Saturday, March 19, 2011

Broadband Linear Power amplifier

A broadband output transformer in lieu of the parallel tuned circuit is also worth a try. The 10-ohm emitter resistor, capacitor C1 and the 10 uH choke all disappear if FETS are employed. This design probably represents an all-time low in parts count to achieve 1 watt of linear power output, with only 8 components. Note, however, that the 2N3906's actually produce 50% more output for very little added complexity. We could stop at this point, connecting our 1-watt powerhouse to an antenna (via a low pass filter, of course!), or use this circuit to drive an a RF power transistor of more substantial proportions, as shown below. Depending on the frequency, we can achieve 4 to 6 watts output with 0,5 to 0,75 amps current consumption from our 13.8 VDC supply, using a Japanese bipolar device (2SC2078) intended for CB radio and similar applications.

Fig. 2: MOSFET's and RF power transistor

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