# What is the value of β in a transistor having IC = 100.2mA and IE = 100mA?

_{C}= 100.2mA and I

_{E}= 100mA?

### Right Answer is:

501

#### SOLUTION

**For common emitter, **the current gain** β **is defined as the ratio of collector current to base current at a constant V_{CE} .

**β = I _{C}/I_{B}**

**For common-base** dc current gain (**α**) is defined as the ratio of the collector current, I_{c}, and the emitter current I_{E} and it is represented by α. The dc current gain α can be expressed as

**α = I _{C}/I_{E}**

**Relation between α & β**

**β = α ⁄ (1 − α)**

Given

Collector current I_{C} = 100.2mA

Emitter current I_{E} = 100mA

α = I_{C}/I_{E }= 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.