JFET

Consider a hose with water pressure on one side (voltage analog) and a water turbine on the other making electricity. You turn on the water and the turbine turns and out come the electricity.....too much electricity, so you squeeze the hose with a pair of pliers to get the correct water flow for the amount of load.

If the pressure is too high the hose ruptures ( peak voltage analog)

Amount of water is the current carrying analog, and so on

The pliers are the JFET

http://en.wikipedia.org/wiki/JFET

Sorry Aurizon -- I didnt read to the very bottom of your post. and I really meant to reply to the original question not your response. I think I need to pay closer attention when I respond.

Try this explanation

http://en.wikipedia.org/wiki/JFET

This is the one which I too found at first. It is also quite basic and simple to understand the whole concept.

Good analogy aurizon. In more technical terms, think of the JFET as a resistor between the source and the drain. In an "N" channel JFET the resistor forms a channel for electrons to traverse. In-between the source and drain is formed a diode junction that is the gate. The JFET is known as a depletion mode device so you apply a negative bias to the gate with respect to the negative source terminal to control the current in the resistive channel between the -source and the +drain. The type of material N & P that form the body and gate determine if it is an "N" or "P" channel JFET and the polarity of the diode that forms the gate. Although the source and drain can be reversed in theory, in actual devices the gate is usually closer to the source and the work better if used that way. At a certain negative voltage (reverse bias of the gate diode junction) you can completely "pinch off" the current flow between source and drain. The maximum current that the JFET will carry is at zero bias and typically varies an order of magnitude for each device type due to difficulties in manufacturing so often they require trimming to work well and this makes them a bit more difficult to use in precision circuits. They are great for very high input impedance amplifiers since the gate is in effect a reverse biased diode junction. They also make very good low leakage diodes. You should be able to find more information at manufacturer's websites.

Aurizon's comments were very good. A FET is used for a high impedance input for an amplifier. It is the solid-state equivalent of a vacuum tube. Woops, I'm showing my age. You probably never heard of a vacuum tube, did you?

Hahaha, yep, i know. Father still keeps the books based on them. Nostalgia i guess,