What is the value of β in a transistor having IC = 100.2mA and IE = 100mA?
Right Answer is:
For common emitter, the current gain β is defined as the ratio of collector current to base current at a constant VCE .
β = IC/IB
For common-base dc current gain (α) is defined as the ratio of the collector current, Ic, and the emitter current IE and it is represented by α. The dc current gain α can be expressed as
α = IC/IE
Relation between α & β
β = α ⁄ (1 − α)
Collector current IC = 100.2mA
Emitter current IE = 100mA
α = IC/IE = 100.2/100 = 1.002
β = α ⁄ (1 − α) = 1.002/(1 −1.002)
β = −501
Here negative just gives information about its direction and nothing else. Because by convention positive current is always defined as flowing into the device. So if you have a PNP common base amplifier and you source current into the emitter input, it will flow out of the collector output. Since the current is flowing out, it is a negative current, hence the gain is negative.