2.1 JFET Small Signal Amplifiers

The JFET can be used to create both voltage amplifiers and voltage followers. In comparison with the BJT, the JFET tends to have less voltage gain potential. On the other hand, JFET circuits offer the possibility of a much higher input impedance, lower noise and better high frequency performance. There are many other similarities with BJT amplifiers. For example, the possibility of swamping still exists as a means of lowering distortion at the expense of voltage gain. Also, the JFET voltage amplifier inverts the signal, just like the BJT version. When it comes to AC analysis, a key element for the BJT is 𝑟′_𝑒. For the JFET, the comparable parameter is transconductance, 𝑔_𝑚.

JFET amplifiers and followers can be used with their BJT cousins. Indeed, the combination of the two, each playing to their strengths, has the potential to outperform a design using only one type of device.

Alongside their use in amplifiers and followers, JFETs can also be used in their ohmic region. This includes applications as voltage-controlled resistors and analog switches. In this mode, the device no longer behaves as a constant current source. Instead, the channel resistance becomes a function of the gate-source voltage and can be used as a control element within a voltage divider. As such, it has the capability of changing resistance value much faster than a mechanical potentiometer.