Final: IRF840

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In my prototype I used IRF840 in the final. Most of the power FET are designed for high voltage operation. At lower operating voltages they saturates quickly 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.

You can directly replace IRF840 with many of the power FET like IRF830, IRF530, IRF540 etc... When using a different IRF, supply voltage should be changed to less than half the maximum drain voltage (Vds). 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. Two numbers of 15 Volt zener diodes are used to keep gate voltage swing below 20 Volts. Specifications for some of IRF series are given below.

IRF 530   75 W    100 V  14 Amps
IRF 540   125 W   100 V  27 Amps
IRF 830   75W     500 V  4.5 Amps
IRF 840   125 W   500 V  8 Amps

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