DSO & AWG Based Curve Tracer - Page 1
Forum section: electronics / projects

author: mawyatt
author_rank: Super Contributor
timestamp: September 28, 2022, 05:35:43 pm
content: While riding hurricane Ian out we decided to prepare a more versatile and complete Curve Tracer concept based upon a DSO and AWG, see Fool'n around thread.https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/msg4421209/#msg4421209This concept worked so well thought maybe a setup fixture which included the ability to do A/B comparisons, deal with SMD as well as leaded DUT devices would be in order. Also, having a higher voltage/current range would be beneficial without the need for an additional buffer amplifier, and using a virtual ground transimpedance amplifier to "sense" the DUT current without creating an offset voltage.With this in mind we set out to utilize a standard DSO and AWG to display and create the waveforms respectively, but also utilizing external PS inputs to power the fixture in an effort to KISS. Along this thinking we wanted to utilize some available cheap components (well, some components were available before the crisis!) to keep the cost down and adaptable to "what's" available.Here's the preliminary schematic.This utilizes a transimpedance amplifier (LM358 for U4) for DUT current sensing with scale factors of 1u, 1m, 1 amp/volt. The amp is buffered with a pair of TO-220 NPN/PNP (TIP122/TIP127). SBD D11 and D12 assure the input doesn't get out of hand above +- ~1V in fault conditions until PTC fuse FB9 opens, under normal closed loop conditions the input is forced to virtual ground by negative feedback formed by U4 and the senses scaling resistors shown with selection switch S/E (Source/Emitter).For the DUT Base current drive a modified Howland Current Source/Sink is employed around U3 (also a LM358), resistors R21,20 & 15 and switch BASE set the current range from 1u, 1m, 1 amp/volt AWG staircased input shown as STEP IN. U3 is also buffered with NPN/PNP TIP41 & 42 for higher current source/sink capability as shown. Note the Howland Current Source/Sink can be converted into a simple voltage amplifier with switch I/V, this allows use with voltage controlled MOS devices.For the Drain or Collector sweep (D/C) we decided to use a simple audio IC amp like the LM1875 or LM3886 setup with a voltage gain of 5 V/V. Switch OUT-Z and attached resistors allow the Sweep D/C Output Impedance to be set from 1, 1K and 100K Ohms.Input power is nominally +-30V (+HV and -HV) as normally available from popular Lab Supplies, but can go higher (LM3886) if required. Standard 7815 and 7915 Linear regulators are employed to supply the regulated VCC and VEE voltages, duals +15V and -15V were selected to distribute the power dissipation rather than utilize a higher capacity single regulator (we had the standard 78 and 79 series in house).The design is based to allow a upgrade to higher voltage ranges like +-150V for the Drain/Collector (D/C) sweep, by only replacing the Sweep Amplifier (PA441).Anyway, here's a quick look at the project. We will follow up with additional details if folks are interested!!Winds are picking up and heavy rain, Ian is getting nearEdit: Updated schematic.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: September 28, 2022, 06:08:43 pm
content: The PCB is designed to fit within an available Aluminum 130mm 110mm 50mm case, same case type we utilized with the LCR Meter DC Bias Adapter. The output is by means of the 16 pin DUT connector shown in the above schematic.For the actual test fixture a concept will utilized a top side PCB which will host a ZIP connector for leaded DUT, a pair of standard 3 pin connectors L-C/B/E & R-C/B/E which will allow wire & clip leads, and a pair of "Toggle Clamps" for SMD DUT types. A pair of Switches L/R-1 & L/R-2 allow A/B comparisons for all configurations and Jumper Setups for D/C (Drain/Collector), G/B (Gate/Base) and S/E (Source/Emitter) which allows routing of the 3 test signals for various DUT arrangements and pinouts.The concept is designed to allow all sorts of either polarity DUT to be evaluated, from Bipolar, MOSFET, JFET, Diodes, Regulators, & LEDs, Triacs to name a few.Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: September 28, 2022, 07:12:19 pm
content: Wow, looks great!It happens that I'm experimenting with something similar this week, an AWG buffer. One of the intents is to use it for curve tracing, except it's not a dedicated tool, for now it's a rat's nest build. Having fun with current-feedback, while using whatever parts happen to find through the scrap boxes. No longer than this afternoon I was burning my fingertips to harvest bigger radiators from defective PC power supplies.I didn't know there's a hurricane there, take care.

