FET’s – FEILD EFFECT TRANSISTORS
FET’s are another one of those mysterious little black things you see on your circuit boards. There are a few varieties, so I’ll keep it simple and work up from there.
FET’s work like a bipolar transistor. They use a small signal to control a bigger one. The difference is that FET’s use a small voltage instead of a small current. Fets have some nice advantages over transistors, no (very low) drive current, lower noise, better power handling, lower distortion. Their disadvantage is that they can
be easily broken if mistreated. Especially static electricity.
To think of it in terms of water. A fet is like a piece of garden hose that will let water flow in it. As we squeeze the hose, it restricts the flow in the hose. The more we squeeze, the less water flows. This is what happens as we apply a voltage to the gate of a fet.
JFET’s. Junction FETs.
There are two types of fet, just like PNP/NPN transistors.
P-Channel and N-Channel. Again, Arrow points “iN” for an N channel.
JFETs are the simplest type of FET.
Lets look at a P-Channel.
The leads on a FET are called the GATE, SOURCE and DRAIN.
The Gate of the FET is zero volts (or open circuit) the JFET allows current to flow from it’s +source to the -drain. As the voltage rises
(positive voltage) on the gate, it restricts the flow drain-source and the current drops.
The reverse is true for an N-Channel.
Current will flow if there is no voltage on the gate.
As the voltage drops (negative voltage) the less current will flow from the +drain to the -source.
*The gate of a fet must not be reverse voltage (bias) as it effectively provides short circuit from the drain and it’s goodbye fet ! :dead:
Just as a short note, vacuum tubes that you find in old radios work just like fets, they just require a bit more power and space. 😯
MOSFETs – Metalic Oxide Semiconductor Field Effect Transistors.
Mosfets or IGFETS (Insulated Gate FET) work in pretty much the same way as a JFET, but just have a bit more fancy way of doing it.
Mosfets can decrease the flow of current between the source and drain just like a JFET can, But they can also increase the flow !
Thinking of our piece of garden hose, we can squeeze it to make the
flow smaller (depletion), but now we can expand it and let it more water flow (enhancement).
There are more combinations of fets but it is unlikely that I’ll be able to describe how the hell they work without having to consult my text books and a large dose of caffine ! It is unlikely you’re going encounter them soon.
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