Field Effect transistors - FETs, Comparison between FET & BJT
Field Effect transistors - FETs
First, why are we using still another transistor? BJTs had a small problem - the input signal was a current IB which was small but not that small. This was the control signal. Typically we would prefer that we control with a voltage directly.
This has an added benefit. If the device control only depends on a voltage signal we can design it so it draws little or NO current! This means that when we attach it to something like a thermocouple, it will not disturb the input since it is drawing no power. In other words the input impedance of the circuit is large.
Comparison between FET & BJT :
FET BJT
Gate Base
Drain Collector
Source Emitter
Gate Voltage Base current
Drain current Collector current
Drain-source voltage Collector-Emitter Voltage
The basic way to think of an FET is that there is a current ID = IS that is flowing through a channel that is controlled by a voltage VGS. Since the channel offers resistance to the flow it has a voltage drop VDS. These three parameters completely characterize the device.