Tech note: making your own V-I plots at home

https://lcamtuf.substack.com/p/tech-note-making-your-own-v-i-plots

by zdw • 5 days ago

85 8 5 days ago

lcamtuf.substack.com

adrian_b2• 4 days ago

And yep, to preempt the inevitable comment: we used to have even more specialized devices, called "curve tracers", that were designed specifically for making V-I plots. They're more or less extinct now because SMUs can do the same job.

The ancient curve tracers, like the widely used Tektronix 576 or 577, could do things for which you would need much more expensive SMUs than that shown in TFA.

For example they could go up to voltages like 1500 V or 1600 V, to see the breakdowns of power transistors or diodes and they could apply very high powers during short pulses, e.g. up to 1000 W with the high current fixture, to see the V/I characteristics up to higher currents, like 200 A.

In general the most interesting parts of the V/I characteristics are towards higher voltages, to see the breakdown behavior, or towards higher currents, to see things like saturation voltages for bipolar transistors or minimum resistances for FETs and to see how the gain drops at higher currents.

A movie showing the use of a curve tracer:

https://youtu.be/bXbGktOHXzs

Nice pictures with the same:

https://www.pa4tim.nl/meetapparatuur/tektronix-576-de-koning...

selimthegrim• 4 days ago

Could they have handled the avalanche transistor posted here a few days ago?

adrian_b2• 4 days ago

Easily.

With a curve tracer you could measure the breakdown voltage of the transistor when connected with the emitter and the collector reversed and with open base, which is close to the breakdown voltage of the emitter-base junction.

Knowing this voltage, one can compute accurately the oscillation frequency of the relaxation oscillator from that article.

However, because that breakdown voltage is low, possibly under 10 V, it could also be measured with the setup from the parent article.

In all measurements of breakdown voltages, one must take care to configure the programmable voltage source for a low current limit, of at most a few mA, and also one must apply the breakdown voltage for only a short time, e.g. a few milliseconds at most, to avoid that the device under test overheats or even enters thermal breakdown, which results in irreversible damage.

Besides avoiding the destruction of the device under test, measuring an avalanche transistor needs an additional precaution.

While the breakdown of diodes and of the emitter-base or collector-base junctions of transistors are easy, because they happen at a fixed voltage, the measurement of the breakdown voltage of a bipolar transistor with open base are a little tricky, because they behave in a similar way with thyristors (SCRs), i.e. during breakdown there is a positive reaction which causes the voltage on the transistor to collapse. Thus there are 2 important values for the breakdown voltage, a maximum voltage and a minimum voltage.

These are easy to determine when you measure manually with a curve tracer, because you rotate the voltage button until the maximum breakdown voltage is reached, and then after the voltage drops you see on the screen the minimum breakdown voltage.

This is a little more difficult to measure when you must write a program to perform the measurement. The easiest is when the programmable source can be used as a current source with high resolution at low currents, when you set a voltage limit high enough and you increase linearly in time the current, from microamperes to at most a few mA, while measuring the voltage on the device. In this way you can measure the complete V/I characteristic, because the voltage is a function of the current, even if the current is not a function of the voltage, because at the same voltage up to 3 different currents are possible (the I/V curve has an S form).

If the programmable source does not have good resolution at small currents, than you can still measure, by raising slowly the voltage and trying to record the maximum reached value, then measuring the voltage that remains stable at a relatively high breakdown current.

skippyfish• 4 days ago

The ancient curve tracers, like the widely used Tektronix 576 or 577, could do things for which you would need much more expensive SMUs than that shown in TFA.

If I'm searching right, Tektronix 576 had an MSRP of $18,000 back in 1970, or $150k in today's dollars. They were very, very expensive.

Of course you can now find them on eBay for much less, but you're buying an ancient device that's living on borrowed time, that's going to take up an unreasonable amount of space in any home lab, and that you will be hauling to the dump because it won't even be worth the shipping cost in another decade or so.

fvwqcecvq• 4 days ago

$150k in today's dollars. They were very, very expensive.

It's very expensive for a hobbyist, but not very very expensive for a professional lab, which I think would be the target market for something like a Tektronix 576 in 1970. It's basically equivalent to buying a mid-tier 10GHz scope today Which is a normal piece of lab equipment in 2026 for a professional lab. It's not even uncommon to see them in University labs.

If a hobbyist wants an I-V curve you generate a 1Hz triangle wave of current with an dual opamp and use X-Y mode on your scope. More than good enough for the overwhelming majority of hobby applications. Unless you're writing a textbook and want fancy plots, then fine. Use an SMU. :)

ThePhysicist• 4 days ago

Takes me back to when I was working on Josephson junctions (JJ) as a student, I-V curves were the first thing we did when characterizing a junction. JJ I-V curves were tricky due to the hysteresis they have in the underdamped regime, so we had some pretty involved stepping logic to ensure we get the curve with the required precision.

That said today everything is pretty much digital, you have Acqiris/Agilent 1 GHz ADCs and all the measurements are done in software, but I still remember using my old 20 MHz HMAG oscilloscope in XY mode with a triangle voltage generator to plot IV curves in real-time. Good old times!

marshray• 4 days ago

Nice article.

Looking at the ONSemi datasheet for the BS170, it is very clear that the "Drain−Source Breakdown Voltage" spec only applies at "VGS = 0, ID = 100 uA DC".

Doesn't it cause problems for designers if the curves measured from actual devices look different from those shown in textbooks?

Not as much as you'd think, because (a) the measured curves are still within the stated parameters, and (b) many basic parameters vary from one part to the next (or as the same part drifts over time) even more.

Look at the values of V_GS used in the graphs: [2.34, 2.5, 2.63, 2.71 V]. They differ by 0.4 V. But the data sheet specifies V_GS(th) (the V_GS threshold needed to just start to turn on) as a range of 0.8 to 3.0 V!

A range isn't even given for the slope of the curve (g_fs). The datasheet only gives a "typical" value at a single operating point.

So the designer cannot rely on this parameter to determine the behavior of the circuit, only its general range and direction. You can't just use the bare transistor alone and expect to get an oscillator or amplifier that behaves predictably.

Any manufacturable circuit must ensure that these variations are completely dominated by negative feedback. This is why actual circuits tend to have networks of resistors and capacitors hanging off of every leg of every transistor.

roger_• 4 days ago

I’m familiar with curve tracers but hadn’t heard about source measure units.

Don’t see any on eBay for dirt cheap, wonder what author paid (he said a fraction of retail).

Tech note: making your own V-I plots at home

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