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# An answer to the Codidact question, “How to plot the I-V curve of a tunnel diode?”

### An answer to the Codidact question, “How to plot the I-V curve of a tunnel diode?”

I wrote this post yesterday to answer the Codidact question How to plot the I-V curve of a tunnel diode? Initially, I did not intend to write an answer but rather wanted to give some guidance to OP through comments. But it turned out this was not desirable there and was treated as a “discussion”… and our comments have been deleted.

My answer is short. My goal is to use it to explain, in the simplest and most convincing way possible, the behavior of the tunnel diode in the negative resistance region… what it does there…

Setup. We can best understand the behavior of the tunnel diode in the region with negative resistance if we imagine it as a self-variable (dynamic) resistor R driven by a variable voltage source V – Fig. 1. If OP has a subtle sense of humor, I suggest we conduct this experiment in the form of a fun (but useful) game where he is the voltage source and I am the “tunnel diode”:)

Graphical representation. The voltage VA across and the current IA through the two elements are the same. So their IV curves can be superimposed on the same coordinate system – Fig. 2. The IV curve of the variable resistor is a straight line (in orange) beginning from the coordinate origin and having a slope depending on the instant (static) resistance R. The IV curve of the input voltage source is a vertical line (in red) shifted to the right from the Y-axis. The intersection point A (aka operating point) represents the instant magnitudes of the current IA and the voltage VA.

Operation. Let’s now see what this “tunnel diode” (I) does when the voltage source (OP) varies its voltage:

1. Low positive resistance region. When OP gradually increases the input voltage from zero to the beginning of the negative resistance region, I keep a low constant resistance R… and OP (i.e., the voltage source) imagines it as a steep line (IV curve).

2. Negative resistance region. When reaching the negative resistance region, I decide to play a trick with OP and begin increasing R vigorously while OP gradually increases the input voltage. As a result, in Ohm’s law I = V/R, both V and R change in opposite directions and with different rates… so the current decreases and OP sees a negative resistance. In Fig. 2, my (R, tunnel diode) IV curve begins rotating clockwise; the operating point A moves down and pictures the negative resistance part of the tunnel diode N-shaped IV curve.

3. High positive resistance region. After the negative resistance region, OP continues increasing the input voltage. My resistor possesses high positive resistance and OP imagines it as a sloping IV curve.

Demystifying the Negative Differential Resistance Phenomenon (Circuit Idea wikibook)

My answer to the SE EE question, “Art of Electronics – Zener Diode Example” (the same explanation about the dynamic resistance but in the case of a Zener diode)negative differential resistancetunnel diode

