RF Oscillator with Three Parts
The Simplest Oscillator Ever

If anyone can successfully disprove this claim, I'll gladly retract it
(Posted September 28, 2007)

     It's pretty well known that "negative resistance" can be used to generate oscillations in electronic circuits.

     What's not so well known is that some ordinary NPN transistors such as the 2N2222, 2N3904 and 2N4401 exhibit negative resistance when reversed biased.

     Note that the base of the transistor is not used; the transistor functions as a diode.

     Output is taken off the emitter. The resonant device is a three terminal 455 KHz ceramic filter typically found in consumer radios.  The input and output of the ceramic filter are tied together.  The filter's center terminal (normally grounded) is connected to the current limiting resistor.

     Oscillation is near 455 KHz, stable within a few Hz.  Oscillation frequency is dependent on the true resonant frequency of the ceramic filter.  Two of the filters I have oscillate at about 453 KHz, one at 455 KHz.

     Waveform is a mildly distorted sine wave.  Some transistors produce cleaner waveforms than others.  Amplitude and wave shape are dependent on input voltage.  The circuit above works best at 10.72 volts.

     A 5 K potentiometer in series with the current limiting resistor can help clean up the waveform.  Further improvement can be achieved by connecting pin 1 of an additional ceramic filter to the emitter, taking output from pin 3 and with the center terminal grounded.  Using a 2N3904 in this configuration, I observed a clean sine wave at 2 V p-p.

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