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A
switch
is a device that is used to 'open' or 'close' a circuit. Opening a
circuit means creating a break in the circuit, preventing current
flow and thus, turning it 'off'. Closing a circuit, on the
other, means completing the circuit path, thereby allowing current
to flow around it and thus, turning it 'on'.
The
bipolar transistor, whether NPN or PNP, may be used as a switch.
Recall
that the bipolar transistor has three regions of operation: the
cut-off region, the linear or active region, and the saturation
region. When used as a switch, the bipolar transistor is
operated in the cut-off region (the region wherein the transistor is
not conducting, and therefore makes the circuit 'open') and
saturation region (the region wherein the transistor is in full
conduction, thereby closing the circuit).
The
bipolar transistor is a good switch because of its large
transconductance Gm,
with
Gm = Ic/Vbe where Ic is the
collector-to-emitter (output) current and Vbe is the base-emitter
(input) voltage. Its high Gm allows large collector-to-emitter
currents to be easily achieved if sufficient excitation is applied
at the base.
To
illustrate this, the simplest way to use an NPN bipolar transistor
as a switch is to insert the load between the positive supply and
its collector, with the emitter terminal grounded (as shown in
Figure 1). Applying no
voltage at the base of the transistor will put it in the
cut-off
region, preventing current from flowing through it and through the
load, which is a resistor in this example. In this state, the load is 'off'.
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Figure
1. A Simple Switch Using an NPN Transistor
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Applying
enough voltage at the base of the transistor will cause it to
saturate
and become fully conductive, effectively pulling the collector of
the transistor to near ground. This causes a
collector-to-emitter current to flow through the load that's limited
only by the impedance of the load. In this state, the load is 'on'.
One
limitation of this simple design is that the switch-off time of the
transistor is slower than its switch-on time if the load is a
resistor. This is because of the stray capacitance across the
collector of the transistor and ground, which needs to charge
through the load resistor during switch-off. On the other
hand, this stray capacitance is easily discharged to ground by the
large collector current flow when the transistor is switched on.
There are, of course, other better designs for using the bipolar transistor
as a switch.
See Also:
Bipolar Transistor;
More BJT Switch Circuits;
Analog Electronics; More
Articles
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