Overall Summary: The FQPF50N06 N Channel Power Switching FET is a cost effective device (available from Farnell etc) in a convenient isolated TO-220 package. It has good voltage and current ratings required in a 12 Volt RF Power Amplifier application. For example, it may be possible to use it single ended to produce 20 Watts PEP at 50% efficiency, i.e. 40 Watts power supply drain for carrier output. This implies a current drain of 3.33 Amps, or possibly 1.5 amps average on voice transmit. This could be provided easily from a 12 Volt, 7 Amp-Hour sealed lead acid "gell cell".
Although the FQPF50N06 NMOS device is listed as a "switching transistor" this does not imply that it switches either on or off. The description just means that the device is designed to operate in switch-mode regulators. This application requires the same characteristics required in an RF power amplifier,
 | High peak drain-source voltage capability (e.g. VDS = 60V) for good VSWR ruggedness. |
| High peak drain current capability (e.g. ID = 31 Amps) since class AB operates at 50% duty cycle, peak drain current is >> than average drain current. |
| Low "on" resistance (e.g. RDS = 0.022 Ohms) assists optimum DC to RF power conversion efficiency (Only 110 mV lost from a 12 V supply at ID = 5 Amps!) |
| Low feedback capacitance (Crss = Cgd = 65 pF at Vgd = 25 V - see graph for Cgd at Vgd = 12 V) allows high power gain and can help stability. |
The NMOS device is still a "linear" component and is based on a standard (although technically advanced) MOSFET construction.
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