then
away. Quite
aMaybe, maybe not, 1 ohm is not unreasonable at 150 to 200 mA max output. I would go larger very slowly, and likely not past 2.7 ohms. Just the same, i do not think i would go below 0.47 ohms. Also i would replace the 806 ohm resistors with 10 mA high compliance fast current sources. Say some Widlar current mirrors biased with a 3.3k resistor.
is
Ahh. I finally remembered, this reminds me of when i had to build a 10X pulse amplifier; input up to 5 V either polarity or both, output 50 V into
50 Ohms following input faithfully with 10 nS max edge transition times. This was in the late 1980's limited parts budget (species not quantity), limited calendar time, and hours limited by two month target date. One design could obtain the edges but not the power (5 nS to 7 nS, but only 25 V), another design could do the power but had 15 nS to 20 nS edges, and i ran out of budgets before i had the third design built. There was an alternative design which required existing but very difficult to obtain parts in the situation.Anyone here care to try to build one today? I am pretty sure that there is still a market (one or two a year, at fire sale prices; maybe leasing is better).
Thanks for the comments.
Probably not enough room left for the current mirrors, on the pcb, for this version. But I will keep it in mind for the next version.
Thanks again.
- Tom Gootee
Wow. Not me.
In this case, for my staircase current/voltage-source system, I have purposely slowed-down the edges (just after the 74HCT393 counter and resistor-ladder D-to-A that create the staircase waveform), to make them as slow as possible, so that they will just barely fit within the maximum allowable window of time before the transition to the next step must be completed (about 4 to 6 uS), to try to keep as much high-frequency "digital"-type noise out of my circuitry, as possible.
- Tom
The circuit you showed here has severe limitations.
First of all you should think about what you want to measure. Today it will be very useful to measure Vcesat, which requires to inject base and collector current simultaneously at a fixed ratio. Ic needs to be much higher (at least 10A) to be useful. Hfe measurements should be done at different fixed Vce with defined Ic. If you build a tracer, grounding the case and collector/drain will avoid short circuits. Unipolar current sources with switchable polarity will avoid damage on the DUT. etc...
Ban,
Thanks, very much. Those are exactly the types of criticisms that I was wanting to get.
Point taken, about the offset resistors! I had only stuck those in there to get near zero offset for further simulations, until I could get around to looking at temperature compensation, and also offset-related issues, in depth. Any ideas about those (and anything else, of course) would be most welcome. Perhaps I could just substitute a thermistor-resistor network, there.
Because I am relatively inexperienced, I will have to do more research on the rest of your comments. But please note that this circuit (however it eventually is designed) will only operate with a relatively-small number of pre-defined, switch-selectable operating conditions: six sweep-voltage (e.g. DUT collector) p-p levels (1, 6,
12, 18, and 30V p-p, with selectable polarity of pos only, neg only, or both), six frequencies (60, 200, 750, 12k, and 22k Hz, currently), and 12 base/gate-step-sizes (with staircase inputs to this circuit of from 0 to 15V or -15V, down to 0 to 200uV or -200uV, only one polarity at a time; or, zero input). [Regarding my modeling of the circuit: I did not think that I needed to post more of it, for this discussion. But I DO have a functioning Spice model for this entire subsystem, that includes all of the driving stages, including the complete staircase generator and its attenuator, buffers, selector switches, etc, and the complete sweep generator, also with staircase's clock-pulse and oscilloscope blanking-pulse generators, ramp generator's dielectric-absorption-compensation feedback subsystem, and more. I also have complete, functioning Spice models for all of the other subsystems of the Curve Tracer. And, in many parts of the models, I also attempted to model effects such as PCB trace inductances, parasitic capacitances of resistors, et al. Everywhere, all capacitors' ESRs are modeled (from specs), too. In LT-Spice, it can start out as an overall block diagram of the whole Curve Tracer. And then I can burrow down into the blocks, all the way to the schematic and component levels, although, of course, I normally simulate just a single sub-system, or less, at one time.]Note that a breadboarded version of the circuit being discussed (but with some modifications that have been discussed), when temporarily integrated into an existing version of the Curve Tracer's PCBs, seems to function fairly well, when tested with various types of transistors-under-test.
The only real "problem" variables that I can see that remain are temperature and load. (Remember, though, that I am relatively inexperienced.) But so far, at least, in the informal, random-device testing that I've already done with it, this particular circuit seems to do a fairly-GOOD job of maintaining constant and accurate step currents, when a BJT-under-test has its base connected to the circuit's output and is exercised with the available sweep signals (and similarly for step voltages, with FETs).
I haven't yet found another type of circuit that can do it as well, although, I HAVEN'T yet had time to test Jim Thompson's new circuit, and am not yet familiar with very many of the possible alternative circuits. But, since there are real objections to this (Howland) topology, I am also still looking for a better way. (And, I haven't yet really begun to look at temperature compensation for this circuit, either.)
Just for completeness' sake, I should note that...: The transistor-under-test will always have a one-ohm current-sense resistor, to ground, on the terminal opposite from where the sweep voltage is applied. And the sweep voltage is applied through a switch-selectable series current-limiting resistor, ranging from 10 Ohms to 5 megOhms, limiting maximium sweep current to 1.5 amps. I also neglected to mention, in earlier messages, that there are two small cooling fans in the unit, one of which blows almost directly onto this circuit.
Thanks again, Ban. More info always welcome.
Regards,
Tom Gootee
Well, current sources are.
John
Eh? Mine will just rail... no oscillations.
...Jim Thompson
Just out of interest, how would you expect this circuit to behave around the zero current point? I mean, the MOSFETs are in class C, so at zero current the opamp output is going to be moving around quite a bit trying to keep the output quiet, no? The MOSFET models are unlikely to say much about what will happen, because AIUI the subthreshold area is incompletely modelled. Would you do anything in a real version of this circuit to reduce the effect?
It'll need proper biasing.
I guess that's why Jim called it conceptual.
Graham
I called it "conceptual" because I didn't detail the multiple power rails, though I did include that in the parameterization of the OpAmps.
The dead band between M8 and M6 might indeed cause hunting, but I don't really see it as an issue, since it's a staircase generator.
...Jim Thompson
It'll be more of a dead band whilst the op-amp output slews to be honest.
Graham
Unless you do something really creative i doubt it is properly patentable. All the math for bias stability for BJT's is over 30 years old.
I assumed Jim was making a joke !
Graham
The *correct* phrase is...
"All the math for bias INstability for BJT's is over 30 years old."
With my method you can apply asymmetric heating and it'll stay stable.
...Jim Thompson
Now why would I make a joke about A-B bias?
I only make jokes about leftist weenies and Democrats ;-)
BTW, News headline this morning in Business Section of the East Valley Tribune...
"...Valley jobless rates up"
We're now at 3.8% ;-)
...Jim Thompson
So what? Many (most?) patents recently have been on gadgets and methods that were in sometimes common use for decades, whose only qualification was that they didn't yet have a patent attached to them. Since the patent bureaucrats don't know anything, the point of a patent now is to use the courts to keep others from using what you want to make money from. The guy who beat Sears was one in a million.
The RTLinux patent, for instance.
John Perry
Hertfordshire - where I live is 3.3% LMAO !!!
Graham
We're the 5th largest city in the US. What are you ?:-)
...Jim Thompson
Gun Free?
martin
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