mosfet as a switch
MOSFET’s make
very good electronic switches for controlling loads and in CMOS digital
circuits as they operate between their cut-off and saturation regions. We saw previously, that
the N-channel, Enhancement-mode MOSFET (e-MOSFET) operates using a positive
input voltage and has an extremely high input resistance (almost infinite)
making it possible to interface with nearly any logic gate or driver capable of
producing a positive output.
We also saw that due to
this very high input (Gate) resistance we can safely parallel together many
different MOSFETS until we achieve the current handling capacity that we
required.
While connecting
together various MOSFETS in parallel may enable
us to switch high currents or high voltage loads, doing so becomes expensive
and impractical in both components and circuit board space. To overcome this
problem Power Field-Effect
Transistors or Power FET’s where developed.
We now
know that there are two main differences between field-effect transistors,
depletion-mode only for JFET’s and both enhancement-mode and depletion-mode for
MOSFETs. In this tutorial, we will look at using the Enhancement-mode MOSFET as a Switch as these transistors require a
positive gate voltage to turn “ON” and a zero voltage to turn “OFF” making them
easily understood as switches and also easy to interface with logic gates.
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