Apex Microtechnoly ~ power amplifiers as a precision voltage controlled current source ...
It is possible to use a power amplifier as a precision voltage controlled current source.
... and there are several possible design options!
The simplest design involves a small value sense resistor outside the feedback loop that turns the controlled voltage into a current. Then, a floating load is placed in series with the resistor so it receives the same precision current.
With this configuration, the load cannot be connected to ground, and the sense resistor must be as close to a pure resistor as possible. Also, the load becomes a part of the feedback loop which can affect the stability of the system.
A second, more stable option, is the Improved Howland Current Pump circuit. This differential amplifier measures across a sense resistor using both positive and negative feedback, and it allows the load to be grounded.
The benefit to using this circuit is that it is incredibly stable and it usually does not require external compensation. Keep in mind that the gain-setting resistors must be closely matched (better than 0.1%) and again, that sense resistor should be purely resistive (keep those traces short!).