Is this circuit a current gain circuit - Page 1
Forum section: electronics / projects

author: Cicada
author_rank: Regular Contributor
timestamp: July 25, 2023, 05:03:40 pm
content: HiI have a circuit board with tightly packed channels of circuits that should function as current input to current output amplifiers with a gain of -100.The output current is measured on another card with a charge recycling integrator i.e. a sort of a delta sigma type converter.I am unsure if the circuit that I have traced is correct. The parts that bother me are:1. R8 resistor in the feedback loop2. The series combination of R1, R2 and R4 tying into the feedback loop. This series combination is relatively low (max 11.68k) compared to the feedback resistor and will cause a big current to flow when the output voltage change on pin 6. I don't see how this part of the circuit is used to change the loop gain or offset leakage currents. The pot R4 is accessible from the front panel. So I assume this is for gain or leakage current offset tuning.3. Which resistors cause the -100 gain?Please let me know if this circuit will indeed function as a circuit that will increase a current of say 1nA to -100nA.RegardsCicada.

author: David Hess
author_rank: Super Contributor
timestamp: July 25, 2023, 10:18:45 pm
content: That will not work at all. This is what you want:https://www.multisim.com/content/8AFxYW2ZGm4c3vp92b2J8h/op-amp-current-controlled-current-source-grounded-load/

author: MasterT
author_rank: Frequent Contributor
timestamp: July 25, 2023, 10:38:16 pm
content: Looks like transimpendance amplifier (TIA). Resistors in question are form of "T-network" in the negative feedback path, comon solution to avoid extremely high values ( and very expensive) >1GOhm. Output of the circuits is voltage, not current, though this kind of "current to voltage converter"

author: Cicada
author_rank: Regular Contributor
timestamp: August 03, 2023, 10:04:12 am
content: Quote from: MasterT on July 25, 2023, 10:38:16 pmHi MasterTYou are correct in your assessment above. I have updated the schematic to better reflect the circuit. See attached pdf.For other readers you can refer to this for more info about the TIA with T network feedback.https://www.ti.com/lit/an/sboa284/sboa284.pdfI would appreciate if you would like to answer my follow up question. Please refer to the post thread for the follow up question.

author: Cicada
author_rank: Regular Contributor
timestamp: August 03, 2023, 10:05:56 am
content: Quote from: David Hess on July 25, 2023, 10:18:45 pmHi DavidThanks for your reply and the link to the circuit. Interesting and a well described circuit. I have added this to my colelction.ThanksCicada

author: Cicada
author_rank: Regular Contributor
timestamp: August 03, 2023, 10:45:18 am
content: HiI have attached version two of the circuit schematic for this follow up question. It describes the whole circuit a bit clearer. I have also attached a picture of a section of one of the boards. There are 5 channels per card.The circuit was built taking into account low leakage design principles. The inverting summing node is isolated from other circuit voltages on Teflon isolated binding posts. I have injected small currents (several pico amps, + and -) with a Keithley 255 current source. I measured the output currents with a Keithley 617 electrometer. There is a little bit of leakage current on the input of some channels but it's very small on most channels. I assume the leakage current is small because of the good low leak design precautions and the covers that kept the boards clean from dust and finger grease.I say offset/leak input currents because at 0 input current I get some output current. For example:Channel 4 results1pA in = -10pA out0pA = 99pA out-1pA in = 200pA outChannel 33 results1pA in = -40pA out0pA = 80pA out-1pA in = 171pA outWould I be able to tune out the "offset currents" by tweaking R10. As far a I can gather this is to trim out the effects of the Offset Voltage?Will untrimmed offset voltage cause effects that will manifest or look like leakage currents on the non-inverting input node?Or is the effects that I see above purely because of leakage currents flowing over the board into the non-inverting input?Or maybe R10 trims out the offset current? Nothing is said in the OPA111 about the offset trim inputs.So R4 is used to change the gain.Thank you for your help on thisCicada

author: MasterT
author_rank: Frequent Contributor
timestamp: August 03, 2023, 01:10:29 pm
content: Seems like offset to me. OPA111 is not in production any more, how old is your stock?OPA has bias current, +-2 pA at max +-0.8 pA typical that is about same as test current. Check out this circuits from DS, and think about purpose of the R connected to non-inverting input

author: Terry Bites
author_rank: Super Contributor
timestamp: August 04, 2023, 01:49:20 pm
content: Avoid trimpots and do a calibration in software if you can.You can lose the input diodes and just use a series resistor- choose it to limit the input current to 10mA under fault conditions.The input didoes of a fet opamp will out perform the PAD5.Put the resistor from ninv to ground as well.High value resistors force you to sue very small value compensation capacitors. Stray capcitance and self capacitance of the feedback resistor will add to your woes.You might use a T network. There are many ways to generate an accurate current output. Do you need bipolar or unipolar? Bandwidth?

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2023, 04:41:13 pm
content: TEE circuits are very useful to avoid unreasonably high resistor values.Be careful, though: the Johnson current noise of such a resistor network depends on the actual resistors used, not on the resulting equivalent resistance.This is important in transresistance amplifiers and electrometers.Also, when used as the feedback resistor across an op-amp, the TEE will give a higher "noise gain" for the amplifier's noise and drift.The increase in resistance should be moderate, not 1000:1, but this is a quantitative question left as an exercise for the designer.