FET stages?

[loren]

What are FET stages?

[petemoore]

Probably gain stages, in series.
 A Jfet, or two [as in Mu Amp], some caps and resistors.
 Wired in a gain stage circuit [look at a 1 transistor booster circuit for examples], an active component [takes power to operate], like a JFet or other transistor will 'amplify' ie, more current and or voltage at the output [drain] than the input [gate].
 See GEO and look for Mu Amp, Jfet, GGG has a Stratoblaster schematic.

[Sir H C]

FET = Field Effect Transistor.

In general these are high input impedance devices compared to the standard BJT (bipolar Junction Transistor), and so you get the advantage of higher input impedance (DC).  They do not have the gain of bipolar transistors and usually have a worse noise response too.