author: mawyatt
author_rank: Super Contributor
timestamp: September 28, 2022, 07:38:11 pm
content: Current feedback is a great type of amp, amazing almost constant BW regardless of gainHere's what the Curve Tracer Concept should look like?Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: September 28, 2022, 08:06:31 pm
content: For those who don't have an AWG for the curve tracer, the AWG can be replaced with a 555 timer.Not kidding, W2AEW made a video about such a circuit. Very ingenious design, entirely analog:#231: Circuit Fun: Stairstep generator using 555 and op amps(schematic shown at minute 1:05)then later he added a ramp generator with a 3rd opamp, like seen here at minute 4:05#232: More Circuit Fun: Simple transistor curve tracer using Stairstep generator circuit

author: RoGeorge
author_rank: Super Contributor
timestamp: September 29, 2022, 02:35:46 pm
content: Took a closer look this afternoon, and made a block diagram so to not get lost myself in all the switches and protections. This is what I understood, and some questions:- 1. the labels for the current range and the functional description in the OP says 1A, 1mA, 1uA, yet the corresponding resistors are 1M/10k/10 and 100k/10k/1. I guess the lowest range is 10uA, not 1uA, is it?- 2. the emitter (in a BJT the DUT) would be kept at a virtual ground by U4.1, but then it will measure Ie, while the curves are usually traced against Ic, not Ie. For high Beta transistors Ic and Ie are about the same, but for power transistors beta can be as low as 10-20 or so. I wonder, would it worth to subtract the Ib from the measured Ie (for example with a resistor from the Howland pump to the U4.2 opamp), so the Y axis would be proportional with Ic?- 3. why the GND points at some pins of the DUT connector, is that for shielding in case of external probing wires?- 4. just got an idea while typing this, would it be possible to use a normal ground instead of the virtual ground of U4.1 and the two Darlingtons?I mean, to tie R29, 31 and 22 to GND, and the other end will go to the emitter of the DUT. A normal voltage amplifier opamp can read the voltage drop from R29/31/22. The Howland pump can be floated instead, by referencing it to the DUT's emitter pin (the R10 now tied to ground, would be tied to the DUT's emitter). In the end, the R29/31/22 voltage drop will be given by the Ic alone (without including the Ib), and the floating of a low current (the Howlan pump) should be easier then floating the emitter, which is a high current, no Darlingtons would be needed. Don't know if the text description makes sense, will scribble a schematic.

author: mawyatt
author_rank: Super Contributor
timestamp: September 29, 2022, 03:26:27 pm
content: 1) Good catch, yes this should be 100ma and not 1a, just changed the text to 100ma. The ranges are in volts per amp, so 100ma across 10 ohms produces 1V. This is for Base current, so many amps should not be required!!2) This is emitter or source current, and since the base current is from the Howland Source, one could adjust the emitter current to subtract the base current if desired. As shown the basic plot is Emitter Current vs Collector Voltage at Base Current Steps. Had thought about putting the current sense in the Collector but this would require the sensing circuit to move around with the collector sweep voltage and decided to use the transimpedance approach. One could certainly include the Collector side current sensing, but likely will be more involved with scaling and such and not as easy with simple components as this approach. This circuit should allow easy sensing of sub-microamp to amp currents without much trouble. If one wanted to get precise results then the a OP-07 or similar OP-Amp could be employed, but decided to just use the LM358 and realize the plot is Emitter current.3) The Grounds are there for shielding the cable and DUT Test fixture if necessary.4) Yes that would work, this is basically what we did with the original DSO & AWG setup. Use various resistor values to sense the emitter current with DSO Ch2 since this is ground referenced, but the collector voltage Vc sensing by the DSO Ch1 is also ground referenced and thus not collector to emitter voltage Vce, but actually collector plus emitter voltage. This works OK if the emitter current sense resistor is small (small emitter sense voltage) and thus Vc is ~ Vce. The virtual ground approach keeps the DSO Ch2 as Vce for the DSO XY plot, since the emitter is forced to virtual ground.Anyway, thanks for spotting the test error and the dialog, we'll post the corrected schematic!!Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: September 29, 2022, 04:12:42 pm
content: This is the DUT emitter-reference Ib and Vce described at point 4. Though, the biggest DUT CE resistors are 100k, that might be big enough that the casual bias current of an average opamp will influence the measurement, not sure, I should look up the datasheet and/or simulate.Looking into alternative arrangements for the DUT to settle what circuitry to add to my AWG current-feedback buffers, so to get from them, apart from other usages, the curve-tracing too.I've stolen already your Howland pump and the idea to turn it into a voltage source for MOSFETs, that's neat!Just for the docs (for myself), adding two links about the Howland pump, one of which is from "The Lightning Empiricist" Vol.12 No.1 1964-Jan-01 from Philbrick Research, and has very clean and intuitive explanations:Impeadance & Admitance Transformations using Operational Amplifiers - D.H. Sheingoldthe other one is "sboa437", from Texas Instruments, ca.2020 colorized:Analysis of Improved Howland Current Pump Configurations - Ignacio Vazquez Lam

