PCS-Electronics

For a 5-15 watt transmitter, the IRF510 is a pretty good choice.
Its Drain-source breakdown voltage is 100V, which gives a reasonable amount of headroom. On mod peaks the supply voltage will be 24V , assuming a 12V power source. The RF voltage on the FET will be something more than 2 times that, say peak voltage of 60-70 Volts.

The input capacitance of the IRF510 is quite low (Ciss ~= 150PF) which makes it easy(er) to drive than larger mosfets with higher gate capacitance.

Why not try to get more out of the IRF510? The problem is Rds - the on Resistance, which is 0.4 ohms for the IRF510. The output impedance seen by the FET is roughly V*V/2P where V is DC supply voltage and P = power out. If P = 10 and V=12 then the output impedance is about 12*12/2*10 = 7.2 Ohms If P = 20 and V=12 then the output impedance is about 12*12/2*20 = 3.6 Ohms

As the value of Rds gets larger relative to the output impedance the efficiency drops and you stat producing more heat and less RF.

At the small end of the scale, you could try MTP3055Es. They are tested at 7 MHz, but they are gate capacitance is about 3 times that of the IRF510, and their breakdown voltage is rated at 60 Watts. If you can drive them you may be able to get more out than an IRF510, assuming you adjust the output network...

73s de R_QRP

Using a series transistor as a class-A modulator works well in some respects eg excellent frequency response, but if you halve the standing voltage to the final to 6 volts and do nothing else, the output power will drop to a quarter. If you re-design the output match to produce the original power level, the load impedance the MOSFET sees will drop and efficiency will fall.

Personally I prefer non-RF MOSfets like IRF510 IRF540 etc because theyre cheap and easily available.

And for what its worth, here's some of my observations using switching type MOSFETS at RF.

The MOSFET gate basically looks like a capacitor. The simplest drive method is to form a paralell tuned cct with the gate capacitance and use this as the load of the driver stage. The gate-source breakdown voltage is about +/- 20 Volts which corresponds to about 14V RMS. About 10V RMS with no bias on the gate should give you good saturated switching.

However a simple tuned cct load from the driver probably wont give you quite this much, You can cheat a bit by adding some DC bias to the gate. The MOSFETS start to switch on at about 3.5 V so if you provide, say 2V DC bias you require a bit less AC signal to turn the FET on and off. Watch out for biasing the FET so it is permanantly conducting, as -
A: MOSFETs turn-on threshold voltage decrease with a temperature increase, so if there is a small quiescent current, the FET gets hot, and the threshold drops so it turns on more , getting hotter , turning on more, getting hotter etc. This can be resoved with tracking bias supplies, but theres no point beacuase :
B: The MOSFET will be close to operating linearly (Class B) at this point, and linear amps don't modulate cleanly.

A lot of collector /drain/plate modulated AM Transmitters don't modulate cleanly because they don't have enough drive at 100% positive modulation. This is easy to spot with an oscilliscope (esp with trapeziod pattern). Without an oscilliscope the best indication is that a averageing type power meter shows average power dropping at high modulation levels.

You can match a FET (or any transistor) running off 12 V to a 50ohm load with a simple 'L' network, but harmonic output will be REAL high.

73s de R_QRP

Care of your IRF-510 MOSFET's
MOSFETs are susceptible to electrostatic discharge damage (ESD). It is important to use proper grounding techniques while handling the amp circuit board and the MOSFETs in particular. While working with MOSFETs you should wear a grounding strap and have an antistatic mat at your feet. At the very least you should frequently ground your hands to the nearest ground point. The IRF510 is a good compromise MOSFET that will work up to 30 MHz but has poor thermal characteristics of 3.5°C/Watt. When used in intermittent SSB and CW service forced-air cooling is not required. Tuning time with full power should be limited to less than 30 seconds with 1-2 minutes between cycles to prevent. Operating at 24VDC does not press the MOSFET to their limits. The Sil-Pad TO220 mounting kit has excellent thermal performance and provides a greaseless thermal interface between the MOSFET and the heatsink. Users report that this amplifier can be safely used for contest CW operation without further cooling. For PSK/RTTY forced air cooling is necessary or a reduction of power by reducing the input drive.

73s de R_QRP