#232: More Circuit Fun: Simple transistor curve tracer using Stairstep generator circuit

in my previous video we took a look at this unique stair-step generator that used a 555 timer and a pair of op amps to create a stair-step voltage waveform and one of the more common questions i got is what would you use this for and there's a number of different applications and one that kind of came to mind by myself and a couple of the viewers was to use this for the basis of a simple curve tracer so so that's what we did so there's a quick example of what that looks like but before we get into that let's take a look at what I added to this circuit to make it a curve tracer so for a very basic transistor curve tracer I wanted to be able to replicate generating this family of curves typically known as the collector curves for a common emitter connected transistor and what it really represents is the collector current as a function of collector emitter voltage and a function of base current so what this is really showing is that for a given base current at a fixed base current as we ramp the collector voltage up what happens to the collector current and what we'll see is we kind of come out of saturation when VCE is very low and then we reach a nearly constant current for the rest of the collector emitter voltage typically though at the higher base currents you're going to start to see a tilt in this waveform as you go up due to an effect called the early voltage but it's just this set of curves that we want to duplicate with a very simple rudimentary curve tracer okay so in order to do this we're going to have each step in the stair step represent or create a different base current for the transistor so that's pretty easy to do and then also during each of those steps during the duration of that step we want to sweep the collector emitter voltage so what we need to do is create a ramp voltage that will essentially drive the collector emitter voltage that ramps up during the stair step and then resets at each step and begins over again so that's those are the two things that we need to add to our stair step generator in order to turn it into a curve tracer now the approach here is pretty minimalist so the responses are not going to be perfect but they're certainly adequate to get a decent response the first thing we'll do is just just take this stair step voltage and apply it to the base of the transistor under test through a resistor it will make this resistor relatively large so it kind of looks a bit more like a current source so we're essentially going to be stepping base current by simply feeding the base through a resistor now to create our collector voltage ramp we're simply going to charge up a capacitor through a resistor from the from the VCC supply here and then using the pulses coming from the 555 timer will periodically discharge that capacitor and let it reset again so what's going to happen now is each time we get that pulse from the 555 timer that increases the step at the same time we're going to turn this transistor on which brings this collector voltage back to zero as soon as that pulse goes away and then the stair-step is steady we've established a new bias current for the transistor and then we're going to ramp up this voltage on this capacitor and we're going to feed that then into a simple non-inverting op-amp gain stage to get the voltage swing that we want to drive the collector voltage here and to give me a nice low impedance drive rather than try to drive it right off that cap ok so let's look at how we've added these components to our stair-step generator I've got them drawn out here on this portion of the schematic and we can simply overlay this on top of our existing stair-step generator and now you can kind of see how we've turned this thing into our curve tracer here's the resistor that's used to drive the base of the transistor under test essentially establish the various steps of base current that we're going to be testing against using the stair-step output the collector voltage ramp is generated by charging up this capacitor through this resistor here and then that's being reset by the pulses coming from the 555 timer using this transistor here and then the voltage here is being buffered and amplified by this simple non non-inverting op-amp amplifier so that's essentially applied ultimately to our collector now there's one more little bit of circuitry I didn't mention before and that's this structure right here and this is really nothing more than a current mirror now the reason we're using this is that the output that we want to see is a collector current kind of on a vertical scale the scope naturally measures voltage with respect to ground so even if we just stuck a resistor from the collector to a positive supply and proved that as the current goes up that voltage would come down and that's not the way we want to represent it we want positive current to be represented by a more positive excursion on the plot so this current mirror structure takes whatever collector current we've got flowing here and mirrors that current flowing down here okay just by time use transistors together in this way so as this collector current increases this current being pushed down here increases therefore the voltage on this resistor will increase as the collector current increases so it just mirrored that current down here so that we give us ourselves a point that we can measure with a voltage probe that gives us a positive response with respect to positive current and then essentially the two voltages that we measure here and here become our X&Y because we're ramping the voltage that appears here this is just a diode drop down below you know the