BJT Emitter Bias Current

Ic ≈ (VB − 0.7) / RE

Quick answer

This calculator solves Ic ≈ (V_B − V_BE) / R_E, V_BE ≈ 0.7 V for Collector Current. With Base Voltage 12 V, Emitter Resistor 0.47 kΩ, the result is 24.04 mA. It solves in 3 directions, so you can enter the values you have and get the one you need.

Calculator

Collector Current (mA)
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Formula

Ic ≈ (V_B − V_BE) / R_E, V_BE ≈ 0.7 V

Description

In the standard voltage divider bias configuration, the collector current is approximately determined by the base voltage (set by the divider) minus the base-emitter junction voltage (≈ 0.7 V for silicon), divided by the emitter resistor. This assumes β is large enough that Ic ≈ Ie, and the divider is stiff (divider current >> base current). This bias technique provides excellent stability against β variation and temperature changes, which is why it is the most common BJT bias configuration in discrete transistor circuits.

Variables

  • Ic — Collector current (A)
  • V_B — Base voltage from voltage divider (V)
  • R_E — Emitter resistor (Ω)

Practical Notes

V_BE varies with temperature at about −2 mV/°C, so Ic will increase slightly with temperature. For germanium transistors, V_BE ≈ 0.3 V. For the bias to be stable, the voltage divider should be designed so that the divider current is at least 10× the base current: (R1‖R2) < 0.1 × β × RE. The collector resistor RC determines the voltage gain: Av ≈ −RC/RE (with emitter bypass cap: Av ≈ −RC×gm).

Symbols and units

Symbol Meaning Default unit Accepted units
Ic Collector Current mA pA, nA, µA, mA, A, kA
VB Base Voltage V pV, nV, µV, mV, V, kV
RE Emitter Resistor Ω mΩ, Ω, kΩ, MΩ

Step by step

  1. 1. What you know: Base Voltage 12 V, Emitter Resistor 0.47 kΩ
  2. 2. The relation: Ic ≈ (V_B − V_BE) / R_E, V_BE ≈ 0.7 V
  3. 3. Substitute: VB = 12 V, RE = 0.47 kΩ
  4. 4. Result: Collector Current = 24.04 mA

Related quantities from the same inputs

  • P = V × I = 0.2885 W

Worked examples

Minimum range

Given: Base Voltage 0.33 V, Emitter Resistor 1 Ω

Result: Collector Current = -0.37 A

Low range

Given: Base Voltage 3.3 V, Emitter Resistor 10 Ω

Result: Collector Current = 0.26 A

Typical

Given: Base Voltage 12 V, Emitter Resistor 0.47 kΩ

Result: Collector Current = 24.04 mA

High range

Given: Base Voltage 0.23 kV, Emitter Resistor 4.7 kΩ

Result: Collector Current = 48.79 mA

Maximum range

Given: Base Voltage 2.3 kV, Emitter Resistor 47 kΩ

Result: Collector Current = 48.92 mA

Solving for each variable

Solve for Collector Current

Base Voltage 0.33 V, Emitter Resistor 1 Ω → -0.37 A

Solve for Base Voltage

Collector Current 100 µA, Emitter Resistor 1 Ω → 0.7001 V

Solve for Emitter Resistor

Collector Current 100 mA, Base Voltage 0.33 kV → 3.293 kΩ

How Collector Current changes with Base Voltage

Base Voltage (input) Collector Current (result)
1.2 V 1.064 mA
3 V 4.894 mA
6 V 11.28 mA
9 V 17.66 mA
12 V 24.04 mA
18 V 36.81 mA
24 V 49.57 mA
48 V 100.6 mA
84 V 0.1772 A
120 V 0.2538 A

The same result in other units

Unit Value
µA 24040
mA 24.04
A 0.02404

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Frequently asked questions

How do you calculate BJT Emitter Bias Current?

Use Ic ≈ (V_B − V_BE) / R_E, V_BE ≈ 0.7 V. With Base Voltage 12 V, Emitter Resistor 0.47 kΩ, Collector Current works out to 24.04 mA.

What is the formula for BJT Emitter Bias Current?

The formula is Ic ≈ (V_B − V_BE) / R_E, V_BE ≈ 0.7 V. In the standard voltage divider bias configuration, the collector current is approximately determined by the base voltage (set by the divider) minus the base-emitter junction voltage (≈ 0.7 V for silicon), divided by the emitter resistor.

How do you find Base Voltage when you know the other values?

Rearrange for Base Voltage: with Collector Current 100 µA, Emitter Resistor 1 Ω, Base Voltage is 0.7001 V.

How do you find Emitter Resistor when you know the other values?

Rearrange for Emitter Resistor: with Collector Current 100 mA, Base Voltage 0.33 kV, Emitter Resistor is 3.293 kΩ.

What units does BJT Emitter Bias Current use?

Collector Current is shown in µA, mA, A. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

All Semiconductors formulas →
MOSFET Conduction Loss MOSFET Switching Loss BJT Total Dissipation Diode Power Loss Zener Regulation Resistor Transistor Current Gain (β) MOSFET Gate Drive Power BJT Emitter Bias Current

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