FET Input for Op-Amp
Junction field effect transistors can give exceptionally low input currents when they are used on the input stage of an operational amplifier for ambient temperatures less than about 70’C. However, since it is no simple matter to diffuse high quality FET’s on the same chip as the amplifier , there is no monolithic FET amplifiers available now. Nonetheless, create a good FET amplifier using a discrete FET pair in conjunction with a monolithic circuit is possible. In circuit diagram below, there is a matched FET pair connected as source followers placed in front of an integrated operational amplifier.
The gain of the composite circuit has roughly the same as the integrated circuit by itself and for unity gain with a 30 pF capacitor is compensate. The circuit leaves something to be desired in applications requiring high common mode rejection although it works well as a summing amplifier. The reasons why this is happens are because resistors are used for current sources and because a good common mode rejection is not available on this FET. [Source: National Semiconductor Application Note]