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Field Effect Transistors (FETs) can be used as amplifiers, much like the Bipolar Junction Transistors (BJTs) studied earlier. The difference is instead of being current controlled, they are voltage controlled. FET’s are just a current source that are controlled by Vgs. The different configurations of FET amplifiers even have similar characteristics to their BJT counterparts. Before looking at the different configurations, lets look at what makes a good FET amplifier, and the formulas used.

The transconductance curve for a FET is a comparison of values of voltage from gate to source (Vgs) to values of drain current (Id). When Vgs is closest to Vgs(off), which means the transistor is no longer conducting, then Id is at a minimum. When Vgs = 0, then Id is at it’s maximum, Idss. Idss is the drain current with the source shorted, which under normal conditions, is the highest amount the transistor will let flow.

Vgs(off) you get Id at a minimum
Vgs = 0 you get Idss (Id maximum)

The different types of JFET’s and MOSFET’s are detailed in my other posts so I won’t go into it too much here. Until further mentioned, I am going to assume a mid-point biased JFET transistor. This gives a bit of distortion due to the transconductance curve, but is acceptable for some applications. When less distortion is needed, D-Mosfets, which operate in either depletion or enhancement mode, the area around Idss can be a fairly linear and therefore a good spot to bias for amplification. For now, I’ll just stick to JFET’s.

The main advantage to FET amplifiers is their high input resistance. Since the gate to source junction is reverse biased, it has as much input resistance as a reverse biased diode.

Gain is still and can always be defined as the ratio of Vout/Vin. In the case of a FET amplifier, it can also be defined as

Av = Vds/Vgs

Gain can also be determined using transconductance (gm) measured in Siemens (S) times the value of the drain resistor (Rd). Remember that in the case of a loaded amplifier, the drain resistor is parallelled with the load resistance (Rd = RD || RL).

Av = gmRd

gm0 is a value given on datasheets, and represents the value of gm measured at Vgs = 0. From this, you can calculate values of gm for different values of Vgs.

gm = gm0 (1-(Vgs/Vgsoff))

There are three main configurations. Common-Source, Common-Drain, and Common-Gate.

In FET Common-Source amplifiers, the DC and AC share a common point at the source of the transistor, and share a lot of the characteristics of the Common-Emitter BJT. The signal at the output has a 180 degree phase shift from the input, and some voltage gain.

In FET Common- Drain amplifiers, the DC and AC share a common point at the drain, and can be compared to BJT Common-Collector amps. Vout is in phase with Vin, and the voltage gain is ~1. They are current amplifiers, and are also referred to as source-followers.

In FET Common-Gate amps, the gate is the common point for DC and AC, and can be compared to Common-Base BJT’s. They have a low input resistance. Common-Gates are mostly used for high frequency circuits, often as the first stage, and sometimes connected directly to an antenna.

Electronics