FETinput

FETinput refers to input stages in electronic devices that use field-effect transistors (FETs) at the input. It is a common designation in datasheets and technical literature to denote high input impedance and low input bias currents. FETinput devices include operational amplifiers, buffer amplifiers, and instrumentation amplifiers employing JFET or MOSFET input stages. The term contrasts with bipolar-transistor (BJT) input designs, which typically offer lower input impedance and higher bias currents.

Characteristics of FETinput stages include very high input impedance, typically in the megaohm to gigaohm range, which minimizes loading of the signal source. Gate bias currents are correspondingly very small, often in the picoampere to nanoampere range. JFET-based inputs generally exhibit low 1/f noise and moderate input capacitance, while MOSFET-based inputs can achieve higher impedance but may present higher voltage noise and greater input capacitance. Temperature affects bias current and offset voltage, and leakage currents or gate-source leakage can influence performance. Input protection considerations, such as ESD and gate protection diodes, are also important.

Applications for FETinput designs include precision instrumentation, high-impedance sensor interfaces, neuroscience and biomedical instrumentation, and audio preamplification where signal integrity must be preserved. They are common in buffer stages, instrumentation amplifiers, and certain chopper-stabilized front ends, where very small signals would be distorted by higher input currents or loading.

In practice, FETinput is a design characteristic rather than a separate device class. Real-world performance depends on whether the input stage uses JFET or MOSFET devices, as well as the overall circuit topology, including noise, offset, drift, and protection considerations.