BJT Emitter-Bias Current

Ic = (Vb − Vbe) / Re

Quick answer

This calculator solves Ic ≈ Ie = (Vb − Vbe) / Re for Collector Current. With Base Voltage 12 V, Base-Emitter Voltage 26.4 V, Emitter Resistor 10 kΩ, the result is -1.44 mA. It solves in 4 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 ≈ Ie = (Vb − Vbe) / Re

Description

In a voltage-divider-biased transistor stage, the emitter current is set by the base voltage minus the base-emitter drop, divided by the emitter resistor. Because this depends on resistor ratios and the stable 0.7 V junction drop rather than on the poorly-controlled beta, it gives a predictable, temperature-stable operating point—the standard way to bias discrete amplifiers.

Variables

  • Ic — Collector (≈ emitter) current (A)
  • Vb — Voltage at the base node (V)
  • Vbe — Base-emitter drop (~0.7 V)
  • Re — Emitter resistor (Ω)

Practical Notes

Set Vb with the R1/R2 divider, then the emitter resistor fixes the current. This negative feedback through Re makes the bias point nearly independent of β, the key to a robust design.

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
Vbe Base-Emitter Voltage V pV, nV, µV, mV, V, kV
Re Emitter Resistor kΩ mΩ, Ω, kΩ, MΩ

Step by step

  1. 1. What you know: Base Voltage 12 V, Base-Emitter Voltage 26.4 V, Emitter Resistor 10 kΩ
  2. 2. The relation: Ic ≈ Ie = (Vb − Vbe) / Re
  3. 3. Substitute: Vb = 12 V, Vbe = 26.4 V, Re = 10 kΩ
  4. 4. Result: Collector Current = -1.44 mA

Related quantities from the same inputs

  • P = V × I = -17.28 mW

Worked examples

Minimum range

Given: Base Voltage 0.33 V, Base-Emitter Voltage 0.726 V, Emitter Resistor 100 Ω

Result: Collector Current = -3.96 mA

Low range

Given: Base Voltage 3.3 V, Base-Emitter Voltage 7.26 V, Emitter Resistor 1 kΩ

Result: Collector Current = -3.96 mA

Typical

Given: Base Voltage 12 V, Base-Emitter Voltage 26.4 V, Emitter Resistor 10 kΩ

Result: Collector Current = -1.44 mA

High range

Given: Base Voltage 0.23 kV, Base-Emitter Voltage 0.506 kV, Emitter Resistor 100 kΩ

Result: Collector Current = -2.76 mA

Maximum range

Given: Base Voltage 2.3 kV, Base-Emitter Voltage 5.06 kV, Emitter Resistor 1 MΩ

Result: Collector Current = -2.76 mA

Solving for each variable

Solve for Collector Current

Base Voltage 0.33 V, Base-Emitter Voltage 0.726 V, Emitter Resistor 100 Ω → -3.96 mA

Solve for Base Voltage

Collector Current 100 µA, Base-Emitter Voltage 0.726 V, Emitter Resistor 100 Ω → 0.736 V

Solve for Base-Emitter Voltage

Collector Current 100 µA, Base Voltage 0.33 V, Emitter Resistor 100 Ω → 0.32 V

Solve for Emitter Resistor

Collector Current 1 mA, Base Voltage 3.3 V, Base-Emitter Voltage 1.55 V → 1.75 kΩ

How Collector Current changes with Base Voltage

Base Voltage (input) Collector Current (result)
1.2 V -2.52 mA
3 V -2.34 mA
6 V -2.04 mA
9 V -1.74 mA
12 V -1.44 mA
18 V -0.84 mA
24 V -0.24 mA
48 V 2.16 mA
84 V 5.76 mA
120 V 9.36 mA

The same result in other units

Unit Value
nA -1440000
µA -1440
mA -1.44
A -0.00144

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Frequently asked questions

How do you calculate BJT Emitter-Bias Current?

Use Ic ≈ Ie = (Vb − Vbe) / Re. With Base Voltage 12 V, Base-Emitter Voltage 26.4 V, Emitter Resistor 10 kΩ, Collector Current works out to -1.44 mA.

What is the formula for BJT Emitter-Bias Current?

The formula is Ic ≈ Ie = (Vb − Vbe) / Re. In a voltage-divider-biased transistor stage, the emitter current is set by the base voltage minus the base-emitter drop, 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, Base-Emitter Voltage 0.726 V, Emitter Resistor 100 Ω, Base Voltage is 0.736 V.

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

Rearrange for Base-Emitter Voltage: with Collector Current 100 µA, Base Voltage 0.33 V, Emitter Resistor 100 Ω, Base-Emitter Voltage is 0.32 V.

What units does BJT Emitter-Bias Current use?

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

Related Concepts

All Discrete Amplifiers formulas →
Thermal Voltage Small-Signal Emitter Resistance Transistor Transconductance Diode Dynamic Resistance Common-Emitter Voltage Gain BJT Alpha–Beta Relationship Darlington Pair Current Gain BJT Emitter-Bias Current

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