author: mawyatt
author_rank: Super Contributor
timestamp: September 29, 2022, 08:04:03 pm
content: That should workThe emitter sense amp could be scaled up with a single resistor from op-amp - to ground, this would help keep the sense voltage low if a good op amp is used (OP-07).However the Howland setup may not work as intended.With the - resistor returned to the emitter junction the supplied current will be:Ibase = (Vawg - Ve)/Re, with Ve as Ie/Re or (Vawg/Rsense)/[1+{Re/Rsense}(Beta +1)]So if Re/Rsense is small then Ibase ~ Vawg/Rsense which is desired.Also, with the switch open now the supplied voltage from the Howland source becomes 2*Vawg - Ve which is not desired, but if Ve is low then Vb ~ 2*Vawg. Noting that Vbase become Vgate for FET use, and Ve is Vsource.This little analysis made me realize that using the other switch contact returned to ground give the result Vgate = Vawg which is what we want, and no need to mentally scale by 2!!Anyway, interesting discussions and keep up the good workEdit: Updated latest version of schematic to reflect mentioned change.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: September 29, 2022, 08:50:48 pm
content: Here's a HV version capable for supplying +-200V for the D/C Sweep. It's based upon the available inexpensive ($20) APEX PA441.Now to see if we can create a PCB the same size to fit, with the included internal HV +- supply. Input power is just ~ +-15VDC and the +-HV is created from the +15VDC Input.Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: September 29, 2022, 09:03:19 pm
content: Quote from: mawyatt on September 29, 2022, 08:04:03 pmOutch, I've implemented wrong the proposed change.The intent was to float the ground for the current source, and substitute its GND with a voltage potential copied from the emitter of the DUT. I've missed to shift the AWG stairs voltage, too, with the same Ve. Might require a voltage follower for the Ve, too, not sure right now, it's 5 minutes to AM hours here. Feels like it should work but I'll think about it tomorrow, and redraw the schematic, then simulate it just to be sure.Thank you for taking the time to point why it won't work correctly.

author: moffy
author_rank: Super Contributor
timestamp: September 30, 2022, 12:40:31 am
content: Just a thought, but I would make allowance for a small cap directly from U4.1 output to its inverting input to provide some more control over stability especially considering that a 1Meg resistor can feed back into the 100pf cap. I see that you have an RC feedback TBD but that might not be enough or in the wrong place.Trying to get my head around your circuit, is it a bit like a network analyser? Sorry it is my first encounter with such a deviceP.S. Have you SPICE modelled the transimpedance amplifier?

author: RoGeorge
author_rank: Super Contributor
timestamp: September 30, 2022, 07:19:21 am
content: Quote from: moffy on September 30, 2022, 12:40:31 amThe circuit makes a Curve Tracer, together with a signal generator and an oscilloscope in XY mode, the schematic will plot how current varies with voltage for a tested component (DUT stands for Device Under Test, can be a transistor, diode, capacitor, resistor, etc.). Displays the I/V curves for the tested components.Something like shown/explained in this video by W2AEW:#290: Vintage Tech: Tektronix 576 Curve Tracer

author: moffy
author_rank: Super Contributor
timestamp: September 30, 2022, 08:26:23 am
content: Quote from: RoGeorge on September 30, 2022, 07:19:21 amThank you, gotcha. So it does a series of DC voltage steps, or a repetitive linear ramp, measures the current and as you stated plots I/V.

author: kripton2035
author_rank: Super Contributor
timestamp: September 30, 2022, 09:14:58 am
content: suscribed.

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2022, 01:20:24 pm
content: Quote from: moffy on September 30, 2022, 12:40:31 amYes we have done some analysis with LTspice. The feedback TBD components should provide the ability to maintain stable operation under the expected conditions since the output buffer has ~1X gain and little additional delay relative to the slow op amp, good point tho and why we included those components!!The buffer is designed to provide added current capability and the cross over should not be a factor since the waveform speed is slow and the added resistor (1K) from bases to emitters allows the op amp to drive the output current until the magnitude is above ~1ma, then the bipolar buffers begin supplementing the required load current.As RoGeorge indicated, this is a device curve tracer for use with a DSO and AWG.Will post some LTspice plots if you are interested.Best,

author: kripton2035
author_rank: Super Contributor
timestamp: September 30, 2022, 02:04:21 pm
content: I don't see the interest in using an external AWG for this. we only need a simple sinus voltage at different frequencies, this could easily be implemented in thecurve tracer itself ?

author: moffy
author_rank: Super Contributor
timestamp: September 30, 2022, 03:25:52 pm
content: Quote from: mawyatt on September 30, 2022, 01:20:24 pmDid a quick sim myself, looks stable as long as you don't mind a limited BW, which really shouldn't be an issue but I would still provide a place for a cap as mentioned just in case. Real world can sometimes be a little more complex.By the way would it be possible to sweep current and just measure voltage? That way you only have to produce the current once without the need to actively sink it. Not sure that would work with DIACs etc.P.S. Not so good for reverse biased diodes either! Guess it's not such a good idea.

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2022, 07:56:58 pm
content: Quote from: kripton2035 on September 30, 2022, 02:04:21 pmHow are you going to produce the Stair-Step Base current (for Bipolar) or Gate Voltage (MOS or JFET) for Transistors and Fets?Sure you could design this and include it, but with the AWG you have the ability to create custom sweep types, ranges, offsets, then the Stair Step waveforms, number of steps, step level, polarity and so on. That's what they are good at, creating Arbitrary Waveforms, and with modern entry level AWGs you get two independent channels!!Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: September 30, 2022, 08:08:39 pm
content: I wonder if it would be a good idea to simplify the current measurement by using the R_load also as a shunt to read Ic, like in the draft schematic (R8/R9/R10).No protections have been included in this draft. Y axis in the captured plot is 10mA/V.

