Simulating a circuit design to produce stable nano ampere

Hi, I am totally unfamiliar with regards to simulating electrical circuit and quite lost on proceeding.

I am thinking of using the ngspice features in kicad to simulate the circuit response.

The attached picture is the schematics of a circuit board I have that is suppose to be able to supply a steady current within nano-ampere range to a 100M ohm thermistor on the left hand side of circuit. The right is the circuit is connected to a 12 voltage power supply and ground. However I am totally stuck on how to simulate the expected current/voltage on the left hand side.

Would appreciate any advice how I can get this simulate properly. It is an old design where I have already the circuit itself physically. It is just that I am unable to test it therefore would love to use simulation to verify its functionality.


I cannot say about the circuit you have chosen. Personally i would have imagined a howland current source. With a lower value resistor like 10k strapped from the op amp output to ground to keep the output transistors biased on.

After some search, I think the problems lies with 2sc1815 FET.

Anyone have some ideas on overcoming this issue?

Perhaps you should have done a little more research, the 2SC1815 is an NPN BJT, not a FET. :wink:

Pardon me, It is a design of more than 20 years old. should have google the component of it. Still I am lost on this. I am suppose to make this old design work(as in produce nano amperage of left side). Though aint so sure how I can simulate it to work properly.

The focus of this forum in mainly on using kicad. The expertise in simulation is a bit limited. You might need to head over to a more specialized forum where simulation is better represented.

Perhaps @holger can recommend a forum?

He/She has the physical circuit but can’t test it so wants to simulate it but doesn’t seem to understand what the circuit is supposed to do. Sounds like homework to me.:man_student: Just do the math.

A suitable forum:

The circuit breaks into 3 parts, The right part with the transistor is just a pseudo voltage reference, The middle is a range switch, The potentiometer allows you to set your node voltage (literally a potential divider) and the left most part is the supply impedance, either direct or via a 1M resistor.

To me this does not comply to a “stable” nano amp source, below is the kind of circuit I would imagine for the task, The potentiometer sets the current, and it regulates that current as best it can. currently is for 0-500nA over the pot range.$+1+0.000005+10.20027730826997+58+5+50 a+256+160+368+160+0+15+-15+1000000+1.3516169805946276+1.3516318483814143 w+368+160+368+96+0 w+368+160+368+224+0 r+256+224+368+224+0+1000000 r+256+96+368+96+0+1000 r+256+96+144+96+0+10000 r+144+224+256+224+0+10000000 w+256+176+256+224+0 g+144+96+144+128+0 w+256+96+256+144+0 w+256+224+256+272+0 s+304+272+304+352+0+1+false w+256+352+304+352+0 g+256+352+256+384+0 w+256+272+304+272+0 x+187+67+217+70+4+24+R1 x+300+67+330+70+4+24+R2 x+185+259+215+262+4+24+R3 x+298+259+328+262+4+24+R4 x+184+320+229+323+4+24+load 174+112+192+144+240+0+10000+0.005+Resistance g+112+256+112+272+0 R+112+192+112+176+0+0+40+5+0+0+0.5 162+256+272+256+352+1+2.1024259+1+0+0+0.000009999999999999999 o+10+64+0+4097+2.5+0.00009765625+0+2+10+3

A tip for better looking responses:
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Basically quite simple to simulate using LTSpice. You need to know the value of the potentiometer - and have a model of the transistor.

Models of historic transistors are a problem.
But you can use any other npn to get the principle.
But everything with this circuit depends on the accuracy of the 12V supply.
This also was a problem in the old days, and the transistor does not improve much.

I guess it’s a MIL or Aerospace project. I have much experience in replacements in this area - and dealing with some people that don’t understand and just stick to some papers. Systems going back to good old 70s.

This circuit (part of it) makes sense, but why opamp circuitry shunting the load (and thus changing current set by the resistor/pot combo and making the circuit potentially unstable)?

zega, That potentiometer is the adjustment of what current you want it to regulate at, I chose a low value just to skip using an op amp buffer for that signal.

The potentiometer will not make it unstable, Its only when moving it, it may cause some weirdness if your using a cheap potentiometer. as its only the set point, once in place the op amp will regulate to that

A possible suggestion if your concerned would be to add a buffering op amp, allowing you to use something like a 100K potentiometer with a small capacitance to ground to smooth out any weirdness when changing set point.

My concern was not with potentiometer , it is with opamp+network that is shunting the load/output…

The op amp is shunting or supplying current to make its output node provide and regulate a given current, E.g. why I included a switch with an LED in that example, It regulates the voltage on that node to provide X amount of current, more or less independant of the load. be it a short, or any value resistor up until the op amps output saturates.

The potentiometer is just acting as a bias (or a set point) seeing as the original circuit implied range switches and a pot to adjust the amount of current, I felt like I should include the same. the one in that example would be 0-500nA

The exact circuit name is a “Howland current source” if you would like to know more about them.

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