Circuit Fantastist’s comments


Circuit fantasist commented on “Cause of Missing Negative Resistance Portion of V-I Curve”

6 hours ago

I have appended these comments to my answer to your question in Codidact


to make them available to more people. We must follow this strategy to promote circuit ideas…

Circuit fantasist
 commented on “Cause of Missing Negative Resistance Portion of V-I Curve”6 hours ago

Thus you have two possibilities to artificially zero the undesired resistance – by a non-inverting configuration or by an inverting one.

I prefer to use the second; the old setup from 90s is implemented this way. It is actually an inverting amplifier with buffered input and output. For your purposes, you have to connect the tunnel diode in the place of R1 and the ammeter (VOM or movement) in the place of R2. If you want to measure the current by a grounded ADC or a microcontroller port, then use the op-amp output voltage as a measure of the current.

The only problem is that it is negative (I = -Vout/R).

The second idea is amazing – instead to increase the input voltage with VR, we decide to add the compensating voltage to the input voltage.

This means to connect an additional voltage source in series to the diode so that its voltage adds to the input voltage according to KVL. For this purpose, its voltage must be negative in regards to ground (you can see for yourself if you travel along the loop).

We can add the compensating voltage in two ways: First, we can make the input voltage source to raise its voltage with the value of the voltage drop. We can do it by applying a series negative feedback to an op-amp and putting (hiding) the ammeter resistance inside the feedback loop. This idea is implemented in Olin’s and coquelicot‭’s answers. A disadvantage of this solution is that the ammeter (ADC) is floating. Also, you can want to test another device by current; then the device will be fliating.

To zero a resistance actually means to zero the voltage drop across it (V = I.R = 0). The natural way is to replace the existing resistance with a “piece of wire”; then really V = I.0 = 0. Only, we cannot do it in this way. However, since we are inventive enough, we decide to do it in an artificial way – by adding a voltage equal to the voltage drop in a series manner, according to KVL. As a result, the total voltage across this network will be zero… as though the resistance is zero. Wonderful, isn’t it?

What is this resistance? First, this is the internal resistance of the voltage source and next, this is the ammeter resistance (as you provably now, VOM measure the current by measuring the voltage drop across a small resistance). How do we do this magic? The “elite” would say, “It is very simple, just apply a negative feedback”. Yes, but we are both not satisfied with ready-made formal explanations and we want to understand the idea behind all this. Here is what it is …

Very useful resource… it is written in a human-friendly manner. Let’s see what is the most important to measure the tunnel diode IV curve. IMO this is not the scope. You have to see that when increasing the voltage across the diode, the current through it decreases. For this purpose, first at all, you have to remove (destroy, neutralize) any resistance in series. Here is my philosophy about how we can do it…

I think the best way to measure and plot IV curves is based on a computer. Apple II was very suitable for this purpose with its graphic features. The monitor software had a function “plot a point” that could be used by an additional program written in Assembler. In this way, in the late 80s, a few students and I created MICROLAB system. The AD periphery consisted of 12-bit DAC and ADC. Later, one of these capable students, created MICROLAB BASIC – an expansion to the embedded BASIC. It consisted of a set of assembler drivers called by commands like “IN”, “OUT”, “PLOT”, etc…

Very useful material. Fig. 7.1 is like my movie about the bi-stable mode of the tunnel diode (https://photos.app.goo.gl/ARfyuT1gzDDRw3Fb8).


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