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2022, 08:15:49 pm
content: Quote from: moffy on September 30, 2022, 03:25:52 pmYou basically have a Voltage and Current Source you can use as desired, so you could use the Howland Current Source to sweep if desired, and use either the AWG drive, or the Output for the DSO Ch1. We tried to create lots of flexibility for various uses.JFETs can be traced by Sweeping the D/C and monitoring the S/E current and using the G/B Current Source switched to a Voltage Source with a a negative StairStep waveform for the Gate Voltage, same for DMOS. TRIACs and DIACs should also be available to be measured.Thought it was interesting in this thread where we were able to show the negative incremental resistance of the reversed biased BE junction of a ordinary 2N3904 NPN in breakdown. Confirmed the incremental negative resistance by adding a simple shunt capacitor which created a relaxation oscillator!!https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/msg4422982/#msg4422982Best,

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2022, 08:23:03 pm
content: Quote from: mawyatt on September 29, 2022, 08:50:48 pmUpdated version with PCB layout. HV driver can be a PA441 or a OPA462.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2022, 08:39:01 pm
content: Quote from: RoGeorge on September 30, 2022, 08:08:39 pmYes, you can do this (use R Shunt) and it should work fine. Certainly simpler than the Transimpedance approach. May require a better differential amp than a LM358 creates though. If you can keep the impedance very high then it may be possible to get to very low current levels which is something we were interested in, with the Transimpedance approach this is possible without much effort (or expense) as the limits are just the Op-Amp bias current as diode leakage which should be very low. The input protection SBD have ~ 0 volts across them under normal operation, so leakage is low and the clamp diodes at the op-amp input aren't even necessary, so they could be eliminated all together since the op-amp input can only "see" the SBD diode drop thru 1K worst case. Output leakage isn't an issue either unless it finds it's way to the input, thru the switch or other paths, but this should be manageable.Anyway, many ways are possible and keep up the great work and dialog!!BTW nice setup & plots with Spice!!!Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: September 30, 2022, 09:28:27 pm
content: Glad you liked the plots, thank you.Quote from: mawyatt on September 30, 2022, 08:39:01 pmThat's a strong argument.I would be very interested, too, in measuring small leakage currents, especially since the same schematic can also be used for DC measurements. Now the HV opamp makes even more sense. First that came to mind would be to measure capacitors' leakage at DC. Or the dark current in a photodiode. Or any other application for a TIA.Wow, I'm fully sold to the idea now, will add the TIA back, that's a great feature, thanks for the idea!

author: mawyatt
author_rank: Super Contributor
timestamp: September 30, 2022, 10:00:28 pm
content: Quote from: RoGeorge on September 30, 2022, 09:28:27 pmThis discussion has created some "thinking out loud" so to speak. Wonder if a better overall approach would be a pair of Howland Sources, each configurable as a bipolar Current or Voltage source and range scaling. One would be a lower voltage type but higher current, say to >|1a|, the other a high voltage but low current type <|50ma|. Add to this the transimpedance amp and you have a very flexible test function capable of covering a wide range of 2 and 3 terminal components over a wide range of currents and voltages!!Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: September 30, 2022, 11:09:06 pm
content: Quote from: mawyatt on September 30, 2022, 10:00:28 pmVery tempting!The difficulty would be the feedback opamp from the high voltage Howland pump, its positive input will see the full HV swing.

author: moffy
author_rank: Super Contributor
timestamp: September 30, 2022, 11:30:32 pm
content: Quote from: mawyatt on September 30, 2022, 08:15:49 pmThanks for the link that was very informative, you adjust your test jig to suit the device, makes sense.

author: mawyatt
author_rank: Super Contributor
timestamp: October 01, 2022, 12:41:13 am
content: Quote from: RoGeorge on September 30, 2022, 11:09:06 pmGreat thinking, that's exactly what I've been looking intoBest,Edit: Attached is a concept that seems to fit the need. It's based upon scaling the Op Amp Vcc and Vee to follow the Output with an Offset set by the zeners.

author: moffy
author_rank: Super Contributor
timestamp: October 01, 2022, 01:59:39 am
content: Just curious, what % accuracy are you looking at for your current source/sink, Howland pump?

author: mawyatt
author_rank: Super Contributor
timestamp: October 01, 2022, 03:18:54 am
content: Overall accuracy should depend on Resistors and Op Amp. Since the DSO is used as a display, anything significantly more accurate than the DSO isn't required. So 3% should work but expect to do much better, but haven't looked much into this yet. The really nice thing about the modified Howland with the positive feedback buffer amp is the removal of the dependance on the sense resistor (ideally), which allows precise current scaling without need for feedback resistor adjustments.With a good Op Amp(s) and Resistors think you can achieve comparable results to other precision current sources.Here's an example with simulations with ideal Resistors, pair of OP-07s, Current Sense R 1K and 1K load. Load attached to swept voltage source and current measured. Shows a 1ma Output Impedance of 250Meg Ohms!!Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: October 01, 2022, 09:22:01 am
content: I had to sleep on it, so here's the new thoughts:- Howland pump comes particularly useful when the load can not float and must be tied to ground.- However, the load is not really grounded, but floated at GND level by the transimpedance amp (TIA).- The voltage output from TIA indicates the current through the load.So all is left to do is to use the voltage output from the TIA in a control loop together with the high-voltage amp (HVA), so to regulate the loop for constant-current (CC) instead of constant-voltage (CV).The advantage would be that the TIA is already referenced to GND, it doesn't have to swing at the full HV range like the feedback opamp from a Howland pump has to. The TIA input was already protected against HV from the very first design, in case it wants to swing more.

