Emitter Follower Output Impedance Calculation
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Reviewed by Calculator Editorial Team
An emitter follower is a common transistor circuit configuration that provides voltage buffering between stages. Calculating its output impedance is essential for understanding how it behaves in a circuit. This guide explains the formula, assumptions, and practical applications of emitter follower output impedance calculation.
Introduction
The emitter follower is a fundamental transistor circuit configuration that provides voltage buffering between stages. It's commonly used in audio amplifiers and other applications where impedance matching is important.
Output impedance is a critical parameter that describes how well the circuit maintains its output voltage when connected to a load. For an emitter follower, the output impedance is determined by the transistor's characteristics and the circuit configuration.
This calculation assumes an ideal transistor model and neglects second-order effects like Early voltage. For precise applications, more detailed models may be required.
Formula
The output impedance of an emitter follower can be calculated using the following formula:
Zout = re || RL
Where:
- Zout = Output impedance
- re = Emitter resistance (re = VT/IE)
- RL = Load resistance
- VT = Thermal voltage (approximately 26mV at room temperature)
- IE = Emitter current
The emitter resistance re is calculated by dividing the thermal voltage by the emitter current. The output impedance is then the parallel combination of re and the load resistance RL.
Calculation
To calculate the output impedance of an emitter follower, follow these steps:
- Determine the emitter current IE based on your circuit design.
- Calculate the emitter resistance using re = VT/IE.
- Identify the load resistance RL in your circuit.
- Calculate the output impedance using Zout = re || RL.
The parallel combination of two resistances is calculated as (R1 × R2) / (R1 + R2).
Example
Let's calculate the output impedance for an emitter follower with the following parameters:
- Emitter current (IE) = 10mA
- