Howland current source

[s.h.bagherzdeh]

Hello everyone
I want to have a VCCS. I used the Howland current source design.
I need to control load current by adjusting input AC voltage. I need a constant current in the frequency of the input. The current 1mA.
Based on the theory, I set the R2=R4=1Kohm and R1=R3=2Kohm. The load is in the range of 100 ohm to 100kohm. The op-amps I picked up are TL072CN and TL074CN. The VCC,-VCC of +/-15 V and +18/-18 V. The max input vo ltage amplitude is 3.75V. The frequency of 10Khz. But when I implemented on the breadboard and tested it, I could not get the the constant value for current source above 500ohm!!!!!!
If you could help me to solve this problem would be great...

 

[KlausST]

Hi,

Use Ohm's law:
1mA on 100kOhms need a voltage of 100V.

Now you talk about AC....therefore I assume you mean 1mA RMS.
This is +/-1.414mA peak and thus +/-141.4V peak.

Now the Opamp needs some headroom. Therefore you need a supply voltage for the OPAMP of about +/-150V.
--> you will hardly find an Opamp for this high voltage.

--> you need to review your requirements.

Klaus

 

[s.h.bagherzdeh]

Thank you for response.
I am wondering even in lower load resistor around 1kohm this current source does not work.
Do I need to change the opamp,? Is there specific opamps IC work better than others?
Do I need to change Resistors design ?
Thanks

 

[crutschow]

Below is the LTspice simulation of your circuit showing a constant current output as the output resistance goes from 1 ohm to about 5.5k ohms.
It starts to clip above that point as limited by the power supply voltage.
I didn't have a model for the TL072 but the op amp shown has similar characteristics.

So I don't understand why your circuit is not working above 500 ohms(?).

if you wanted to go to a higher voltage for a higher load resistance, you could use an HV op amp such as an LTC6090.
Just be aware of the resistors' power dissipation at higher voltages.