Playing With Negative Resistance
About a month ago I got it into my head to begin playing with negative resistance and the Lambda diode. I first ran across the Lambda diode in the 1980s as a Technical Correspondence in the July 1974 issue of the IEE Transactions on Electron Devices and I’ve always been fascinated by the concept. If the amount of information available is any indication, I’m not the only one.
The most popular implementation seems to be the one described by Ramon Vargas Patron, shown on the left below. On the right is shown the original Lambda diode circuit for comparison. Variations change the particular transistors used but the operating voltage and biasing resistors typically remain the same.
As an experiment I put together a Lambda diode using a J310 and 2N3906 as the active elements. After measuring the I-V characteristics to confirm the negative resistor behavior I built an oscillator using the standard design and found that while it worked well on a breadboard, it didn’t oscillate when the Lambda diode circuit was moved to its own printed circuit board (PCB). Eventually I figured out that a bypass capacitor added across the 100K bias resistor restored the expected operation. The updated circuit is shown below.
The addition of the bypass capacitor makes sense if you compare the original Lambda diode circuit using N and P channel JFETs to the one using the PNP bipolar transistor. It appears to provide an RF short from the PNP transistor base to the JFET drain as is done in the original N-P JFET circuit.
So, playing with this has been interesting, but what is to be done with it? Since it’s behavior is similar to that of a tunnel diode, perhaps that’s the place to start. A good reference for this is the GE Tunnel Diode Manual. It not only discusses the theory of tunnel diodes, it shows a variety of circuits using them including amplifiers, oscillators, and logic elements. Perhaps the most intriguing use of a tunnel diode I’ve run across is in Michael Rainey’s tunnel diode receiver, where a tunnel diode oscillator is used as the pump for a varactor audio amplifier. There doesn’t seem to be any reason a Lambda diode couldn’t be substituted in any of these circuits.