In this article we are going to study Current Gain in Transistor of Common Base Transistor and Common Emitter Transistor. Along with the formula of current gain in transistor.
Current gain in Common Base Transistor
- Large signal current gain (α)
- D.C. current gain (αdc)
- Small signal current gain (α‘or hfb).
Large signal current gain (α)
α is known as large signal current gain of a common base transistor. Since IC and IE have opposite signs, so α is a positive quantity. The value of α lies between 0.90 to 0.995.
Note. α is not constant. It varies with the emitter current IE and collector base voltage VCB.
D.C. current gain (αdc)
Since IC >> ICO,hence eqn. (1) can be written as
which is known as d.c. current gain. αdc is a positive quantity. Its value is less than 1 as IC < IE.
Small signal current gain (α‘or hfb).
Small signal current gain is defined as the negative ratio of the small change in collector current (ΔIC) to small change in emitter current (ΔIE). That is,
α‘ is a positive quantity as IC and IE have positive signs.
Current gain in a Common Emitter Transistor
- D.C. current gain (β or hFE)
- Small signal current gain (β‘ or hfe).
D.C. current gain (β or hFE) or Forward current transfer ratio
D.C. current gain of common emitter transistor is defined as the ratio of collector current (IC) to the base current ( IB. That is,
The value of β lies between 20 to 200.
Small signal current gain (β‘ or hfe).
It is defined as the ratio of small change in collector (ΔIC) to the small change in base current (ΔIB) at constant Collector Emitter voltage (VCE).
β‘ can be determine from the output characteristics of Common Emitter Configuration.
Relation between Current Gain in Common Base Transistor and Common Emitter Transistor
It is also known as the relation between α and β.
We know, emitter current IE in term of base current IB and collector current is given by
Put this value in eqn. (1), We get
Since in active region of transistor, IB >> ICO. Hence neglect the term containing IC0 in eqn. (2)
Collector Current in term of β
Using eqn. (3) in eqn. (2), we get
When base is open circuited (i.e. IB=0) and junction is reverse biased, then IC =ICEO. Hence eqn. (4) becomes
Using eqn. (5) in eqn. (4), we get