BJT Voltage-Divider Bias
Resistor-divider bias for an NPN common-emitter stage: solve R1/R2/Rc/Re from Vcc, target Ic, hFE, and collector / emitter voltages, then check the operating point, gain, and I/O impedance.
Key points
BJT Voltage-Divider Bias: Resistor-divider bias for an NPN common-emitter stage: solve R1/R2/Rc/Re from Vcc, target Ic, hFE, and collector / emitter voltages, then check the operating point, gain, and I/O impedance. Formula: Ve = Ie·Re | Vb = Ve + Vbe | R1/R2 set by 10×Ib divider current | Av ≈ Rc/Re. Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. The divider current should be much larger than base current (often 10×Ib) so the base voltage is set by R1/R2 and the operating point is insensitive to hFE. Re provides DC negative feedback and stabilizes the operating point. Larger Re is more stable but reduces swing; bypassing Re increases AC gain.
Inputs
Ve = Ie·Re | Vb = Ve + Vbe | R1/R2 set by 10×Ib divider current | Av ≈ Rc/ReCalculation model (updates with parameters)
| Emitter resistor Re | 240 Ω | ideal 237.6 Ω, by E24 取整 |
| Collector resistor Rc | 1.2 kΩ | ideal 1.2 kΩ |
| Upper base resistor R1 | 20 kΩ | ideal 20.2 kΩ（Vcc → 基极） |
| Lower base resistor R2 | 3.9 kΩ | ideal 3.8 kΩ（基极 → 地） |
| Base current Ib / divider current | 50 µA / 500 µA | 分压current取 10×Ib 左右 |
| Actual operating point Ic / Vce | 4.575 mA / 5.402 V | Vc 6.511 V，Ve 1.109 V |
| Voltage gain Av (Re bypassed) | 5.0 × | 发射极旁路Capacitance对交流短路时 Av ≈ Rc/Re |
| Voltage gain Av (Re not bypassed) | 4.9 × | Av ≈ Rc/(Re + re)，re = 26mV/Ie |
| Input impedance Zin | 2.881 kΩ | R1∥R2∥β(re+Re)（未旁路） |
| Output impedance Zout | 1.2 kΩ | 共射近似等于 Rc |
| BJT dissipation | 24.71 mW | 按 Vce·Ic 估算，核对器件额定dissipation |
Engineering notes
- The divider current should be much larger than base current (often 10×Ib) so the base voltage is set by R1/R2 and the operating point is insensitive to hFE.
- Re provides DC negative feedback and stabilizes the operating point. Larger Re is more stable but reduces swing; bypassing Re increases AC gain.
- Small-signal re = 26 mV/Ie at room temperature. Without bypass, gain ≈ Rc/(Re+re); actual gain also depends on load, Early effect, and frequency.
FAQ
- What is BJT Voltage-Divider Bias for?
- Resistor-divider bias for an NPN common-emitter stage: solve R1/R2/Rc/Re from Vcc, target Ic, hFE, and collector / emitter voltages, then check the operating point, gain, and I/O impedance.
- What is the formula for BJT Voltage-Divider Bias?
- Core formula: Ve = Ie·Re | Vb = Ve + Vbe | R1/R2 set by 10×Ib divider current | Av ≈ Rc/Re. The tool returns results instantly using this formula and the selected parameters.
- What engineering notes should I keep in mind when using BJT Voltage-Divider Bias?
- The divider current should be much larger than base current (often 10×Ib) so the base voltage is set by R1/R2 and the operating point is insensitive to hFE. Re provides DC negative feedback and stabilizes the operating point. Larger Re is more stable but reduces swing; bypassing Re increases AC gain. Small-signal re = 26 mV/Ie at room temperature. Without bypass, gain ≈ Rc/(Re+re); actual gain also depends on load, Early effect, and frequency.