Unusual Components and Applications

Posted on Feb 5, 2014

A common NPN transistor can be turned into a negistor by wiring it in reverse. The emitter is connected to the power source through a resistor and the collector is connected to ground. When wired this way the transistor exhibits a phenomenon known as negative resistance, also known as avalanching. In the diagram we see a capacitor across the transistor

Unusual Components and Applications
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charges until the reverse breakdown voltage is reached. At this point the collector to emitter resistance actually reaches a resistance of less than zero and then the capacitor is discharged through transistor. Once the capacitor is discharged the transistor returns to its resistive state and the capacitor starts to charge again. Not all transistors will operate in this mode, however, a surprisingly large number do. Even transistors that test bad may operate as negisters. Amazingly, the circuit is extremely stable and produces a linear ramp wave. It can feed a speaker directly or, an amplifier stage can be added to use the oscillator as a function generator. Its wide range and stability make it suitable for use in music synthesizers. The components used in the circuit are just a starting point, for different transistors will react differently and you may have needs that require other component values so experiment. It is advisable to use a regulated power source of at least15 volt. The avalanche mode can also be useful in the production of white noise. The illustration shows a transistor Q1 connected differently than the oscillator. Instead of the emitter lead, the base of the transistor is used and since there are no frequency-selecting components, a cascade of random frequencies is generated. Transistor Q2 amplifies the signal using resistor R2 as a current limiter, while R3 and capacitor C1 smooth response and increase...

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