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

53 points by zdw

by adrian_b

2 subcomments

> 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...

by marshray

0 subcomment

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.

by ThePhysicist

0 subcomment

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!

by briomd

1 subcomments

> The book is meant for the inquiring hobbyist, including those who have tried to learn the craft and hit a wall before. With 420+ pages and 290+ meticulously-crafted color illustrations, The Secret Life of Circuits prioritizes modern problem-solving and true intuition over plumbing analogies or cryptic formulas.
IMHO this is a wasted opportunity to write a solid book on the subject. The poking at existing literature and offering a “true” intuition is tiresome at best.