2.7 Summary

JFETs can be used to create both voltage amplifiers and voltage followers. The common source configuration is similar to the BJT’s common emitter configuration. It offers voltage gain with signal inversion. The amplifier can be built upon any of the bias schemes presented in the preceding chapter. Biasing circuits that made use of a source resistor, such as self bias and combination bias, may also use swamping. Swamping will decrease available voltage gain but reduce distortion.

The JFET voltage follower, or source follower, is similar to the BJT’s emitter follower. It offers a voltage of gain of nearly unity without inversion, a high input impedance and a low output impedance.

In general, JFETs do not offer as high of a gain as BJTs. The parameter comparable to the BJT’s 𝑟′_𝑒 is the transconductance, 𝑔_𝑚. Further, they tend to offer very high input impedance values compared to BJTs. This is due to using a reverse biased junction instead of a forward biased junction.

JFETs can also be used in their ohmic region to create voltage-controlled resistances and analog switches. A key parameter in these applications is the minimum channel resistance, 𝑟_{𝐷𝑆(𝑜𝑛)}.