Saturday, February 27, 2010

a low-cost MOSTFET CW RF power amplifier, using an IRF710 power mosfet

Someone gave me a sleeve of IRF710's, so, this week i decided to try
to build an RF power amplifier out of one of them. These transistors
were never intended for RF applications, they were built for motor
control, solenoid control, and other high power switching applications.

Making this amplifier was remarkably easy. Without impedance
matching or feedback neutralization i was able to make a 7 watt out
(200 mW in) RF power amplifier at 7 MHz. The amplifier was likely
operating in class C mode. Perfect for QRP operations!

I should be able to achieve much higher output power and greater
efficiency when i properly match the input and output. Many articles
have been written on this topic in QST, this is nothing new. Some
authors have written that they have achieved power added efficiencies
upwards of 75% at high frequencies (in the Mhz range) operating in
class D mode.

From this result i am very encouraged, and i am considering building
a CW transmitter using one of these and pushing it as hard as i