Mumbai University > FE > Sem 1 > Basic Electrical and Electronics Engineering

Marks: 4 M

Year: Dec 2013

In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as $Ie = Ic + Ib.$

As the load resistance (RL) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib. A transistors current gain is given the Greek symbol of Beta, (β).

As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of α. Note: that the value of Alpha will always be less than unity.

Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current ( Ib ), will result in a much larger change in the collector current ( Ic ).

Then, small changes in current flowing in the base will thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors.

By combining the expressions for both Alpha, α and Beta, β the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as:

$$Alpha,(\alpha)=\dfrac{I_C}{I_E} \ \ \ and \ \ \ Beta,(\beta)=\dfrac{I_C}{I_B} \\ \therefore I_C=\alpha.I_E=\beta.I_B \\ as: \alpha=\dfrac{\beta}{\beta+1} \ \ \ \ \beta=\dfrac{\alpha}{1-\alpha}$$