The transistor is a three-layer semiconductor device
consisting of either two n- and one p-type layers of
material or two p- and one n-type layers of material.
The former is called an npn transistor, while the latter
is called a pnp transistor
The common-base terminology is derived
from the fact that the base is common to both the
input and output sides of the configuration.
In addition, the base is usually the terminal closest
to, or at, ground potential.
BJTs can be used as amplifiers, switches, or in oscillators.
The arrow in the
defines the direction
of emitter current
flow) through the
The emitter base
junction is forward
biased while collector
base junction is
• For fixed values of collector voltage(Vcb), as the
base-to-emitter voltage increases, the emitter
current increases in a manner that closely
resembles the diode characteristics.
• Ignoring the effects of changes in VCB and changes
in slope of the graph and then applying piece-
wise linear model we get the approximation VBE
=0.7V i.e. once a transistor is in the “on” state,
the base-to-emitter voltage will be assumed to be
• In the active region, the BJT is used as an amplifier.
• In the active region the collector-base junction is
reverse-biased, while the base-emitter junction is
• VCB has negligible effect on collector current for the
• As the emitter current increases above zero, the
collector current increases to a magnitude essentially
equal to that of the emitter current as determined by
the basic transistor-current relations
• The curves clearly indicate that
• The current Ico is so small (microamperes) in magnitude
compared to the vertical scale of Ic(milliamperes) that it
appears on virtually the same horizontal line as Ic =0
• The cutoff region is defined as that region
where the collector current is 0 A.
• In the cutoff region the collector-base and
base-emitter junctions of a transistor are both
• The saturation region is defined as that region
of the characteristics to the left of VCB =0 V
• In the saturation region the collector-base and
base-emitter junctions are forward-biased.
• Alpha ( ) is a quantity that relates IC and IE
• Even though the characteristics would suggest
that =1, for practical devices the level of alpha
typically extends from 0.90 to 0.998
• For most situations the magnitudes of ac and
dc are quite close, permitting the use of the
magnitude of one for the other.
• The ac alpha is formally called the common-base,
short-circuit, amplification factor,
Alpha ( ) in the AC mode:
• Electronic devices and circuit theory – Robert
Boylested and Louis Nashelsky, 7th edition,
Printice Hall publishers