Negative Resistance Oscillator with Tunnel Diode

Posted on Oct 14, 2012

This circuit can be easily powered from a 1.5 volt battery. One characteristic of N type negative resistance devices is that they typically require a very low bias source resistance in order to keep the bias voltage stable within the negative resistance region. With too high of bias resistance, the voltage, as it enters the negative resistance region, will have a tendacy to suddenly jump past it. This is why a gap is seen in the curve (see photo). This homemade device, with its narrow negative resistance region, requires an even lower bias source resistance than a typical tunnel diode. A typical tunnel diode can be biased within the negative resistance region with a bias resistance of around 20 ohms. This device works best when the bias resistance is 1 ohm or less.

This device, like many others that I have been making, has a symmetrical curve in both the positive and negative direction and can work with the battery connected either way. Just for the sake of being consistant, I did most experimenting with the aluminum biased positive with respect to the zinc covered wire. The low (approx 250 millivolt) bias voltage for this device was produced across a .47 ohm resistor, connected to the battery through a 5 ohm pot. This means drawing 500 to 600 milliamps from a 1 1/2 volt battery in order to supply approx 35 ma to the device. A 5 ohm pot is not as common as a 50 k ohm pot but can be easily obtained at a surplus outlet. It might also be easy to improvise a 5 ohm pot from something like a pencil lead. It was easy to run the circuit from a single AA cell but of course a D cell is much more suitable when drawing this much current. An emitter follower circuit could be a much more efficient way to bias this device but I like to have this circuit completely void of any commercially made transistors or other active devices. I wanted to be absolutely sure, that this homemade device is indeed what is actually producing the oscillations. Below are schematics and waveform pictures of an LC oscillator and a relaxation oscillator. Adjustment of these circuits can be tricky but easily done if things are set up rite. I have had very little success in getting the circuit to oscillate while trying to...

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