FET input operational amplifier

Photocoupler

Junction Field-Effect Transistors (JFETs) are known for their ability to deliver 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 high-quality FET amplification on a single chip is not straightforward. As a result, there isn't currently a single FET amplifier that can match the performance of more complex integrated designs. Nevertheless, it's possible to construct a high-performance FET amplifier by using a split JFET pair within a monolithic circuit.

In the circuit diagram shown below, a matching JFET is connected to the op-amp as a composite source follower placed at the front end. This configuration helps improve the overall performance and stability of the amplifier system.

The gain of this composite circuit closely matches that of the integrated circuit itself, and unity gain is compensated using a 30 pF capacitor. This design eliminates the need for a summing amplifier that typically requires high common-mode rejection. The reason for this lies in the current source used in the resistor network, which works effectively with the FET due to its excellent common-mode rejection ratio. This makes the FET-based amplifier a reliable and efficient choice in many applications.