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

• I_E is the emitter current
• I_C is the collector current
• I_B­ 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 A_V is the ratio of output voltage to input voltage:

Where

• α = I_C / I_E
• R_L is the load resistance
• R_{in ­}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 I­_C to the input current i.e. the base current I_B.

β_F = h_FE = I_C/I_B

Where

• Β_F is the forward current gain
• I_C is the collector current
• I_B 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.

• I_E = I_C + I_B
• I_E = I_C / α
• I_E = I_B (1+ β)

Collector Current:

The collector current for BJT is given by:

• I­_C = β_FI_B + I_CEO ≈ β_FI_B
• I­_C = α I_E
• I_C = I_E – I_B

Where

I_CEO  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:

V_CE = V_CB + V_BE

Where

• V_CE is the collector-to-Emitter voltage
• V_CB is the collector-to-base voltage
• V_BE 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 A_i of common collector BJT is given by the ratio of output current I_E to input Current I_B:

• I_E = I_C + I_B
• A_i = I_E / I_B
• A_i = (I_C + I_B) / I_B
• A_i = (I_C / I_B) + 1
• A_i = β + 1

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