author: mawyatt
author_rank: Super Contributor
timestamp: October 01, 2022, 01:39:16 pm
content: You could use the feedback from the TIA to "force" the HV Output Amp to become a current source, might be tricky with all the delays and such and various unknown applied loads.Just for fun included the HV Follower shown above with the Modified Howland Source as a HV Current Source.We don't have a model handy for a HV Op Amp like the PA441 or OPA462 so used the OP-07 Buffered Driver Amp. Also, no HV transistor models so used the 2N3904 and 2N3906 since the Transistors and Op-Amp models used don't seem to include voltage breakdown effects.Anyway, here's the schematic and pulse response.Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: October 01, 2022, 04:47:20 pm
content: That could work, but won't be easy:- ideally would need for the mirrors 2 pairs of matched HV transistors of at least 300mW- the bootstrapped supply for the opamp has to swing a lot, about 275V, stray capacitances will see that swing- will burn some power, 3mA from the mirrors plus, say 2mA opamp idle current, 1.5W of heat to evacuate- because of the HV swing, will be slower and less stable than a TIA based CC (didn't check yet, only a guess)A TIA near GND looks more appealing, and I think it would be faster because all its nodes will be close to GND. There shouldn't be any delays in current, once entered in the DUT through the HV terminal, the current has to go out through the other terminal at the speed of light.The only case where the already existing TIA can not be used would be when testing a 3 terminals DUT with both sources in the CC mode. To overcome this, each CC source will have to have its own TIA, so 2 TIA, one for each CC source to sense their corresponding current individually, each maintaining their own virtual potential.Seems like it should work, but to be sure I'll have to draw the 2 TIAs.Another thing, I've just checked out of curiosity how sensitive is the Howland pump against resistor's tolerance. I know it's supposed to be very sensitive, but never looked into the actual numerical values. Did a simulation today, and even for as little as 1% resistors tolerance, the worst case is close to unusable.The input stair voltage is 100mV per step. The worst mismatching case with 1% resistors and 20Vpp swing on the load looks very, very bad. The current error in the DUT (induced by the DUT's own voltage swing) is as big as it would be when varying the input control voltage with 4 steps (400mV in this numeric example). Same results with an ideal opamp instead of LM358.I'm thinking of replacing the Howland CC with something else.

author: mawyatt
author_rank: Super Contributor
timestamp: October 01, 2022, 07:18:43 pm
content: The mirror just needs to consume slightly more than the op amp base current and the output current.The R1/R2 and R3/R4 ratio needs to be matched not the absolute value, only Rs determines the scale error with the mentioned matching. To keep op amp offset error low use as high a Rs as practical which requires a high input voltage.0.1% resistors are available and not expensive and easily matched to much better if desired.Agree this is still a somewhat messy solution with all the HV biasing and such.Best,

author: moffy
author_rank: Super Contributor
timestamp: October 02, 2022, 07:39:04 am
content: Borrowing ideas from RoGeorge, mawyatt and of course J.Williams, I thought I'd contribute my WAY OVERDESIGNED and complex HV precision transimpedance amplifier. It doesn't rely on any Vbe matching for current mirrors, that would be too simple, it needs 2 pairs of well matched resistors R4/R5 and R7/R8 and limits the error currents by using darlingtons. Only the difference between Q5/Q6 and Q7/Q8 collectors appear at the output, so load currents like the supply current for U1 are eliminated, not original but worth stating.The complexity comes from the mirrors which need their own opamp and supply, but in the sim with a 4vp-p input it managed 80db linearity or 0.01% at 1kHz. The component choices are just for the sim, not suggested for real life. The current mirror also can provide current amplification as shown easing load and power requirements on U1 and the HV supply.P.S. Added .asc file as per RoGeorge's recomendation.P.P.S. It is of course a Transadmittance not Transimpedance amplifier as RoGeorge has pointed out.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 02, 2022, 09:53:35 am
content: Looks good. I like those two opamps controlling the current-mirrors!Quote from: moffy on October 02, 2022, 07:39:04 amThat always happens to me. I start with something very simple, say, adding a series resistor and call it a current-source, then replace it with an active current source, then make it a voltage-controlled current source, then add a DAC to it, then add an ARM microcontroller to control the DAC, then add Linux and write software to control it, then design a GUI for it, then optimise the boot time, then estimating how long it will take to complete all these...Then, after agonizing for a full week about all that, just go back to the initial schematic and add a series resistor to it.P.S. When adding LTspice printscreens it's a good practice to also attach the .asc file, so others can simulate it, too.

