Negative Resistance – Quick Rescript

In electronics, negative resistance is an oddball property which occurs in a few nonlinear components. Just think that an increase in voltage across a device’s terminals results in a decrease in electric current through it. Even though this contrasts with a common resistor in which an increase of applied voltage causes a proportional increase in current (positive resistance), under certain conditions it can increase the power of an electrical signal, amplifying it.

This may not be an interesting topic for today’s electronics students, makers, and hobbyists, as they are unlikely to get negative resistance components outside the realm of a surplus parts store. However, knowing something of how components work in a particular circuit should be part of the learning process of anyone with a keen interest in electronics, and is thus worth taking a moment to look at. All things are strange which are worth knowing!

Negative Resistance Devices

The best example of an electronic component with negative differential resistance is the Tunnel Diode. Another great piece that exploits negative resistance is the Gunn Diode. The list also includes some unijunction transistors (UJTs), NPN bipolar transistors (NPN BJTs), thyristors, etc.

Note that the term “negative resistance” almost always signifies negative differential resistance. Negative resistance can be either static or differential. Both are measured in Ohms, and of course, they conform to Ohm’s law. Static resistance is another term for the ordinary resistance that conforms to Ohm’s law. Differential resistance, also known as dynamic resistance, is the derivative of applied voltage with respect to measured current. Differential negative resistance happens when the electrical energy is in the form of alternating current.

Tunnel Diode: As said above, Tunnel Diode also known as Esaki diode, after its inventor, is probably the best-known device that exhibits negative resistance. The tunnel diode is a type of semiconductor that is capable of very fast operation, well into the microwave frequency region, made possible by the use of the quantum mechanical effect called tunneling.

This is a useful post describing Tunnel Diodes

Also see the 1N3716 Tunnel Diode Datasheet

Gunn Diode: The Gunn diode, also known as transferred electron device (TED), is a semiconductor device with negative resistance that converts the energy of a constant-voltage power source into the energy of high-frequency oscillations due to the formation of a strong field region. The internal construction of a Gunn Diode is other diodes in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. It therefore does not conduct in only one direction and cannot rectify alternating current like other diodes, which is why some sources prefer the term TED.

The Gunn diode forms an easy method of generating microwave signals anywhere from around 1GHz up to frequencies of possibly 100GHz using a single diode element. Because of its high frequency capability, Gunn diode is mainly used at microwave frequencies and above. A Gunn Diode can produce some of the highest output power of any semiconductor devices at these frequencies. Although the most common use is in oscillator circuits, it is used in microwave amplifiers to amplify signals as well. Go through this link to see what Wikipedia says

This is the datasheet of a 40GHz Gunn Diode -40GHz.pdf

You may not have enough opportunities to play with these odd diodes today, so I am not going into project ideas based on them now. You can find several “negative resistance” oscillator circuits across the web to test yourself. See below links:

Negistor Oscillator

The concept of negative resistance is interesting and worth doing some experiments on negative resistance devices. Sadly, this does not seem to be very practical in the absence of requisite components that are readily available.

But do not worry! In this session you will find a little experiment that anyone can perform with a handful of cheap electronic components. This quick experiment will demonstrate you how a very simple oscillator can be built with a component which shows a negative differential resistance effect if used it properly.

As you may know, some common transistors when wired into a circuit in reverse have a negative resistance like Tunnel Diodes. If you already have a few of them on hand, probably then you can start your Negistor experiments.

Put, when a regular NPN transistor is used in a way that its base is not connected at all, and collector-emitter junction is wired in reverse direction, we can call it as a Negistor (most signal transistors break down at about 8 volts in this mode and in the avalanche region exhibit some negative resistance).

Okay, so much for the intro. Let’s reach into the drawer for an appropriate transistor. There are several types of bipolar junction transistors among which Negistors can be found. The most easy and quick pick will be the 2N2222 transistor. I also found a randomly picked 2N4401 and the obsolete BC107 transistors worked quite well. But not all transistors have this behaviour, so, choose a trial-and-error method that works for you.

The most quick and easy play I could think of is a basic Negistor Oscillator setup as shown below.

When powered by regulated 12VDC, my breadboarded negative differential resistance (NDR) oscillator – the Negistor oscillator prototype – starts to oscillate gently at about 35kHz and remains stable. Beautiful!

Below you could catch a glimpse of the hurried breadboard setup:


So, that wraps up this quick revision about negative resistance/negative differential resistance. I admit, this post has not delved deeply into theory or practice, but rather is intended to be a quick rescript just to wake you up. Be sure to let me know in the comments if you have a shareable experience with Tunnel diodes & Gunn diodes or devices alike, especially if you can get a budget Gunn diode to go from any online storefront. I want to see one of these things in action!

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