Micro-Amps Measuring Instrument - Page 1
Forum section: electronics / projects

author: leninchakra
author_rank: Newbie
timestamp: October 03, 2025, 09:56:17 am
content: Good day,I am working on a small project. The final goal is to plot the BJT characteristic curveIb Vs Icusing an Arduino.The overall idea is to convert microamperes to millivolts using an OPA277 op-amp. Finally, The measuring circuit will be connected to a BJT's base and measureIb.Before building the hardware, I planned to simulate the circuit in SPICE (Micro-Cap 12 Simulation Software). Initially I ran a simulation using an ideal constant-current source with the OPA277 and it worked as expected (with_const_current_source.jpg) . Next, I replaced the constant-current source with a current mirror (with_current_mirror.jpg). When I made that change I got the following error:Error: Failed to converge in specified number of iterations at time=0.During a transient DC operating point, or the operating point calculation prior to an AC analysis, or during a DC analysis, if the number of iterations required to solve the network equations exceed the specified maximum, this message is issued.Please help me understand this issue and how to fix it.I assumed that if the measuring circuit works with a current mirror, then I could also use it with a BJT amplifier.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 03, 2025, 11:02:37 am
content: The feedback resistor on X2 seems sto be too small to me.

author: leninchakra
author_rank: Newbie
timestamp: October 06, 2025, 06:33:31 am
content: Thank you for helping out.The resistor value is chosen, 10uA -> 6.01 V.Could you suggest any other idea, i could try ?

author: Terry Bites
author_rank: Super Contributor
timestamp: October 06, 2025, 12:28:11 pm
content: The MCP601 model in microcap does'nt work in any simulation. Its broke, I think its a pin numbering issue.You could build a new model from scratch using the component editor in MC(Use the add new component wizard). Call the part MCP601_new or something.I'm not sure how this circuit connects to a transistor.You're applying Ib from a controlled current source, then measuring Ic and Vce?Current mirrors are great on paper but a royal pain in practice.Avoid these text book things, there are better ways. I've shown some practical mirrors.You dont have to use mosfets if high precision isn't required. At low currents 3906/3905 would probably be fine.I've added some illustrations of a current source, a current sense amplifier and a bipolar current to voltage converter.The opamps can't drive more than a few mA so some transistors can be used boost the current range.3906/3904 cost nothing so why not.The current source feedback resistor values are not critical but the individual resistors should be well matched.You can easily change the polarity of the source by swapping over the drive voltage (V4) and the ground.The input is fully differential, analog switch?The sense amplifier range is simply set by one feedback resistor. It can be used with currents from from uA to A.At low currents the buffer is not needed, but again, 3906/3904 cost nothing so why not.Re is optional.For sensing currents collector currents higher than 100mA, I'd be using ZTX651/751 and you need Re to keep the magic smoke contained.After that we'll need to make a more sophisticated fireproof buffer!