author: moffy
author_rank: Super Contributor
timestamp: October 02, 2022, 10:02:24 am
content: Quote from: RoGeorge on October 02, 2022, 09:53:35 amAdded the .asc file as you suggested. Thanks.P.S. Simulated output impedance is 143M ohms. For a 10V step on the output driving 9.998ma the output current changed by 70na.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 02, 2022, 12:46:31 pm
content: Quote from: moffy on October 02, 2022, 10:02:24 amWow, gave it a run and it works rock solid, congrats!That's SMU grade performance. Gonna use the same idea as a standalone AWG buffer, thank you. A simple switch can turn the same circuit from a transadmitance amplifier, like it is now (Iout/Vin), into a current-feedback voltage amplifier (Vout/Vin). Perfect!

author: moffy
author_rank: Super Contributor
timestamp: October 02, 2022, 11:12:56 pm
content: Quote from: RoGeorge on October 02, 2022, 12:46:31 pmHope it will be of use. Just be careful of C1 it might degrade the AC performance acting as a source/sink for currents rather than the darlingtons. Might want to reduce it in size, not sure.

author: mawyatt
author_rank: Super Contributor
timestamp: October 02, 2022, 11:49:56 pm
content: Here's the concept described by RoGeorge (we believe, please correct if incorrect). Using the TIA as a current sense and negative feedback to a HV Amplifier to make the HV Amp "look" like a HV current source. Note this technique does not require a HV op amp for current sensing because the input voltage is forced to virtual ground by the TIA negative feedback and thus the voltage levels are just within the range of Iload*Rsense. Also the resistors are not critical, and Rsense with the ratio of R4/R3 determine the TIA gain which because of the overall negative feedback by the HV op amp is ~ G(Iload/Vin), where G is Rsense(R4/R3). We are just using a buffered OP07 model which has no voltage supply limit for the HV amp, normally this would be proper HV amp, a OPA462 or PA441 for example. Seems to work well as a current source, but no detailed effort has been applied just yet, so an opportunity for some folks to jump in and provide a more detailed analysis!!BTW Rsense and Rload were swept from 100 to 100K and 100 to 10K respectively for the plot.Best,

author: moffy
author_rank: Super Contributor
timestamp: October 03, 2022, 12:10:21 am
content: Quote from: mawyatt on October 02, 2022, 11:49:56 pmIt is a very nice concept, but might under certain load conditions run into stability issues because of its closed loop nature. I am sure there are things that could be done for stability, the major difference with my concept is that it is open loop but more complex as a result(to maintain precision), but stability with load should be a given. From the discussion I assume you would be analysing loads other than resistance? Some that might have even negative resistance.

author: moffy
author_rank: Super Contributor
timestamp: October 03, 2022, 08:54:21 pm
content: I was unhappy with my previous design in one particular aspect: The input amplifier U1 couldn't be decoupled at the supply pins properly without compromising the current flows and matching for the transadmittance amplifier. Odds are that a fast op amp could go unstable so I am proposing a slight change in the input topolgy that overcomes this limitation. Comparing the schematics should explain it all, if not I could explain any detail needed.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 04, 2022, 12:53:58 am
content: Quote from: moffy on October 03, 2022, 08:54:21 pmC1 doesn't have to be there at all. It shouldn't oscillate without C1, and anyways, those are not power pins any more. Those pins are now part of the active circuit, and is essential that no other circuit branching can steer away any current from those former power pins. All the current has to go to the R4/R5, in the external mirrors.The new power pins are the terminals of the R4/R5/1k at the high voltage. Decoupling "near pins" has to be done now between the HV pins of those resistor and GND, at the node between R4-V4 and GND, and the other one between R5-V5 node and GND. When a symmetric supply is used, two decoupling are needed, one for each supply.Another thing, on my LTspice simulation hangs at 100% without going to the next steps, so I had to add in the schematic an ".options srcsteps=0". The minus of V4 would be better to be tied at GND, same the plus of V5. In simulation it doesn't matter, but the physical circuit will be better with V4 and V5 tied to the ground, and not in series with V1 and V2.Quote from: mawyatt on October 02, 2022, 11:49:56 pmYes, indeed, thank you!I did try to do that yesterday and failed, it was oscillating all the ways.My try was with 2 opamps, and instead of U3 I've tried to sum the TIA output voltage directly to the U1. Should have worked, but it didn't.I have a serious lack of skills when it comes to loops stability.Then I have to leave it, and today spent some time with the moffy's version of buffer, trying to modify it from a Voltage-Controlled Current-Source (VCCS) into a Voltage-Controlled Voltage-Source. I was hoping to do that by using current-feedback (CF), so to keep it fast (as an AWG buffer)With CF the results were not very good. It was stable, but the Vout didn't follow Vin very well with low impedance loads. With 100+ ohms it was OK. For some reason I couldn't reduce enough the output impedance. I have to understand CF better before fiddling again with it.The moffy's schematic resembles very well the topology of a CF amplifier (I know them as Norton opamps, not sure if this name is widespread), like shown in Fig.5b here:https://archive.org/details/edn-1989_01_05/page/164/except moffy's doesn't have the second voltage follower at the output.For a VCCS the output voltage follower is not needed, but to make it VCVS I had to add a voltage follower. It's enough to buffer the voltage for the feedback network only, so a low power opamp should be enough (the very same opamp that measures/display the Vout on the R_load). However, to make the VCVS immune to load variations I had to drop the CF idea.The schematic is drawn with the switch in CV mode. Notice that for the CC mode, there is no global feedback from load to input, and also no current measurement in the load!Hope I'm not wrong with this one, but I think there is no need to have a global feedback in for CC mode, because of the Kirchhoff law in the node "out", and in the node-opamp "U1".The two current mirrors have their own local feedback loop. The current-mirrors driven by the hanging rail-to-rail opamps in the moffy's initial schematic (instead of a classic transistors pair mirror) also solve the thermal runaway, and make it easy to set the idle current in the power transistors.Another advantage would be that nothing swings in high voltage, except the "out" node. And all is current controlled, which means induced noise and Miller or the stray capacitance's bad influence are kept to a minimum.Or at least that's how I believe it is (please correct me if wrong).If it will work in practice as well as in simulation, then two buffers like those can make a dual channel SMU like you said, one with HV and lower max currents, the other with lower voltage and high I, and bot can be tied to ground to know Ib and Ic .OTOH, would be a pity to not use a ready made HV opamp like that APEX.

