Transistor Bias Calculator Formula

Understand the math behind the transistor bias calculator. Each variable explained with a worked example.

Use the Transistor Bias Calculator

Formulas Used

Base Resistor

base_resistor = (vcc - vbe) / ib_a

Base Current

base_current_ua = ib_a * 1000000

Transistor Power

power_mw = vcc * ic_ma

Variables

VariableDescriptionDefault
vccSupply Voltage (Vcc)(V)12
vbeBase-Emitter Voltage(V)0.7
ic_maTarget Collector Current(mA)10
hfeCurrent Gain (hFE)100
ib_aDerived value= ic_ma / hfe / 1000calculated

How It Works

BJT Transistor Biasing

In common-emitter configuration a base resistor sets the operating point.

Formula

I_B = I_C / hFE

R_B = (Vcc - V_BE) / I_B

Where hFE is DC current gain and V_BE is typically 0.7 V for silicon. This simple bias is beta-dependent; voltage divider bias provides better stability.

Worked Example

Bias a transistor with hFE=100 for 10 mA collector current from 12 V.

vcc = 12vbe = 0.7ic_ma = 10hfe = 100
  1. 01I_B = 10 mA / 100 = 0.1 mA = 100 uA
  2. 02R_B = (12 - 0.7) / 0.0001 = 113,000 ohms (113k)
  3. 03Power = 12 x 10 = 120 mW

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