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JFET Design Spotlight: Negative Resistance in a Semiconductor

4 days ago
2 min read

JFETs are commonly used in amplifiers, current sources, variable resistors, and oscillators, but sometimes the most interesting designs come from using them in ways you wouldn’t normally expect.


This week’s Design Spotlight explores one of those “odd uses”: creating a negative resistance effect with complementary N-channel and P-channel JFETs.

 

“Use of JFETs in typical applications range from amplifiers to variable resistors and oscillators, among many other applications, but then there are some odd uses to consider.

 

1902 was the year that a high-powered dynamic method of exciting a resonant circuit for continuous Radio Frequency generation was proposed by Valdemar Poulsen. It consisted of a circuit with a Carbon/copper poled arc within a Hydrogen or Methane gas environment cross to an electromagnetic field in a series connection. This created a critically managed Negative resistance that could resonate an L/C tank continuously. Noted in Moorcroft Principles of Radio 1921, some crystal diodes at the time exhibited negative resistance but were deemed unreliable for any practical use. Long later, an electron tunneling diode effect was invented by Leo Esaki in 1957, having useful negative resistance properties for high frequency use.

 

Negative resistance happens when an increase in current through a two-terminal device results in decreased voltage, which brings us to a curious Negative resistance effect that can be made with N and P channel JFETs. JFETs are known to be normally on and proceed to cutoff with a channel depletion voltage applied to the gate. If one were to place a P-channel and N-channel JFET part in series, drains common, and connect the gates to each other’s source voltage polarities, channel conductance starts at 0 volts and increases with applied voltage. At some point, however, conductance with increasing voltage will peak and begin to decrease thereafter as the gates deplete both JFET channels. The combined cutoff voltage of both FETs with continuing applied voltage reduces channel conductance to zero. This negative resistance slope can then be used as an active gain function for the purpose of generating oscillations with a two-terminal connection or even as an amplifier for voltage. This JFET pair has a Negative resistance midpoint of ~4 volts and is ~2Kohms in use as an active gain element. Not that there is a great use for such a circuit, so it’s simply presented here as the oddity that it is to think about. A practical use: I have a couple of these on the bench to just randomly test L-Cs for resonant frequency. Conveniently, the NJ and PJ pair does not reverse to a positive conductance after the negative slope ends, like a Tunnel Diode does, as shown in the DC sweep.

 

Fig 1) DC V/I profile for the example JFETs

Fig 2) Negative resistance amplifier

Fig 3) Neg Res Amplifier Frequency response

Fig 4) Negative resistance oscillator


DC V/I profile for the example JFETs

 

Negative resistance amplifier


Neg Res Amplifier Frequency response


Negative resistance oscillator


As Usual …. Kirkwood Rough”

 

Sometimes an unusual device characteristic opens the door to an entirely different circuit approach.

 

Linear Systems offers a broad range of N-channel and P-channel JFETs for precision analog applications and design experimentation. Interested in evaluating a device for your next design? Contact us for FREE engineering samples.

 

 

 

 
 
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