Emitter Resistor Calculator

Created: 19/03/2026Updated: 13/06/2026By: Mert
Emitter Resistor Calculator
RE = VE/IE
VE = VB − VBE
Stability: VE ≈ 1–2V
Emitter Resistor Calculator
VCC Supply Voltage
VB Base Voltage
VBE (typically 0.7V)
IC Target (mA)
β (optional, for IB)
Enter values
Emitter Resistor Calculator Results
RE Required
—
Ω
VE (Emitter)
—
V
IE
—
A
VE/VCC Ratio
—
%
Thermal Stability
—
 
Power in RE
—
W
Emitter Resistor Calculator Input Output

Figure 1: Emitter Resistor Calculator — enter values to calculate the result with step-by-step workings.

Table of Contents
Fundamentals
  1. Why Every BJT Amplifier Needs RE
  2. The Formulas
Worked Examples
  1. 12V Supply, 1 mA (Good Stability)
  2. 5V Supply, 2 mA (Tight Headroom)
Deep Dive
  1. The Stability Rating
  2. Bypassing RE for AC Gain
Reference
  1. Frequently Asked Questions
  2. Related Calculators

Why Every BJT Amplifier Needs RE

Without an emitter resistor, a BJT amplifier is thermally unstable. As the junction heats up, IC increases, which heats the junction further — a positive feedback loop that can end in thermal runaway. RE breaks this loop by converting collector current changes into emitter voltage changes that oppose the original shift.

The calculator above finds RE from your target VE and IC, and rates the thermal stability based on the VE/VCC ratio. The Transistor Biasing Calculator shows the complete bias circuit where RE is one of four resistors setting the Q-point.

The Formulas

VE = VB − VBE
RE = VE / IE
Stability: VE/VCC — above 10% = Good, 5–10% = Moderate, below 5% = Poor

Example: 12V Supply, IC = 1 mA

VCC = 12V, VB = 2.7V, VBE = 0.7V, IC = 1 mA, β = 100

VE = 2.7 − 0.7 = 2.0 V

IE = 1.0m + 0.01m = 1.01 mA

RE = 2.0 / 1.01m = 1,980 Ω → nearest: 2.0 kΩ

VE/VCC = 2.0/12 = 16.7% — Good stability

The BJT Gain Calculator provides IE = IC + IB used in this calculation.

Example: 5V Supply, IC = 2 mA

VCC = 5V, VB = 1.7V, VBE = 0.7V, IC = 2 mA

VE = 1.7 − 0.7 = 1.0 V

RE = 1.0 / 2.02m = 495 Ω → nearest: 470 Ω

VE/VCC = 1.0/5 = 20% — Good stability

At 5 V, spending 1 V on RE leaves only 4 V for VCE + RC drop. The Op-Amp Gain Calculator eliminates this headroom problem — op-amps self-bias internally.

The Stability Rating

The VE/VCC ratio measures how much of the supply is “spent” on thermal feedback. Above 10% is good: VE dominates over VBE variations (~2 mV/°C), so IC is stable. Below 5% is poor: VBE changes are a significant fraction of VE, and IC drifts with temperature. The Shockley Diode Calculator models the VBE temperature coefficient that RE compensates.

Bypassing RE for AC Gain

Unbypassed RE reduces voltage gain to Av ≈ −RC/RE. Adding a bypass capacitor (10–100 µF) across RE shorts it at signal frequencies, restoring gain to Av ≈ −RC/re where re = VT/IC. DC stability is preserved because the capacitor blocks DC. The 555 Timer Calculator uses capacitors for a different purpose (timing) but the same RC filtering principle applies.

Frequently Asked Questions

Why do I need an emitter resistor?
RE provides negative feedback for thermal stability. Without it, rising temperature increases IC, which heats the junction further — thermal runaway can destroy the transistor.
How much VE should I drop?
VE should be 10–20% of VCC for good stability. 1–2 V is typical for 5–12 V supplies. More voltage = better stability but less headroom for signal swing.
Does RE reduce gain?
Yes. Unbypassed gain = −RC/RE. Bypassed gain = −RC/re. Add a capacitor across RE to restore AC gain while keeping DC stability. The LM317 Resistor Calculator uses a resistor divider for a similar reason — stability via fixed ratios.
How do I choose VB?
VB = desired VE + 0.7 V. Then design a voltage divider (R1, R2) to produce that VB. The calculates the complete divider circuit.
What if stability is rated Poor?
Increase RE (drop more voltage across it) or increase VB. If VCC is too low for adequate VE, consider a different circuit topology. The MOSFET Calculator analyses MOSFET amplifiers which do not suffer from the same thermal runaway risk.
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Last updated: March 2026