Through proper design transistors
can be used as switches for computer and control
applications.
When the input voltage VB is high
( logic 1), the transistor is in saturation ( ON). And
the output at its collector = VCE
is almost 0V( Logic 0)
Transistor as a
switch
When the base voltage VB is low(
logic 0), i.e, 0V, the transistor is cutoff( Off) and IC
is 0, drop across RC is 0 and
therefore voltage at the collector is VCC.( logic 1)
Thus transistor switch operates
as an inverter.
This circuit does not require any
DC bias at the base of the transistor.
Design
When Vi ( VB) is 5V, transistor
is in saturation and ICsat
Just before saturation, IB,max =
IC,sat /βDC
Thus the base current must be
greater than IB,max to make the transistor to work in
saturation.
Analysis
When Vi = 5V, the resulting level
of IB is
IB = (Vi – 0.7) / RB
= ( 5 – 0.7) / 68k
= 63μA
ICsat = VCC / RC = 5/0.82k
= 6.1mA
Verification
( IC,sat / β) = 48.8μA
Thus IB > ( IC,sat / β) which
is required for a transistor to be in saturation.
A transistor can be replaced by a
low resistance Rsat when in saturation ( switch on)
Rsat = VCE sat/ ICsat (VCE sat is
very small and ICsat is IC,max is maximum current)
A transistor can be replaced by a high resistance
Rcutoff when in cutoff ( switch on)
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