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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.

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    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.Conventional Current Notations in a Common Emitter BJT Configuration, Image  

    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:


    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).

    Silicon Transistor Collector Characteristics in the Common Emitter Mode, Image   BJT Common-Emitter Configuration, Base Characteristics, Image   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.


    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.

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    Common-Emitter Example Circuits

    Rain Sensor: This circuit can be used as a rain indicator and serves as a nice little rain warning device.Simple Rain Sensor Circuit Diagram, Image  

    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.Simple Delay-On Timer Circuit Diagram, Image  

    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.Simple Day Night Switch Circuit Diagram, Image  

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    Book: Electronic Devices and Circuit Theory By Robert L. Boylestad and Louis Nashelsky.

    Department of Semiconductor and Optoelectronic Devices: