Hi this is a simulated circuit that I have ran on my multisim.Image 1

According to this circuit, my current will stay at a constant 1.41mA given an input of 1Vp. I get a stable current until my Load Resistance R8 goes above 6k ohms, then it starts to drop.

My questions are -

1) Is there a way to increase my Load Resistance R8 above more than 20k ohms, while obtaining a constant current of 1mA, without changing the op-amp used. [I have tried increasing Vin, but it doesn't work]

2) I have rebuilt this circuit on an actual circuit board, but my results that I obtain vs the simulation are totally different. According to this circuit, above 6k~7k ohms, I would obtain a saturation (squarish) waveform of +11V -to- -1V in the oscilloscope at the probe 4 connection as shown below in image 2. However, in my actual connection, I am only obtaining a 2Vpp sinewave across that same point for a 10k ohm connection. I have changed the IC chip and still obtained the same issue. I was wondering if anyone knows why such a different result is obtained from the simulation.

Image 2

This project is based on the Electrical Impedance Tomography Design. I have to build a constant current source that outputs a 1mA(rms) current across a body(tank) of water. I have built this circuit, but it does not seem to work according to how I hope it would. If you have any ideas or alternate circuit designs to suggest, I am open to listening also.

  • \$\begingroup\$ until my Load Resistance R8 goes above 6k ohms, then it starts to drop. Your very first step is then to figure out why that is happening. Hint: look at the output voltages of the opamps. You want to design a circuit EXCELLENT :-) But that comes with the responsibility to understand how the circuit works. It seems to me that you're just "trying things" and believe me, you can do that until the end of times and still not have a solution. \$\endgroup\$ – Bimpelrekkie Feb 23 '17 at 10:51
  • \$\begingroup\$ but it does not seem to work according to how I hope it would. You can rest assured that circuits almost never work like you hope they should work. Circuits work how they're supposed to work depending on their design. The trick is to design a circuit and then make it work according to your design. In your case, the circuits are in charge and you get a mess, you should be in charge ! \$\endgroup\$ – Bimpelrekkie Feb 23 '17 at 10:53
  • \$\begingroup\$ Thanks @FakeMoustache So far, from theoretical calculations, I understand how the circuit works until Vout (at probe 1, before R5), I believe it is just an inverting amplifier + voltage follower design. I am still trying to figure out how the increase of R8 to approximately above Vsat(12V)/2 = 6k ohms, causes my waveform to be driven to a 'squarish' waveform. I hope I can have some insight to why that happens. \$\endgroup\$ – Lenny n Carl Feb 24 '17 at 6:30
  • \$\begingroup\$ Think about the Vsat voltage you just mentioned there and how that relates to the voltage you're feeding the opamps with. Do you expect these opamps to be able to output a higher voltage than what they're supplied with ? Understanding a circuit's formulas and calculations is only part of understanding how a circuit works. There are many other factors which are not in those formulas. Like physical limitations of opamps. \$\endgroup\$ – Bimpelrekkie Feb 24 '17 at 6:45

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