FET input operational amplifier

Photocoupler

Junction Field-Effect Transistors (JFETs) are known for their extremely low input currents, making them ideal for use in the first stage of an operational amplifier, especially when operating at ambient temperatures below approximately 70°C. However, achieving a high-quality FET amplifier on a single chip is not a straightforward task, and as a result, there isn't a single FET amplifier available off-the-shelf today.

Despite this limitation, it's possible to build a high-performance FET amplifier by using a pair of JFETs in a monolithic circuit configuration. This approach allows for better control over performance characteristics and can significantly improve the overall quality of the amplifier.

In the circuit diagram provided, a matching JFET is connected to the op-amp as a composite source follower, placed right before the main amplification stage. This configuration helps maintain a high input impedance while ensuring good signal integrity.

The composite circuit achieves a gain similar to that of the integrated circuit itself, and a 30 pF capacitor is used to compensate for unity gain. This design eliminates the need for a separate summing amplifier with high common-mode rejection, which is typically required in other configurations. The reason for this is the use of a current source in the resistor network, combined with the inherent strong common-mode rejection capability of the FET.

This kind of setup is particularly useful in precision analog circuits where noise and distortion must be minimized. By leveraging the unique properties of JFETs, engineers can design more stable and accurate amplifiers, even in complex or high-temperature environments.