Voltage Controlled Current Source

[Cannonball2134]

Hello Everyone,

I am trying to create an enhanced howland voltage controlled current source. I have created a multisim file (attached) which works for a little while, then after a certain time the output current sin wave begins  to loose its shape (shown in oscilloscope window), and the rms current begins to change,when it should be stable at 400uA. I noticed this occurs faster as i increase the input signal frequency.

Can anyone suggest a way of fixing this? Do i need to add a capacitor somewhere?

[Josh_E]

Hi Cannonball2134,

I've replicated your circuit with a 741 Op-amp and it works fine, which means that the problem is due to the THS4021CD High Speed Op Amp. 

Placing a 1nF capacitor in parallel with the load does seem to fix the problem with the THS4021CD, however the simulation is still slowed down as before.

Have you tried this circuit in real life with successful results? It would be interesting to see if this behaviour is down to the actual component, or a potential mis-model in Multisim.

All the best,

[Cannonball2134]

Hi Josh,

Yes, I came to the same realization when I was experimenting with a different opamp today. What is interesting is that I replicated the exact circuit that was used in a paper, and their results were stable. Unfortunately I don't have a real THS4021CD to test on, so i can't confirm if it was a model problem.

Thanks for your help Josh.

Regards

James

[Josh_E]

Hi James,

Thanks for your reply!

Even using the THS4021 as a voltage follower leads to a heavily distorted output, which leads me to believe there is a bug with this specific component in Multisim.

I'm going to look further into this for you, and I'll update as soon as I find more information.

All the best,