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FIELD EFFECT TRANSISTOR (FET)
OPERATION
An FET is a type of transistor. FETs are semiconduc-
tors well suited for high-frequency switching.
Drawing A above shows an FET in a passive mode.
There is no gate signal, zero volts relative to the source
and, therefore, no current flow. The drain terminal of
the FET may be connected to a voltage supply; but
since there is no conduction, the circuit will not supply
current to downstream components connected to the
source. The circuit is turned off like a light switch in the
OFF position.
Drawing B above shows the FET in an active mode.
When the gate signal, a positive DC voltage relative to
the source, is applied to the gate terminal of the FET, it
is capable of conducting current. A voltage supply con-
nected to the drain terminal will allow the FET to con-
duct and henceforth supply current to downstream
components. Current will flow through the conducting
FET to downstream components as long as the gate
signal is present. This is similar to turning on a light
switch.
THEORY OF OPERATIONS
E-8 E-8
INVERTEC STT
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SOURCE
TERMINAL
GATE
TERMINAL
DRAIN
TERMINAL
DRAIN (N)
N CHANNELSUBSTRATE (P)
SOURCE (N)
(0 VOLTS)
GATE
TERMINAL
(+ 6 VOLTS)
DRAIN (N)
ELECTRONS
SOURCE (N)
B. ACTIVE
A. PASSIVE
FIGURE E-8 – FIELD EFFECT TRANSISTOR OPERATION