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Emitter Follower Circuit Calculation

Reviewed by Calculator Editorial Team

An emitter follower is a common configuration in bipolar junction transistor (BJT) circuits 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?

The emitter follower is a fundamental BJT circuit configuration where the emitter of the transistor is connected to the input signal. This configuration provides several important characteristics that make it useful in many electronic applications.

Key features of an emitter follower include:

  • High input impedance
  • Low output impedance
  • Voltage gain close to 1 (unity gain)
  • Phase inversion (180° phase shift)

Emitter followers are often used as impedance matching stages between high-impedance sources and low-impedance loads, as well as for voltage buffering applications.

Key Characteristics

Three primary characteristics of an emitter follower circuit are:

  1. Voltage gain (Av): Typically close to 1 (unity gain)
  2. Input resistance (Rin): High, determined by transistor parameters
  3. Output resistance (Rout): Low, determined by transistor parameters

Voltage gain formula:

Av = β/(β + 1)

Where β is the current gain of the transistor

Calculation Method

To calculate the parameters of an emitter follower circuit, you need to know:

  • Transistor current gain (β)
  • Transistor parameters (hfe, hie, hre)
  • Load resistance (RL)

The calculations involve using transistor hybrid parameters and solving the circuit equations. The calculator uses these formulas:

Input resistance:

Rin = hie * (1 + (hfe * RL)/(hie + hre * RL))

Output resistance:

Rout = (hie + hre * RL)/(1 + hfe)

Example Calculation

Consider an emitter follower with the following parameters:

  • hfe = 100
  • hie = 2.5 kΩ
  • hre = 100 Ω
  • RL = 1 kΩ

Using these values, we can calculate:

Parameter Calculation Value
Voltage gain (Av) 100/(100 + 1) 0.99
Input resistance (Rin) 2.5k * (1 + (100 * 1k)/(2.5k + 100 * 1k)) 2.52 kΩ
Output resistance (Rout) (2.5k + 100 * 1k)/(1 + 100) 25.25 Ω

This example demonstrates how the emitter follower provides near-unity voltage gain while maintaining high input resistance and low output resistance.

Practical Applications

Emitter followers are used in various applications including:

  • Audio amplifiers
  • Impedance matching between stages
  • Voltage buffering
  • Signal level shifting

In audio applications, emitter followers are often used as output stages to provide low output impedance while maintaining good voltage gain.

FAQ

What is the main advantage of an emitter follower?
The main advantage is its high input impedance and low output impedance, making it ideal for impedance matching and voltage buffering applications.
How does the voltage gain of an emitter follower compare to other BJT configurations?
Emitter followers typically have voltage gain close to 1 (unity gain), which is lower than common emitter configurations but higher than common collector configurations.
What are the key limitations of emitter follower circuits?
Key limitations include phase inversion, limited current gain, and the need for careful bias design to maintain stable operation.
Can emitter followers be used with field-effect transistors?
No, emitter followers are specifically designed for bipolar junction transistors and cannot be directly implemented with field-effect transistors.
What factors affect the performance of an emitter follower circuit?
Performance is affected by transistor parameters, load resistance, bias stability, and temperature variations.