our ramp voltage here so that's essentially the voltage being applied to the collector and then the current is being represented by the voltage that appears across this resistor of course you know understanding how this part of the circuit is working where we've got the stair-step voltage being applied here and that creates a different base currents that's pretty simple to understand let's take a look at how this circuit is working here and what I've got on the scope here is the yellow trace is plotting the voltage that appears at the capacitor here actually it's going after the amplifier but it's essentially the same thing and then also the cyan trace channel 2 is probing the output from the 555 timer and we can see that the 555 timer out but when it goes high it turns on this transistor and it Yanks all the charge off of this capacitor thereby dropping that voltage all the way down to ground and then as soon as that pulse finishes this transistor turns off this capacitor is now allowed to charge up now of course the Wii doesn't get fully charged all the way to VCC before the next pulse comes along and because we're kind of cutting that charge cycle off early we get something somewhat linear in terms of that charge because this would continue up in an exponential fashion ultimately but we're just terminating that before it kind of finishes so this gives us our VCE ramp that we're applying to the collector of our test transistor okay so now we can take a look at these two guys here plotted on channel 1 and channel 2 look at the voltage being applied to the collector and the resulting collector current and each step will be at different steps of base current that's what we're looking at here so this is the voltage that's being applied to the collector and then so that's ramping up and then resetting ramping up again resetting and after each of these steps the base current is being slightly increased so the collector current that's flowing is being increased and increased increased with each step and we can kind of zoom all the way through this and see how that process increases all the way up now of course to turn this into a curve tracer we just need to change the display mode to be XY so that will use the collector emitter voltage as our X input and the collector current or this voltage that represents collector current to be Y so we'll just go into our XY mode here turn that on and now we've got our curve tracer response I'll just change our scale here so that we get a more familiar family of curves with our test transistor in place now as I mentioned the response is not going to be ideal because we're doing this in a very minimalist fashion with a minimum number of components the voltage is being applied to the collector is you know being altered a little bit by our current mirror here but not a dramatic amount that's just going to make a change to the shape of the curves right at the very beginning here the curve tracer but but overall gives us a basic shape if you want to use this to try to match some transistors you certainly could do that just as an example I've got a transistor in here now who's got a beta of about 300 350 or so now we've got to get a good idea of what those curves look like for that transistor we yank that transistor out of our little socket here got another NPN transistor here who's got a gain of about half of that about 150 and we can see that the same set of drive levels give us much less collector current than the previous transistor that had in there so these are different transistor part numbers so they're going to be very different but you could use this to take transistors of a similar part number and try to match up the curve for multiple devices if you had circuits that relied on the transistors being matched I hope you enjoyed this look at not only our our simple analog stair-step voltage generator but how to add just a couple of components to this and turn it into a reasonably good bipolar transistor curve tracer now of course you know I wouldn't if I was going to set out to build a curve tracer I probably wouldn't start with this but it's a very simple circuit to go build for you know kind of this dedicated purpose if you want to do trace out like like n-channel MOSFETs here I'd probably lower this resistor value so that we could charge up the gate voltage a little bit faster and maybe even add a bleeder resistor to to be able to pull that charge off of the gate when you're stepping the thing up or down but you know certainly there's a lot of things that we would modify if we were actually making a true curve tracer but I just wanted to show you know how you can simply just add a few components here to get this kind of unique function with the stair-step generator you know I hope you enjoyed that if you did give me a big thumbs up on the video here if you haven't subscribed please subscribe to the channel tell your friends and thanks again as always for watching and I'm glad you enjoyed this

Channel: w2aew

Views: 47,234

Rating: 4.9853587 out of 5

Keywords: W2AEW, Tek, Tektronix, curve tracer, transistor, op amp, 555, 555 timer, stairstep generator, circuit, tutorial, schematic, integrator, collector current, NPN, base current, voltage, current, XY mode

Id: ZOLLoa2fH24

Length: 11min 13sec (673 seconds)

Published: Thu Mar 03 2016

Reddit Comments

[u/ArseneLupinII]

dude, unbelievably perfect timing with this. I just got asked by my teacher to look into making a transistor characteristic curve chart thingee earlier this week.

She was thinking about using arduino and LabView to control a dc voltage generator but this seems like a damn good idea as well.

[u/dago_joe]

Awesome as always. Thanks for another great video.