Posted on Dec 10, 2012 7715
This current source has continuous control of the magnitude and polarity of its amplifier gain and needs only one voltage reference. The circuit includes reference ~, voltage-amplifier circuit A1 with gainsetting resistor Rs, and bootstrap-follower amplifier A2. The bootstrapping converts the circuit to a current source and allows the load to be grounded.
Any voltage developed across load ZL feeds back to the reference and voltage amplifier, making their functions inunune to that voltage. Then the current -source circuitry floats, instead of the load. The voltage reference is connected to both the inverting and noninverting inputs of A1; this provides a balanced combination of positive and negative gain. The inverting connection has equal feedback resistors, R, for a gain of -1, and the noninverting connection varies according to the fractional setting, X, of potentiometer R vÂ· X controls the noninverting gain and adjusting it counters the effect of some of the inverting gain. The value of X is the portion of Rv"s resistance from the noninverting input of A1 to the temporarily grounded output of A2. Between potentiometer extremes, the current varies with X Â±1 mA.