精密电流源：Precision Current Source

精密电流源：Precision Current Source
The precision current source is shown in Figures 17, 18. The configurations shown will sink or source conventional current respectively.
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.
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 17, 18 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.

图17、18分别给出了精密电流源电路在吸入和输出电流两种情况下的电路结构。
在实际应用本电路的时候必须注意，（OP输出端的）电压范围为外部晶体管的BVCER到比输入电压VIN低约1V（多）。
从原则上说，电流源电路在小电流状态下内阻是无穷大的。在VIN大于VOS，并且IO比OP的偏置电流大得多的情况下，可以认为输出电流是精确的。
图17、18中使用了一个FET来驱动输出级的BJT。当需要大的输出电流的时候，可以选用达林顿来取代这个输出电路。并且其（栅极）输入电流不会对电路带来显著的误差。
本电路使用的OP必须对单位增益进行补偿。并且根据负载特性和输出级电路的特性可能需要额外的补偿。