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Bipolar Junction Transistor (BJT) – Formulas and Equations

Formula & Equations for Bipolar Junction Transistor “BJT “

Bipolar Junction Transistor (BJT) - Formulas and Equations

Bipolar Junction Transistor:

Current Gains in BJT:

There are two types of current gain in BJT i.e. α and β.

Current gains in BJTWhere

  • 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:

Common Base Voltage Gain


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

Related Posts:

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


  • 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


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 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 Ito input Current IB:

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

Related Formulas and Equations Posts:

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