RADIO 2 Watt 433 MHz Output stage using diode biasing with only 400 mV input

This seems better than the resistor based biasing. Although it produces the same gain, the quality of the sine wave is better than is that of the other biasing network. Additionally, the diodes provide protection from excessive input voltage. This may be due to losses that are inherent with a design that uses low value resistors to ground. The diodes set the DC bias point of the base in this to just above a volt; this produces excellent results with a 300 mV input. As the diodes are facing down, the variable capacitance that you can get from diodes positioned at the base of the transistor isn't a problem (Large 1n4001 type diodes are often used as vaicaps in oscillators to vary the frequency and for FM modulation.) Using the diode volt drop like this is an idea I got from Paul Harden and other HAM RADIO folk. The result is a class C style amplifier stage that is very usable.

published 10 years ago