BJT-IntroductionThe 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 derivedfrom 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.

Common-Base ConfigurationThe arrow in the graphic symbol defines the direction of emitter current (conventionalflow) through the device.

BI

CI

EI

The emitter base junction is forward biased while collector base junction is reversed biased

minorityCOI

majorityCICI

Input Characteristics

• 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 0.7V

Output Characteristics

• 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 forward-biased.

• VCB has negligible effect on collector current for the active region

• 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

EI

CI

• 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 reverse-biased.

• 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,

EI

CIα dc

Alpha () in the AC mode:

EI

CIαΔ

Δac

References

• http://en.wikipedia.org/wiki/Bipolar_junction_transistor

• Electronic devices and circuit theory – Robert Boylested and Louis Nashelsky, 7th edition, Printice Hall publishers