the bias circuit.

[X-Rated]

Just make sure they're all from the same batch. I have not seen any signs of one device conducting harder than the rest.

For reliable operation up to 30V, use an output transformer secondary of 3 turns. For low voltage e.g. 14v, high power operation use 4 turns then you can get 200w+ out at 13.8V.

The voltage limit is 30V due to use of a 5V regulator (35V max) in the bias circuit.

Beyond matching the transistors from the same batch, I was concerned about matching the phase inputs to the transistors as well. I am not sure what the schematic looks like on this thing, but the phase delays that occur will naturally be more obvious at the higher frequencies.

[mw0uzo]

Beyond matching the transistors from the same batch, I was concerned about matching the phase inputs to the transistors as well. I am not sure what the schematic looks like on this thing, but the phase delays that occur will naturally be more obvious at the higher frequencies.

I don't know. It's not really usable above 18Mhz. I have been trying to increase the frequency response but not got further than that. Any tips would be most welcome!!

Is it Linear? If so, is it class E?

Its class AB, linearity seems to be ok - it transmits SSB nice and clean with no splatter :)

[ab1ga]

I don't know. It's not really usable above 18Mhz. I have been trying to increase the frequency response but not got further than that. Any tips would be most welcome!!

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.

73,

[mw0uzo]

Gain is in no way flat ... for 5w in, 300w out operation up to 20m I have had to include a switchable first order high-pass filter in the input attenuator. Not perfect, but it works :)

[NY3V]

Got another amplifier project on the go to learn more about them, this time made completely from scratch.It uses 8 x IRF520N mosfets and works up to 20m and will deliver a significant amount of power. Its designed for 250W, but I have had over 400W out of it!!

View of completed module


Drilled heatsink, nicked from an old audio amplifier. And thermal pads.

Underside of amplifier


Start of development of amplifier controller


The amp has an RX/TX circuit on there so is usable at the moment with a suitable LPF for the band required. Developing a controller with ADCs etc for monitoring SWR and current and TX sequencing. Once tested and working with this smaller amp, it will be used to control the big EB104.

Output transformer is made from ferrites that were clipped to power leads in a dead switch mode audio amp.

It runs off any voltage up to 30V, 24V has been chosen for high power mobile use at 300W. At 13.8V, over 100W of power is available up to 20m.

Total cost very low, £50?

Is it Linear? If so, is it class E?