Easy Howland Current Source
The Difference Amplifiers from a number of vendors can be deployed as a precision current source -- with Linear's LT1990 you only need a sense resistor. However, the LT1990 is also limited to about with f3 of 100kHz. This circuit uses the Analog AD8276 which has an f3 of 550khz -- but requires another resistor. Vin/R1 = I out. In this case, I show 10mA which is about the limit for the AD8276.
Diff amps are great in this application since the resistors are cut with high precision:
Or you can do a boosted current source -- I've done this with the LT1990 -- and the driver transistors are on hand:
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