Talk:Circuit Idea/Parallel Connected Negative Resistor

Starting the discusiion
I would like to start the discussion about this exotic negative resistance application by inserting my comment to the EDN's article Negative-Resistance Load Canceller Helps Drive Heavy Loads. In this story I have revealed the secret behind negative resistance phenomenon and its most popular implementation - negative impedance converter (NIC). Circuit-fantasist (talk) 08:16, 26 December 2007 (UTC)

A reader's comment to a relative EDN's article
(reader's comment to the EDN's article Negative-Resistance Load Canceller Helps Drive Heavy Loads)

This exotic circuit solution seems to be original; actually, it is very, very old. For the first time, I saw it in the book "L’amplificateur opérationnel et ses applications Masson, Paris, 1971" by J. C. Marchais; then I was a student and was trying without success to grasp the idea behind the odd circuit). In spite of that, I have not yet seen any reasonable human friendly explanation of this clever circuit arrangement (including the book mentioned).

Believe me, it is not sufficient to say that "a negative resistance -R connected in parallel with a positive resistance R gives an infinite resistance". In order to really understand the circuit, readers (as human beings) need to know what problem the op-amp solves, why it is connected there, what it actually does in the circuit, how it does this magic, etc.

The idea is extremely simple; we may see it in the daily round when someone (something) helps someone (something) else to such extent that the latter needn't do anything. Examples of this "exact helping": parents help children to such extent that they needn't do anything, a husband supports his wife to such extent that she needn't do anything:), compensating the load by an anti-load (anti-weight) used in the lift systems and cranes, etc.

In the circuit discussed, the op-amp (or, more precisely, the whole NIC converter acting as a negative resistor) does exactly the same - it "helps exactly" the input reference voltage source by injecting the whole current needed through the load. In this way, it serves as an "exact helping" source, which has actually replaced the input source. As a result, the load does not consume any energy from the input source since it is supplied completely by the "helping" source. There is no current flowing between the output of the reference source and the load since the whole rest part of the circuit (the load + the NIC) behaves as a dynamic load with infinite internal resistance. Actually, this is the famous "bootstrapping" technique; it is put in practice for the first time by Baron Munchhausen who was using his own boot straps to pull himself out of the sea:)

In this circuit, the parallel connected negative resistor -R "neutralizes" the positive one R giving the whole load current. Then, in another implementation of this powerful idea - the famous Howland current source (and Deboo integrator as well) - the negative resistance absorbs a part of the positive one (the positive resistance dominates); it adds only a "correcting" supplementary current, in order to keep a constant current through the load. Finally, if the negative resistance dominates (it has completely absorbed the positive one and some negative resistance is left) the circuit will act as a negative resistance amplifier (in this case, the input voltage source has to have an output resistance).

Cyril Mechkov (cyril@circuit-fantasia.com) Circuit-fantasist (talk) 08:48, 26 December 2007 (UTC)

An extract from a related Wikipedia article
(after Wikipedia's Negative resistance)



...Compensating the load resistance. In nature, real sources (for example, human beings) have a limited power. Therefore, if they are loaded (for example, by a weight), they droop. A similar problem exists in electronics (electricity) when imperfect voltage sources are loaded. For example, the simplest varying voltage source (Fig. 7) consists of a steady voltage source VIN and a potentiometer P (a voltage divider r1-r2). If there is no load connected, this real voltage source works well - VOUT = r2/(r1 + r2). However, when a load RL is connected, it "sucks" a current IL and the output voltage VL drops. The classic solution is to connect an op-amp voltage follower (a unity-gain amplifier acting as a buffer amplifier) before the load, in order to decrease the current IL (to increase the load resistance RL). Unfortunately, this remedy introduces some errors inherent for this circuit. In mechanics, there is another powerful idea that is referred to as anti-weight or anti-load (Fig. 7). It is widely used in the lift systems, cranes etc. In electronics, this exotic solution is implemented by a voltage-driven negative resistor (a negative load); it is connected in parallel to the "positive" load (Fig. 7), in order to "help" the imperfect voltage source ,.



The compensating voltage source BH (a non-inverting amplifier with K = 2) produces two times higher "helping" voltage VH= 2VL. It makes a current IH = (VH - VL)/R = (2VL - VL)/RL = VL/RL = IL flow through the load. In this way, the whole load current IL is provided by the "helping" current source IH (the negative resistor -RL) instead by the real voltage source. The load does not consume any energy from the input source since it is supplied completely by the "helping" source. In order to put this circuit into practice, they use usually a negative impedance converter acting as a voltage-driven negative resistor,. There is a striking resemblance between the mechanical and electrical versions, which helps understanding the phenomenon. According to this intuitive viewpoint, the load "pulls" the point A down toward the ground while the resistor R "pulls" the point A up toward the voltage VH. As a result of this "stretching", the point A "experiences" weightlessness (as it pulls itself up) and it follows easily the point B. There is no current flowing between the point B and point A since the whole right part of the circuit (RL, R and VH) behaves as a load with infinite internal resistance. This phenomenon is referred to as bootstrapping and it is put in practice for the first time by Baron Munchhausen (the legend says that he was using his own boot straps to pull himself out of the sea)... Circuit-fantasist (talk) 09:02, 26 December 2007 (UTC)

Resources
Negative resistance A heuristic approach to teaching negative resistance phenomenon Understanding negative impedance converter (NIC) - reveals in three consecutive steps the basic idea behind NIC. How to Compensate Resistive Losses by a Parallel Connected Negative Resistor Negative-Resistance Load Canceller Helps Drive Heavy Loads - an interesting material from Electronic Design Negative Resistor Cancels Op Amp Load (AN 1868) - a similar material from MAXIM