Question

The current gain $\alpha$ of a transistor is defined as
A. the ratio of change in collector current to the change in emitter current for a constant value of collector voltage in a common base arrangement.
B. the ratio of change in collector current to the change in base current for a constant value of collector voltage in a common collector arrangement
C. the ratio of change in collector current to the change in base current for a constant value of collector voltage in a common emitter arrangement
D. the ratio of change in emitter current to the change in collector current for a constant value of collector voltage in a common emitter arrangement

Answer 1

Hint: Usually there are two types of current gains defined for a transistor. They are denoted by $\alpha \text{ and }\beta \text{.}$A transistor is three element device namely collector, base and the emitter. One element has to be always common to the input and the output circuits and this terminal is connected to the ground and serves as a reference point for the entire circuit. The two current gains are possible when a constant value for collector voltage in a common base arrangement or a constant value of collector voltage in a common emitter arrangement is held. Further with the above information let us define alpha i.e. current gain.

Complete step by step answer:
Current gain is basically the amplification of current when held in a particular configuration. As mentioned in the hint there are two types of current gain. Now let us define them.
Common base current amplification factor: It is defined as the ratio of small change in collector current to the small change in emitter current when the collector-base voltage is kept constant i.e. $\alpha =\left[{\displaystyle \frac{\mathrm{\Delta }{I}_C}{\mathrm{\Delta }{I}_E}}\right]\text{ for }{\text{V}}_{\text{CB}}\text{=constant}$
Common emitter current amplification factor: It is defined as the ratio of small change collector current to the small change in base current when collector-emitter voltage is kept constant i.e.
$\beta =\left[{\displaystyle \frac{\mathrm{\Delta }{I}_C}{\mathrm{\Delta }{I}_B}}\right]\text{ for }{\text{V}}_{\text{CE}}\text{=constant}$

So, the correct answer is “Option A”.

Note:
Given below is the figure showing the transistor configuration in the common base(CB) circuit.

Above we defined the current gains for common base configuration i.e. $\alpha$ and common emitter configuration i.e. $\beta$. The value of $\alpha$ is about 0.95 to 0.99 and $\beta$ is about 20 to 100. Hence the CE configuration is frequently used as it gives high current gains. It is also to be noted that the above current gains are dimensionless as they are the ratio of current with current itself.