This article is about the general subject of ‘negative resistance’ and about a particular incarnation called the ‘Lambda Diode’.
Contrary to what you might be thinking, ‘negative resistance’ isn’t some crackpot, physics-defying, perpetual motion scheme. It’s a very real characteristic of some solid state devices and circuits that have an operating region where current actually decreases as voltage increases. This is, of course, the direct opposite of how common linear devices, such as resistors, behave. My first practical encounter with circuits that exhibit negative resistance was about 25 years ago when I read about a circuit called the ‘lambda diode.
The lambda diode can be implemented in several configurations, but the easiest one for weekend experimenters to build looks like this, drawn in LTSpice:
If the voltage source, V1, is increased from 0V to about 20V, the current response looks like this, plotted by LTSpice:
The increasing voltage is dimensioned on the X-axis and the resultant current draw of the circuit is dimensioned on the y-axis. Notice that, in the region from about 2.5V up to over 14V, the current is actually decreasing as the voltage increases. If we pick any 2 points (V2 and V1) in this strange region and look at the corresponding currents (I1 and I1), then the differential resistance is (V2-V1)/(I2-I1). If you plug in some numbers, you’ll come up with a negative result!
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