2008-02-29
Negistor Applications
The "Negistor" is a favourite toy of the Free-Energy Freaks. (More charitably known as the Over-unity Experimenters) Unfortunately most over-unity experimenters don't seem to grasp the conceptual difference between power, voltage and current. Neither do many seem to understand that average power is an integral and most test equipment is not true RMS, and that even true RMS multimeters have bandwidth limitations. Build something that will indefinitely deliver more DC power than what it receives from all sources and I'll start taking you seriously...
Anyway, enough OU-bashing, the negistor is a common NPN transistor, biased with its emitter more positive than its collector and the base open-circuit. Most signal transistors break down at about 8 volts in this mode and in the avalanche region exhibit some negative resistance. Negative resistance is nothing magical of course, just a non-monotonic region in the device transfer function, but it lets us build somewhat exotic-looking oscillators.
By placing a tank across the negistor, oscillations up to about 1-2 MHz can be achieved. Even just placing a capacitance across the device will cause oscillations, the circuit functioning somewhat like a NE-2 Neon relaxation oscillator.
PY2OHH utilises this curious circuit as a BFO oscillator! This ingenious application inspired my interest in the circuit, especially after I realised it could produce pretty linear sawtooth ramps. I was hoping to use it as an minimal component external quench oscillator, but the output is very small in amplitude and a unijunction oscillator is better suited. With amplification it could be used as a simple sweep generator instead of a unijunction, NE555 or Op-Amp circuit.
The most immediate application I could think of is a simple sidetone oscillator for QRP rigs. Most QRP rigs are powered from 12 Volts, so the supply voltage is sufficient, and the resulting circuit has quite a low parts count. A piezo squeaker is used to actually make the noise, and just three other components are needed, four if you include a keyline diode and maybe five for a shaping capacitor to soften the edges.
You could add a 1k pot for a volume control if you wanted. You might like to pipe the sidetone out via the RX AF path instead of via a piezo. In which case you can just capacitive couple it into the AF amp input.
Anyway, it works. Its simple, if kinda exotic. Possibly the weirdest code practice oscillator you might like to build too.
Notes
- Try using the piezo capacitance alone for optimum 3-component minimalism.
- Base voltage affects its properties, one negistor oscillator can FM another... Siren applications?
- Some transistors work much better than others. I found an old BC109 that worked quite well.
- Can you make a regenerative receiver by putting a variable +ve resistance in the tank and tuning it for near-zero affective resistance?
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Attachments
| title | type | size |
|---|---|---|
| Negistor Circuits Source | application/postscript | 14.431 kbytes |