BJT Bias Resistor Calculator (Voltage Divider)
Design R1, R2, Rc and Re for a stable common-emitter NPN bias point.
Voltage-divider bias, the "stiff" way
This is the standard four-resistor common-emitter bias network: R1/R2 set a fixed base voltage from Vcc, and Rc/Re set the collector current and operating point. The design starts from what you want the transistor to do — Ic and Vce — and works backward to the resistor values.
First pick an emitter voltage VE (rule of thumb: ~10% of Vcc) so KVL around the collector loop gives the voltage left for Rc: VRc = Vcc − Vce − VE. With Ie ≈ Ic (Ib is small): Rc = VRc / Ic and Re = VE / Ic. The base sits one diode drop above the emitter, VB = VE + 0.7 V.
For R1/R2, the trick is making the divider "stiff": the current flowing through R2 (I2) is set to a multiple of the base current Ib = Ic/hFE — typically 10×, so the divider barely notices the base current being tapped off and VB stays put even as hFE varies between transistors. Then R2 = VB / I2 and R1 = (Vcc − VB) / (I2 + Ib).
The point of Re + a stiff divider together is temperature/β stability: if Ic tries to rise (hotter transistor, higher-β part), VE rises too, which reduces Vbe and pulls Ic back down — negative feedback that a fixed-base-current design doesn't have. This is a DC bias-point calculation only; a real amplifier stage also needs an emitter bypass capacitor (for AC gain) and coupling capacitors, sized separately.
Values
VE% and divider stiffness are design choices, not measured values — 10% and 10× are the standard stable-bias rules of thumb. Raise VE% for better thermal stability at the cost of headroom.