A bipolar junction transistor consists of a collector, emitter and base. A large percentage of the current enters the collector terminal, goes through the base region and exits from the emitter. A smaller percentage of current enters the base region and exits through the emitter. The collector current increases slightly with an increase in voltage. This slight dependence of collector voltage on collector current is called the "Early effect." The Early voltage is the distance between a point chosen on the I-V characteristic curve and the intersection of its tangent and the horizontal axis.
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Determine the majority carrier charge in the base or "Qb" using the formula: Qb = qAcNb*wb where "wb" is the quasi-neutral base width, "Ac" is the collector area, "Nb" is the base doping and "q" is the magnitude of electric charge on the electron. Doping introduces either electrons or holes into the silicon crystal lattice which in turn introduces mobile charge. An increase in doping levels increases conductivity. A narrower base width indicates a larger charge gradient and an increase in collector current.
Divide the value obtained for "Qb" with the base collector capacitance to obtain the value for "Va" or the Early voltage using the formula: Va = Qb/Cj,bc. "Cj,bc" is the base collector junction capacitance. Early voltage ranges between 15 and 150 volts depending on the type of device.
Measure the distance between a point on the I(collector current)-V(collector-emitter voltage) characteristic curve and the intersection of its tangent with the horizontal axis to graphically calculate the Early voltage value.
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