author: moffy
author_rank: Super Contributor
timestamp: October 04, 2022, 01:35:45 am
content: Quote from: RoGeorge on October 04, 2022, 12:53:58 amUnfortunately they are the power pins for the opamp and I was even able in LTSpice to get opamp power supply oscillations depending on the opamp I used. The LF412 when it first came out was notorious for power supply oscillations if not properly decoupled, and many mid to high BW opamps these days have the same issue. There is another issue with using the darlingtons to provide power to the opamp. The emitters are modulated by the varying current which then can appear across the load depending upon the PSRR of the opamp and the frequency of modulation, this introduces distortion into the output. I was able to reduce the distortion level by 20db by moving the opamp supply out of the current path at 10kHz, from -60db to -80db.P.S. Meant the LF351 instead of the LF412, memory.

author: rhb
author_rank: Super Contributor
timestamp: October 29, 2022, 01:34:06 am
content: @mawyattI wish you could appreciate the trouble you’ve caused me. I’m in the process of installing ABEC 7 bearings in a small lathe in place of the ABEC 1 taper roller bearings which have ~0.0015” TIR. The tolerances for the ABEC 7 bearings are +0.0/-0.0002” for the spindle and +0.0/-0.00035” the housing bore. Not easy for anyone. Very intimidating if you’ve never done that. So I’ve been rather focused on thatSo after a long hiatus from EEVblog I wander by and am confronted by your work. OMG! Color me extremely impressed. In one sense you’ve pointed out the obvious, but until someone does, it is not obvious. Well done! But now I have another project.Have Fun!Reg

author: mawyatt
author_rank: Super Contributor
timestamp: October 29, 2022, 01:28:09 pm
content: Hi Reg,Thanks!!Sorry about that!! Never tried anything like your bearing installation, but seems very difficult indeed!!And yes, many things are "hiding in plain sight" so to speak, and are obvious after the discovery!!This whole episode of the Curve Tracer endeavor has led us to acquire an old Tek 577 and refurb it back to life and good working order.https://www.eevblog.com/forum/repair/old-tek-577-return-to-life-story/msg4479919/#msg4479919The issue of software development (we're not very good at this) & some reoccurring family issues has kept us from going forward with the full custom approach, so this is on "hold" at the moment, and the refurbed 577 has filled the immediate need for the Curve Tracer.Here's another related post using a DSO for Impedance Plots you might find interesting.https://www.eevblog.com/forum/testgear/capacitive-impedance-plots-with-sds2104x-plus-bode-function/msg4335745/#msg4335745Best,

author: iMo
author_rank: Super Contributor
timestamp: February 09, 2025, 07:10:36 pm
content: Quote from: mawyatt on February 08, 2025, 09:18:15 pmInterestingly this has jumped off YT at me today (6 transistors + DUT):Also a replica:https://www.eddybergman.com/2017/06/simple-but-effective-transistor-curve.htmlOriginal from 1980:https://archive.org/details/ElektorMagazine/Elektor%5Bnonlinear.ir%5D%201980-09/page/n37/mode/2up

author: iMo
author_rank: Super Contributor
timestamp: February 09, 2025, 07:11:14 pm
content: It was created/published by B. Darnton in 1980 (see the Elektor Magazine article in my above post).EddyB replicated it in 2017..Quote from: mawyatt on February 09, 2025, 05:59:59 pmAnd of course here is my sim, as usual..Btw, pretty sensitive on the values used (the picture on the scope will be different as the sim shows many runs)..

