To continue the metaphor, the voltage of the supply is the water pressure, and the water
flowing from the supply is the electrical current. The Source (S) connection of the FET is
the output of the faucet. The Gate (G) of the FET is the control pin, and like the handle on
the faucet, it controls the amount of electrical current through the FET from the Drain to the
Source. For the FET, this control is a function of the relative voltage between the Gate and
Source pins. For an N channel FET, raising the Gate positively with respect to Source
increases the current flow.
Yes, I know some of you are thinking that maybe the Source should be on top and the Drain
on the bottom, but they’re not. You might not want to call a quantum mechanic if your
plumbing stops up.
The idea that the current going through FET transistor is controlled by the voltage between
the Gate and the Source pins remains the key idea, and if you have that firmly fixed, we can
leave the waterworks metaphor behind.
FETS come in different types. There are two polarities, N channel and P channel. There
are different voltage, current, and power ratings, and different semiconductor processes
resulting in JFETs and MOSFETs. In all of them, the current from the Drain to Source is
controlled by the voltage between Gate and Source.
A FET is a three pin device, and there are three ways to amplify with them. Figure 2
illustrates these with an N channel FET:
Common-Source (CS) is the connection which can give us both voltage and current gain in
a circuit. The input voltage (shown as a little graphical sine wave) goes to the Gate, and the
output is taken from the Drain and appears across a resistor between the Drain and the
supply voltage. The Source is grounded, and doesn’t show a signal voltage, and that’s why
it’s called Common-Source. Note that the output voltage is inverted in phase from the input
voltage.
Common-Drain (CD) gives current gain only, and is also known as Source follower,
because the output voltage across the Source resistor is nearly identical to the input voltage
at the Gate. While the Drain is usually attached to a DC voltage value, the AC voltage is
ideally zero, and so it is called Common-Drain.
Common-Gate gives non-inverted voltage gain only, with the input signal going into the
Source and coming out the Drain. The Gate is grounded.
Figure 2 only shows what happens to AC signals, but it doesn’t illustrate the DC voltage and
current values that the FETs need in order to operate. These DC values are often referred
to as the bias of the device, and you will hear that word a lot with respect to amplifiers.
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