Transistor Emitter Follower Calculator

An emitter follower is a common transistor amplifier configuration that provides voltage buffering between stages. This calculator helps you determine key parameters of an emitter follower circuit including voltage gain, input resistance, and output resistance.

What is an Emitter Follower?

An emitter follower is a type of common-emitter amplifier configuration where the emitter of the transistor is connected to the output. This configuration provides several important characteristics:

High input impedance (resistance)
Low output impedance
Voltage gain close to 1 (unity gain)
Good current amplification

The emitter follower is commonly used as a buffer stage between other amplifier stages to maintain signal levels and provide impedance matching.

Emitter followers are often used in audio amplifiers and signal processing circuits where stable voltage levels are required between stages.

How to Use the Calculator

To use the transistor emitter follower calculator:

Enter the transistor parameters: β (current gain), Vcc (supply voltage), and Re (emitter resistor value)
Select the appropriate units for each parameter
Click "Calculate" to compute the emitter follower characteristics
Review the results including voltage gain, input resistance, and output resistance
Use the chart to visualize the relationship between parameters

The calculator will display the calculated values and explain their significance in the context of your circuit design.

Formula Explained

The key parameters of an emitter follower are calculated using these fundamental formulas:

Voltage Gain (Av)

Av = β / (β + 1)

Where β is the current gain of the transistor

Input Resistance (Rin)

Rin = (β + 1) * Re

Where Re is the emitter resistor value

Output Resistance (Rout)

Rout = Re / β

These formulas are derived from the fundamental transistor equations and represent the key characteristics of an emitter follower configuration.

Worked Example

Let's calculate the parameters for an emitter follower with:

β (current gain) = 100
Vcc = 12V
Re = 1kΩ (1000Ω)

Calculations:

Voltage Gain: 100 / (100 + 1) = 0.99
Input Resistance: (100 + 1) * 1000 = 101,000Ω
Output Resistance: 1000 / 100 = 10Ω

This example shows a typical emitter follower configuration with near-unity voltage gain, high input resistance, and very low output resistance.

FAQ

What is the purpose of an emitter follower?: An emitter follower provides voltage buffering between amplifier stages, maintains signal levels, and offers good current amplification.
How does the emitter follower differ from a common-emitter amplifier?: The emitter follower has the emitter connected to the output, providing different impedance characteristics compared to the common-emitter configuration.
What factors affect the performance of an emitter follower?: Key factors include transistor β (current gain), emitter resistor value, and supply voltage. Higher β values generally improve performance.
Can emitter followers be used in audio circuits?: Yes, emitter followers are commonly used in audio amplifiers as buffer stages between different amplifier sections.
What is the typical voltage gain of an emitter follower?: The voltage gain is typically close to 1 (unity gain) for most practical emitter follower configurations.