convert voltage to current

This does not make sense !!

Please explain what you really need.

thanks

donald

======================= Resistor in feedback loop of an opamp voltage follower?

He's talking about a transconductance amplifier, which has a voltage input and current output. You can make discrete circuits that do this, as Tony Williams and I have written about here on s.e.d., but an transconductance IC would be much easier. Burr-Brown offers OTA ICs, or operational transconductance amplifiers. The opa660 was a favorite of mine, because of its current programmability, but TI has just replaced it with the opa860 and opa861 parts.

They have a raft of application notes, which are most easily found by searching on opa660. I find that OPA ICs can be very useful in making precision integrators, ramps, log converters, gyrators, etc., but Burr-Brown's app notes concentrate on making amplifiers, etc., which to my mind is better done with ordinary opamps.

An op amp with feedback off a current sensing resistor. D from BC

In addition to OTA, one can use a pair of current mirrors, op amp, and sense resistor.

Feed program voltage to (+) input of op amp; connect (-) input to output, and run a resistor R from output to ground. Input of current mirror (two matched transistors, or three if you want to be fancy) connects to each of the power leads to the op amp. Connect both the current mirror outputs to the load, and of course feed the current mirrors from appropriate +/- power supplies.

Compliance should be within a volt of power supply, bipolar.

The idea is that the imbalance in the current from the op amp power pins is due to the output current (in that sense resistor R). The sum of the current mirrors is equal to the difference of those two currents, thus equal to the current in the sense resistor, which has GND at one end and your signal (program voltage) at the other (because the op amp is configured as a voltage follower of the input signal).

Alas, it isn't just 'an IC'; it takes at least two chips.