1. tlfong01December 6, 2020 at 2:51 PMI understand what you mean by “The IV curve of the variable resistor is a straight line (in orange) beginning from the coordinate origin and having a slope depending on the instant (static) resistance R”. For newbies, I would say the following: “For a particular linear (static) resistor obeying Ohm’s Law, the relationship of current I, a dependent variable, depends on the independent variable V can be shown as a straight line originating from 0, 0 of the graph. For many resistors with different resistance (R = V/I) values, we see a bunch of such straight line starting from the V, I origin 0. 0.REPLYDELETE
2. Circuit fantasistDecember 6, 2020 at 3:09 PMThis comment has been removed by the author.REPLY
1. tlfong01December 6, 2020 at 3:15 PMHa, adding the moving potentiometer wiper bit makes thing much more clear. Perhaps you should add a picture of the pot, with a hand moving it, .. 🙂DELETE
3. tlfong01December 6, 2020 at 3:11 PMThis is very confusing: “The IV curve of the input voltage source is a vertical line (in red) shifted to the right from the Y-axis.” for the following reason: If I-V curve denotes the I depending on V relationship, then how come the vertical lines denote many voltage sources. Or perhaps I was misled in the very first beginning. Or perhaps I should interpret your description as “A variable resistor can be represented by a bunch of orange lines, different resistances with different slopes.” But then do you mean the red vertical line represent different voltages of the same voltage source? This new one-variable-resistor, one-variable-voltage-source seems to make more sense. But I still find it confusing. Perhaps another presentation using moving pictures by animated GIF or Flash/HTML5, showing a time sequence. Of course an oscilloscope display in slow motion is even better. Actually this is what we can do using your 50Hz sine signal to the tunnel diode and display by a scope. The only thing I wish to do is to show the events happening in the NDR region, …REPLYDELETE
4. Circuit fantasistDecember 6, 2020 at 3:12 PMLet’s try to say it even more simply to the newbie: “You should already know Ohm’s law, right? Then you know that the IV curve of a resistor is a straight line starting from the origin of the coordinate system and its slope depends on the resistance R. So, in our case, when you move the potentiometer wiper, its IV curve will rotate around the origin (clockwise when you increase the resistance and v.v.)REPLY
5. tlfong01December 6, 2020 at 3:16 PMAh, need to go now, see you later. Nice chatting with you. Cheers.REPLYDELETE
6. Circuit fantasistDecember 6, 2020 at 3:24 PMOK, I will explain it… IV curves of elements behaving as voltage stabilizers (voltage sources, diodes, circuits of voltage stabilizers, etc.) are measured by varying the current and measuring the voltage. So, in the case of a variable voltage source, its IV curve will move horizontally (translates) when its voltage varies. I have Flash movies where this is animated (e.g., https://www.circuit-fantasia.com/my-students/ske2004/classes/class1/v-to-i-old-page6-3.html). But how do I show it on a sheet of paper? Of course, as a family of IV curves… a bunch of red vertical lines… Look how I had drawn it in 2004 by the help of the Corel Draw…REPLY
7. Circuit fantasistDecember 6, 2020 at 3:58 PMHere is an interesting laboratory experiment from the early 90s, where the diode IV curve is measured and plotted on the screen by a computer-based curve tracer:

https://photos.app.goo.gl/qobqm7Fwp3fTEA4o7

Since it is created not by a “humble engineer”:) but, in addition, a teacher, inventor, thinker, dreamer, fantasist:)… it shows not only the final result – the diode IV curve, but also the way how it is obtained. The computer and its AD periphery serves as a variable voltage source whose IV curve (vertical line) is drawn on the screen. When I vary the voltage (by the arrows on the keybord), this line moves left or right and the operating point draws the diode IV curve on the screen. Isn’t it great? Moreover, it was made 30 years ago… How much -1s would I get in SE EE if I showed it there? I think at least a dozen:)REPLY
1. tlfong01December 7, 2020 at 1:40 PMWell, I would give 8/10 marks to this computer aided diode curve tracer experiment report if published 30 years ago in a BBS (Bulletin Board System). Actually it was in early 1990”s when I was working in a university research project on CIS (Clinical Info System), and I setup up in an IBM２８６ＰＣ　（ｓｏｒｒｙ，　ｆｕｎｎｙ　ｃｈａｒａｃｔｅｒｓ　ｓｙｓｔｅｍ　ｂｕｇ），　a BBS, using a BBS system ｐｒｏｇｒａｍ　called Remote BBS. At that time I had several phone lines and use 9k6 baud modem for communication and sharing software (linux 0.9 need 12+ 3,5″ 1.44MB floppies. In those years a 170MB hard disk was very expensive (HK\$1,700) . And in the 1980’s 40MB hard disk costed HK\$4,000.

But this afternoon I went to the local computer centre and bought a SamSung T7, 1TB SSD for only HK\$1,250.

So it is in this historical background that I think your curve tracer should got 8/10 marks.DELETE
2. tlfong01December 7, 2020 at 1:48 PMThe funny characters seem disappeared. So let me continue. So let us fast forward from 1990 to 2020 now. We still do diode curve tracing, but the hardware is completely different. Let me go though my suggested versions.

(1) Version 1 – Voltage source is a manual button adjustable in mV steps of a digital PSU, form 0V to 1V. Measuring meters are cheapy multi-meters.

