Bipolar Junction Transistor (BJT) – Formulas and Equations
Formula & Equations for Bipolar Junction Transistor “BJT “
Bipolar Junction Transistor:
Current Gains in BJT:
There are two types of current gain in BJT i.e. α and β.
Where
- IE is the emitter current
- IC is the collector current
- IB is the base current
Common Base Configuration:
Common Base Voltage Gain
In common base configuration, BJT is used as voltage gain amplifier, where the gain AV is the ratio of output voltage to input voltage:
Where
- α = IC / IE
- RL is the load resistance
- Rin is the input resistance
Related Posts:
- What is NPN Transistor? BJT Construction, Working & Applications
- What is PNP Transistor? Construction, Working & Applications
Common Emitter Configuration:
Forward Current Gain:
It is the ratio of output current i.e. the collector current IC to the input current i.e. the base current IB.
βF = hFE = IC/IB
Where
- ΒF is the forward current gain
- IC is the collector current
- IB is the base current
Emitter Current:
The emitter current is the combination of collector & base current. It can be calculated using any of these equations.
- IE = IC + IB
- IE = IC / α
- IE = IB (1+ β)
Collector Current:
The collector current for BJT is given by:
- IC = βFIB + ICEO ≈ βFIB
- IC = α IE
- IC = IE – IB
Where
ICEO is the collector to emitter leakage current (Open base).
Alpha α to Beta β Conversion Formula:
The gain alpha & beta are inter-convertible, & they can be converted using,
- α = β / (β + 1)
- β = α / (1- α)
Collector-to-Emitter Voltage:
VCE = VCB + VBE
Where
- VCE is the collector-to-Emitter voltage
- VCB is the collector-to-base voltage
- VBE is the base-to-emitter voltage
Related Post: How to Check a Transistor by Multimeter (DMM+AVO) – NPN & PNP
Common Collector Configuration:
Current Gain:
The current gain Ai of common collector BJT is given by the ratio of output current IE to input Current IB:
- IE = IC + IB
- Ai = IE / IB
- Ai = (IC + IB) / IB
- Ai = (IC / IB) + 1
- Ai = β + 1
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