Introduction
Linear Systems provides a variety of FETs (Field Effect Transistors) suitable for use in low noise amplifier applications for photo diodes, accelerometers, transducers, and other types of sensors. In particular, low noise JFETs exhibit low input gate currents that are desirable when working with high impedance devices at the input or with high value feedback resistors (e.g., ≥1MΩ). Operational amplifiers (op amps) with bipolar transistor input stages have much higher input noise currents than FETs. In general, many op amps have a combination of higher noise and input capacitance when compared to some discrete FETs. For example, a typical FET input op amp may have input capacitances of about 20 pF, whereas many discrete FETs have input capacitances of less than 5 pF. Also, there are few low noise FET input op amps that have equivalent input noise voltages density of less than 4 nV/ 𝐻𝑧. However, there are a number of discrete FETs rated at ≤ 2 nV/ 𝐻𝑧 in terms of equivalent Input noise voltage density. For those op amps that are rated as low noise, normally the input stages use bipolar transistors that generate much greater noise currents at the input terminals than FETs. These noise currents flowing into high impedances form added (random) noise voltages that are often much greater than the equivalent input noise.