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IRF840
Power MOSFET, see application drawing below; Transistor IRF840 N Channel 500 Volt 8 Amps

$1.95

cat#  IFR840

In the prototype an IRF840 is used in the final. Most of the power FET are designed for high voltage operation. At lower operating voltages they can saturate , limiting the output power. I had given 120 V for IRF840 it takes 1 Amp at peak. Gate voltage is fixed at 1V. Heavy head sink is essential for IRF. My heat sink measures 30 cm * 6.5 cm. Use mica insulator and heat sink compound for fixing IRF.   RF Power Amplifier using IRF840 used in 7MHz SSB Transceiver.
Two numbers of 15 Volt zener diodes are used to keep gate voltage swing below 20 Volts.  A zener diode rated slightly higher than the twice the supply voltage connected across drain and source can prevent drain source breakdown. Peak to peak gate voltage of magnitude more than 20 Volts can destroy the FET instantaneously. Bifalar transformaer T1 is wound with 8 turns 26SWG on 1.4 x 1 balun core. The coil on the drain of IRF-840 is 3 turns 20 SWG wound on 4 number of T13,.9 torroids (two torroids are stacked to form a binacular core). The RFC at the Vcc line is 20 Turns 20 SWG wound on T20 torroid.

Drawing below is broadband RF amplifier for 40 meter QRP transceivers

 


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