The flow analogy can also be used to model how a transistor operates in a
logic circuit. The transistor is an amplifier. It uses a small amount
of energy to control a larger energy source, just as a valve controls a
high-pressure water source. There are two kinds of transistors: bipolar
and field-effect transistors (FETs).
HPK taruh disini
We will look at bipolar
transistors first; these amplify current. A small amount of current
flows in the control circuit (the transistor base emitter circuit) to
turn the transistor on. This control current is amplified (multiplied by
the gain or beta of the transistor) and allows a larger current to flow
in the output circuit (the collector emitter circuit). Once again, the
device is not perfect because of the resistance, current, gain, and
leakage limitations of real transistors. Bipolar transistors come in two
polarities, NPN and PNP, with the difference being the direction in
which current flows for normal operation. A bipolar PNP transistor is
shown and modeled in Figure 1-7.
For most of the illustrative circuit examples in this site, we will be using NPN transistors, as shown in Figure 1-8.
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