Precision Current Source
The precision current source is shown in Figures 1, 2. The configurations shown will sink or source conventional current respectively.
Figure 1. Precision Current Sink
Caution must be exercised in applying these circuits. The voltage compliance of the source extends from BVCER of the external transistor to approximately 1 volt more negative than VIN. The compliance of the current sink is the same in the positive direction.
Figure 2. Precision Current Source
The impedance of these current generators is essentially infinite for small currents and they are accurate so long as VIN is much greater than VOS and IO is much greater than I bias.
The source and sink illustrated in Figures 1, 2 use an FET to drive a bipolar output transistor. It is possible to use a Darlington connection in place of the FET-bipolar combination in cases where the output current is high and the base current of the Darlington input would not cause a significant error.
The amplifiers used must be compensated for unity-gain and additional compensation may be required depending on load reactance and external transistor parameters.
15358 10 December 2007
The circuit gives the highest input impedance of any operational amplifier circuit.