Understanding Simple Transistor Circuits that use the Emitter
as the Common Termination
written by: Swagatam • edited by: Lamar Stonecypher • updated: 9/7/2011
Bipolar junction transistors (BJTs) are probably the most extensively used devices in electronic circuits and within
the ICs, and the most common configuration which are employed with them is the common-emitter mode. Here we
find out how it's evaluated using derived formulas and circuit diagrams.
Deriving Common-Emitter Modes in BJTs
Probably the most commonly and widely implemented transistor configuration in most electronic circuits is the
common emitter mode, which is called this because hhe emitter lead of the transistor becomes the common
reference to both the input and the output of the transistor, i.e. the input trigger at the base and the output signal from
the collector are both with respect to the emitter’s potential.
The figure shows the actual conventional paths or the directions of current flow through the base, emitter, and the
collector terminals of a transistor connected in the common emitter mode.
The achieved relation between the base, emitter, and the collector currents can be written as:
Ie = Ic + Ib and Ic = αIe,
Where α is the magnitude which relates the levels of collector and emitter currents in DC mode due to majority
carriers, and is expressed as:
α(dc) = Ic/Ie
where Ic and Ie are the current levels at the point of operation. Although normally the value of α is assumed to be 1,
practical devices typically exhibit alpha levels from 0.9 to 0.99 (mostly toward the higher range). Since this
primarily involves majority carriers, is given as:
Ic = αIe + I(cbo)----------------------------------------------------------------------------(1)
The plotted graph shows typical common-emitter characteristic curves of the output current (Ic) against the output
voltage (Vce) for different input current ranges (Ib).
The second graph (on the right) shows the plotted characteristics of the input current (Ib) against the input voltage
(Vbe) for different output voltage ranges (Vce).
We can see that the curves are not completely horizontal, which indicates the affecting tendencies of
collector/emitter voltage over the magnitude of collector current.
We can also witness the upper right portion of the quadrant to be fairly linear, which actually becomes the active
region of a typical common-emitter transistor configuration. The region almost produces a straight line and also
looks pretty uniformly spaced.
Therefore, with common-emitter transistor amplifiers the collector-base junction in the active region is always
reverse-biased.
Interestingly for common-base configurations the collector-base junction is reverse biased in the active region while
the base-emitter is forward-biased.
For common-base configurations Ic ~ Ie---------------------------------------------(2)
The difference in the collector characteristics may be analyzed through proper evaluation of the expressions (1) and
(2):
As per equations (1) and (2) Ic = αIe + I(cbo) and Ic ~ Ie
Substituting and rearranging the terms appropriately gives:
Ic = αIb/(1 - α) + I(cbo)/(1 - α)
Considering the case where Ib = 0A and α typically = 0.996, the consequent collector currents yields the following
result:
Ic = α(0)/(1 - α) + I(cbo)/(1 – 0.996)
= I(cbo)/0.004 = 250I(cbo)
Also, if I(cbo) would be 1µA, with Ib = 0A, the yielding collector current would be 250(1µA) = 0.25 mA (refer
graph).
Tired of boring evaluations? Relax, the following section shows some cool circuits involving some practical
common-emitter configurations that you can build.
Diagrams: Drawn By Swagatam - Courtesy: Electronic Devices and Circuit Theory By Robert L. Boylestad and Louis Nashelsky.
Common-Emitter Example Circuits
Rain Sensor: This circuit can be used as a rain indicator and serves as a nice little rain warning device.
Delay-On Timer: Another common-emitter sample circuit shown below may be used to produce short time delays
and can be used in places where delay-On feature is required.
Day-Night Switch: This important common-emitter configuration finds useful application as a light activated switch
and can be used as a cheap automatic day-night switch for activating porch lights, street lights etc automatically
when dusk falls.
Source:
http://www.brighthubengineering.com/diy-electronics-devices/117387-understanding-simple-transistor-circuitsusing-emitter-as-common-termination/