 # 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.

• Iis the emitter current
• Iis 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 Ais the ratio of output voltage to input voltage: Where

• α = IC / IE
• RL is the load resistance
• Rin ­is the input resistance

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### Common Emitter Configuration:

#### Forward Current Gain:

It is the ratio of output current i.e. the collector current I­to 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
• Iis the base current

#### Emitter Current:

The emitter current is the combination of collector & base current. It can be calculated using any of these equations.

• I= IC + IB
• I= IC / α
• I= I(1+ β)

#### Collector Current:

The collector current for BJT is given by:

• = βFI+ ICEO ≈ βFIB
• α IE
• I= 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

### Common Collector Configuration:

#### Current Gain:

The current gain Ai of common collector BJT is given by the ratio of output current Ito input Current IB:

• IE = IC + IB
• A= IE / IB
• A= (I+ IB) / IB
• Ai = (I/ IB) + 1
• A= β + 1

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