(2)Ｖｅｒｓｉｏｎ　２　ｓｏｒｒｙ，　ｆｕｎｎｙ　ｃｈａｒｓ　ａｇａｉｎ．Ｎｅｅｄ　ｔｏ　ｔａｋｅ　ａ　ｂｒｅａｋ．Ｃｈｅｅｒｓ．　DELETE
3. tlfong01December 7, 2020 at 2:04 PMVersion 2 – Using programmable triangular wave signal generator as the voltage source to power the (tunnel) diode, …

Version 3 – Using AD5933 Impedance Converter – This is what I am exploring now. AD5933 can frequency sweep an impedance and return two integers, real and imaginary part of the impedance. In other words, we can measure the resistance and inductance of a (tunnel) diode.

I want to measure inductance at any particular frequency, becuse I want to know

(1) which frequency the tunnel diode would oscillator,

(2) which (external) inductance would force the tunnel diode to crawl down the slope after the peak, instead to jumping over. (According to the GE TD 1962 User Guide, these parameters need careful tests to find.)DELETE
8. Circuit fantasistDecember 6, 2020 at 5:07 PMThis comment has been removed by the author.REPLY
9. Circuit fantasistDecember 6, 2020 at 5:10 PMWell, once we started this topic, let’s move on to the other way of investigating elements – by varying the current and measuring the voltage. As I said above, it is more suitable for testing diodes. Here is a video of such an experiment from the distant past:

https://photos.app.goo.gl/Rng6drGK63s1BjhN8

Now the computer (Apple II manufactured in 80’s in Bulgaria under the name “Pravetz”) and its AD periphery serves as a variable current source whose IV curve (horizontal line) moves up and down when I vary the current by the arrows on the keybord.REPLY
1. tlfong01December 7, 2020 at 1:22 PMAh, this diode voltage Vd vs diode current Id of testing diode characteristics is interesting. I guess you can do this curve plotting in two ways: software and hardware.

(1) To do it in software we know the diode equation Id = f(Vd) , and so Vd = g(Id) where function g( ) is the inverse of f( ),

(2) To do it in hardware, we can use a variable current source, controlled by hitting one key, say “i” to increase current magnitude, and another key “d” to decrease current magnitude.

I agree this can be done using Apple II and associated hardware, current source, voltage and current meter.DELETE
10. Circuit fantasistDecember 7, 2020 at 9:03 PMTL Fong, I wonder why there are so few normal people like you and me and there are so many idiots on the web … I think that Q&A sites are one of the reasons for the idiocy of people because the normal form of communication between people is not the monologue (asking and answering questions) but rather the dialogue (discussion)… ie, what we both do here… However, let us not forget the fact that they are very popular and what we write there becomes available to many more people than here. So I suggest you exchange some of these valuable thoughts also there under my answer to your question. Olin should not react negatively to this because it is directly related to the topic and is not written under someone else’s answer.REPLY
11. Circuit fantasistDecember 7, 2020 at 9:21 PMFrom what you have written above, I understand that you have many years of experience in complex computer systems and that it is very important for you that the implementation is virtuoso. You pay special attention to the form and the means … you want them to be modern so that you get a product with a sales value. I, on the contrary, am interested in the basic idea in its most general form, which does not depend on the concrete implementation and therefore does not die … it is eternal and immortal. I guess this is a popular view of the world in the Eastern teachings and philosophies you mentioned to me …

So I want, when discussing, to distinguish the idea (1) from its implementation (2) and the measuring means for its investigation (3)…REPLY
12. Circuit fantasistDecember 7, 2020 at 9:54 PM… not to oppose them to each other. Also, I want to distinguish between the functional notion about the element (NDR) and the notion about the specific element (tunnel diode).The idea is the most important to me and least important to most people. The vast majority of people (including the “elite” of Q&A sites) think specifically … “they see the trees but do not see the forest” …

So, as you can see in my answer to your question in CD, actually I do not explain the behavior of the tunnel diode; I explain the behavior of the N-shaped negative differential resistor. That is why, I can replace it by a humble potentiometer (rheostat) and show how it operates. This is possible sinve there is a common idea between them…REPLY

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