BJT voltage divider bias calculator
Design the classic four-resistor (voltage-divider) bias for an NPN BJT: enter supply, β, target collector current and the operating point comes out.
Bias design
Results
R1 — divider upper
R1 = R_B·Vcc ÷ Vb
R2 — divider lower
R2 = R_B·Vcc ÷ (Vcc − Vb)
Emitter resistor (Re)
Re = Ve ÷ Ic
Collector resistor (Rc)
Rc = (Vcc − Vce − Ve) ÷ Ic
Divider current
Vcc ÷ (R1 + R2) — ~10× the base current
Operating point Ic
Simulated with nearest-E24 parts
Operating point Vce
Simulated with nearest-E24 parts
β sensitivity
Ic holds within ±10%| β | Ic (mA) | Vce (V) | vs nominal |
|---|---|---|---|
| 100 | 4.1 | 6.08 | +8.56% |
| 200 | 4.49 | 5.54 | 0% |
| 400 | 4.71 | 5.22 | +4.91% |
Ic stays within ±10% when β is halved or doubled — the divider bias is doing its job.
About the model
The four-resistor network sets a Thevenin base voltage with R1 and R2 while Re degenerates the emitter. Because the divider current is roughly ten times the base current (R_B = β·Re/10), the base sits near a fixed voltage and Ic ≈ Ve/Re barely moves with β — the classic trade-off between bias stability and the power the divider wastes. Rc is then sized from the headroom left between Vcc, Vce and Ve.
Real parts spread β far wider than the datasheet typical, so prototypes should always be measured. The operating point above is simulated with the nearest E24 resistors.
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Frequently asked questions
How do I design a four-resistor voltage divider bias for an NPN BJT?
The tool sets the emitter at Ve = Vcc / S (1.2 V with the defaults Vcc = 12 V and S = 10), sizes Re = Ve / Ic, and sets the base through a Thevenin divider where R1 = R_B × Vcc / Vb and R2 = R_B × Vcc / (Vcc − Vb). Rc is then sized from the headroom left between Vcc, Vce and Ve.
Why is the divider base resistance set to β × Re / 10?
Making R1 ∥ R2 = β × Re / 10 keeps the divider current roughly ten times the base current, so the base sits at a near-fixed voltage and Ic ≈ Ve / Re barely moves with β. It is the classic trade-off between bias stability and the power the divider wastes.
How does the tool check bias stability against β spread?
It re-simulates the operating point with the nearest-E24 resistor values at β/2 and 2 × β and compares Ic with the nominal point. If the collector current stays within ±10% in both cases the design is flagged as stable, and the β sensitivity table shows the exact numbers.
ICBOMS provides this tool for reference only.