Voltage control bidirectional constant current source

[DanioIO]

Hi,
I need to make a voltage control bidirectional constant current source for impedance measurement. Unidirectional circuit is simple one op-amp and a transistor, but i cannot pass direct current through my stuff (corrosion and other stuff) is there any circuit that can provide bidirectional current and still allow control using voltage?

[Andy Watson]

Does your current source float with respect to the load? If so - place the load in the feed-back loop of the op-amp. If not, look-up Howland current source.

[retrolefty]

So basically you are asking for an AC constant current source? Is their a specific frequency you require?

[DanioIO]

I would need about 100kHz.

[David Hess]

You did not say what current levels are required.

High frequency operation is going to make it more difficult to get high output impedance because of shunt capacitance.

Basic ways include the Howland current pump which you can search for online however the Howland design requires matched resistors.

Page 65 of Linear Technology application note 47 shows a simple and fast design.  A slower but more precision version may be found in the LT1102 datasheet.

For the highest performance, I would use a pair of singled ended current sources.  If one fixed current source is placed in parallel with a variable current source of the opposite polarity that goes from zero to double the current, then the output will be a bidirectional current.  Cascode output stages can then be used to raise the output impedance and operation into the 10s or 100s of MHz will be possible.