Common-emitter configuration
The most frequently encountered transistor configuration is commonemitter configuration.
As in CB configuration the emitter current is:
IE = IC + I B
and
I C = αI E
For the C-E configuration the input characteristics represented by a plot
of the input current (IB) versus the input voltage (VBE) for a range of
values of output voltage (VCE).
The output characteristic is a plot of the output current (IC) versus output
voltage (VCE) for a range of values of input current (IB).
The magnitude of IB is in microamperes, compared to milliamperes of IC.
VCE will influence the magnitude of IC.
- Active region is represented by the area where IC is directly
proportional to IB . In the active region of a common-emitter amplifier
the collector-base junction is reverse - biased, while the base-emitter
junction is forward-biased. In this region, the transistor is mainly used as
an amplifier.
- Saturation region is represented by the area where IC is not proportional
to IB. This region represents the area for VCE ≤ VCEsat
- Cut off Region is represented by the area below IB = 0. In both
saturation region and cutoff region the transistor is mainly used as
electronic switch.
For the CB configuration, when the input current IE was equal to zero,
the collector current was equal only to the reverse saturation current
ICBO, so that the curve IE= 0.
As in common-base configuration, for the common-emitter
configuration the same approach for the value of VBE can be taken,
resulting in the approximate equivalent of Figure below.
Example:
(a) Using the output characteristic curve, determine IC at IB=30 µA and
VCE=10 V.
(b) Using input and output characteristic curves, determine IC at
VBE=0.7 V and VCE =15 V.
Solution:
(a) At the intersection of IB= 30 µA and VCE = 10 V, IC = 3.4 mA.
(b) Using the input characteristic curve, IB = 20 µA at VBE = 0.7 V.
From output characteristic curve we find that IC = 2.5 mA at the
intersection of IB = 20 µA and VCE = 15 V.
Beta (β)
In the dc mode the levels of IC and IB are related by a quantity called
beta and defined by the following equation:
The level of typically ranges from about 50 to over 400.
A relationship can be developed between β and α using the basic
relationships introduced thus far.
Using
β = IC/IB
we have
IB = IC/β
and from
α = IC/IE
we have
IE = IC/α.
Substituting into
………(1)
………(2)
Then by substitution (2) in (1)
In addition, recall that
as indicated on output characteristic curve, Beta is a particularly
important parameter because it provides a direct link between current
levels of the input and output circuits for a common-emitter
configuration. That is,