Bias Voltage Divider Calculator
Design and analyze a BJT voltage-divider bias network using VCC, R1, R2, emitter resistance, transistor beta, and base-emitter voltage.
The calculator estimates the ideal base and emitter voltages, emitter and collector currents, divider stiffness, and can reverse-size either divider resistor for a target base voltage.
Engineering tool
Bias Voltage Divider Calculator
Calculate a BJT voltage-divider bias point or reverse-size R1 or R2 for a target base voltage.
DC supply feeding the bias network.
Resistance from VCC to the base node.
Resistance from the base node to ground.
Emitter resistance used for DC feedback.
Gain used to estimate base current for divider loading.
Expected transistor base-emitter voltage.
Recommended bias status
Excellent
Divider current is at least ten times base current, providing strong resistance to beta variation.
Result console
- Base voltage (VB)
- 2.11236V
- Emitter voltage (VE)
- 1.41236V
- Emitter current (IE)
- 1.41236mA
- Collector current (IC)
- 1.41236mA
- Divider current
- 449.438202µA
- Divider voltage ratio
- 17.60%VB / VCC
- Divider-to-base current
- 31.821798× IB
Formula reference
BJT Voltage-Divider Bias Formulas
These equations use the ideal unloaded-divider approximation; the divider-to-base current ratio indicates whether that approximation is suitably stiff.
Base voltage: VB = VCC × R2 / (R1 + R2)Emitter voltage: VE = VB - VBEEmitter current: IE = VE / RECollector current: IC ≈ IEDivider current: Idivider = VCC / (R1 + R2)R1 = (VCC - VB) × R2 / VBR2 = VB × R1 / (VCC - VB)Variable definitions
- R1
- upper resistor from VCC to base
- R2
- lower resistor from base to ground
- RE
- emitter feedback resistor
- Divider rule target
- Idivider ≥ 10 × IB
Worked Example
12 V NPN Bias Network
VCC = 12 V, R1 = 22 kΩ, R2 = 4.7 kΩ, RE = 1 kΩ, β = 100, VBE = 0.7 V
VB = 12 × 4.7 / (22 + 4.7) ≈ 2.112 V
VE = 2.112 - 0.7 ≈ 1.412 V
IE ≈ IC = 1.412 V / 1 kΩ ≈ 1.412 mA
Idivider = 12 V / 26.7 kΩ ≈ 449.4 µA
IB ≈ 14.1 µA, so divider current is about 31.8 × IB: Excellent.
Engineering Notes
Use the 10× rule
Divider current should generally exceed estimated base current by about ten times for a stiff bias node.
Balance stability and power
Increasing divider current improves bias stability but wastes power and lowers input impedance.
Account for VBE temperature
VBE changes with current and temperature, shifting emitter current and the operating point.
Expect beta variation
Transistor beta varies greatly between devices and operating conditions, even within one part number.
Use emitter feedback
Bias should not rely solely on transistor gain; emitter resistance provides stabilizing negative feedback.
Support reference
FAQ
What is voltage divider bias?
Voltage-divider bias uses two resistors to establish a BJT base voltage and an emitter resistor to create DC negative feedback. This provides a more predictable operating point than a single base resistor.
Why use two resistors?
R1 and R2 define a base-voltage ratio from VCC. When divider current is sufficiently larger than base current, the base voltage depends less on transistor beta.
Why is divider current important?
Base current loads the divider. A divider current around ten times base current is a common design target that improves stiffness, although it consumes additional power.
Does beta affect bias stability?
Yes. Beta determines the base current required for a given collector current. A stiff divider and emitter feedback reduce, but do not eliminate, sensitivity to beta variation.
Why is emitter resistor used?
The emitter resistor develops a voltage proportional to emitter current. If current rises, emitter voltage rises and reduces VBE drive, creating stabilizing negative feedback.
Documentation
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