Posted on Dec 10, 2012

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.

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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 v· 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 ±1 mA.

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