精密電流源：Precision Current Source

精密電流源：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.

圖17、18分別給出了精密電流源電路在吸入和輸出電流兩種情況下的電路結(jié)構(gòu)。
在實際應(yīng)用本電路的時候必須注意，（OP輸出端的）電壓范圍為外部晶體管的BVCER到比輸入電壓VIN低約1V（多）。
從原則上說，電流源電路在小電流狀態(tài)下內(nèi)阻是無窮大的。在VIN大于VOS，并且IO比OP的偏置電流大得多的情況下，可以認為輸出電流是精確的。
圖17、18中使用了一個FET來驅(qū)動輸出級的BJT。當(dāng)需要大的輸出電流的時候，可以選用達林頓來取代這個輸出電路。并且其（柵極）輸入電流不會對電路帶來顯著的誤差。
本電路使用的OP必須對單位增益進行補償。并且根據(jù)負載特性和輸出級電路的特性可能需要額外的補償。