author: iMo
author_rank: Super Contributor
timestamp: March 07, 2025, 09:03:15 am
content: ..some improvements attempts:

author: mawyatt
author_rank: Super Contributor
timestamp: April 24, 2025, 05:37:20 pm
content: We finally got back to the Curve Tracer and one of thing we wanted to do is verifyIFthe conceptstolenborrowed from the way auto-balancing LCR meters use a transimpedance amp to measure the DUT current by creating a virtual ground at the DUT lower terminal (emitter/source for our case) by injecting an equal but opposite current into the node and forcing the node to ~zero volts. In our Curve Tracer case this allows the voltage across the DUT to be driven and sensed single ended and no need for a floating or high side current sense circuit, and allows the base drive circuit (Howland Current source) to work against a virtual ground referenced DUT Emitter terminal at the Base. For MOS devices this will be simply a single ended Gate Voltage Source and thus Vgs is simply the drive voltage as the Source is at virtual ground.Since our goal for a Curve Tracer would cover at least 1ua/div to 1a/div (6 decades) for the DUT ranges we would typically require and handle both DUT polarities, we decided to kludge up a protoboard to "see" how this virtual grounded transimpedance concept behaves.Here's the schematic sketch and kludge protoboard. We didn't have any Darlingtons handy, so just used a TIP41 and TIP42 as the LM358 buffer. Current ranges were selected by changing Rsense from 1MΩ (1ua/V) to 1Ω (1a/V). Since the AWG can't supply the higher currents, we used a TIP41 as an Emitter Follower Buffer (on heatsink) for the higher current NPN DUT currents.We melted a scope probe tip as things get hot fast at the 100ma and 200ma DUT current ranges, so be careful if you decide to play around. When/If we ever implement the full Curve Tracer of course proper heat sinks will be employed, but not for the kludge setup.We are thinking about a full range Curve Tracer, actually 2 types a HV and HC type. The HV type will range from at least 1ua to 100ma DUT current and up to +-160V DUT voltage, the HC type will range from 1ua to 1A up to 60V. Also will forgo the AWG inputs in favor of built-in generation. Various Source Impedances, Current Scale factors and DUT Base/Gate Ranges are envisioned, as well as Voltage and Base/Gate continuous variability. Also considering built in HV supply generation rather than external +-175V requirement, the HC will require an external Supply (+- 32/64V at 6/3A). If folks are interested in participating let us know!!Anyway, here's some results from the tests with various devices and current/div ranges indicated. Seems the virtual grounded current sense concept works, judge for yourself!!PNG 5-8 2N3904 at 1ua (Rsense 1MΩ), 10ua (100KΩ), 100ua (10KΩ), 1ma/Div (1KΩ)PNG 9 D569A at 10ma/Div (100Ω)PNG 11-13 TIP41 at 10 (100Ω), 100 (10Ω) and 200ma/Div (1Ω)Best

author: RoGeorge
author_rank: Super Contributor
timestamp: April 25, 2025, 02:53:33 pm
content: Sounds like a good plan!Quote from: mawyatt on April 24, 2025, 05:37:20 pmI would have expected slowly increasing slopes after the knee of the plot (all pointing to the Early voltage), similar with the ones seen in the rest of the captures, but the above plot is different.- Why the peak, then a valley? They don't seem like segments that will meet at the Early voltage.- Also curious, what are the 5 small doubling "dents" in the middle trace, the one right under the horizontal axis?

author: mawyatt
author_rank: Super Contributor
timestamp: April 25, 2025, 04:34:53 pm
content: Quote from: RoGeorge on April 25, 2025, 02:53:33 pmGood questions!!This was created using a 10MΩ Base Resistor and 1MΩ Sense Resistor (in Transimpedance Amp), so scale factor is 1ua/Volt Y Axis and base currents are in few nano amp regions. Being kludged on a Protoboard with all sorts of parasitics and an LM358 based Transimpedance amp are likely candidates for this behavior. Also the "bumps" are from the 60Hz Mains beating against the 80Hz Sweep Frequency and 10Hz Step Frequency, we proved this later by changing the Sweep to 120Hz and the Base Steps to 15Hz and watched as the "bumps" would "walk" along the plots. They don't appear in the higher current settings which hints it's associated with the effective gain of the Transimpedance Amp.On a proper PCB (and maybe better Op-Amp) hopefully these artifacts will disappear or be significantly reduced.Another concern other than how this Virtual ground approach to DUT current sensing behaved, was how would the Base Current Source based upon an Enhanced and Buffered Howland Current Source work driving into a DUT Base Emitter junction tied to the Virtual ground Transimpedance Input. With this in mind we kludged up a Howland Current source based upon another LM358 as shown in the photo below.This quick assessment didn't show any unexpected behavior between the Virtual Ground and the Current Source plot 14 shows 100ma/V scale factor.We also took a look (PNG 16) at the negative resistance region of a reversed biased Emitter to Collector (Base open so Emitter to Base Breakdown current flows thru to Collector), 1ma/V scale factor and this is from a TIP41 which was handy.Best

author: rhb
author_rank: Super Contributor
timestamp: May 11, 2025, 10:50:49 pm
content: OK I’ve been pretty much off line for quite a whiley. However, I have a 4145A and a bunch of 2N3904s. The display doesn’t seem correct. I’ve never seen a droop after the knee. This makes me question the validity of the data shown or the condition of the transistor.I'll append screenshots tomorrow.Have IFun!Reg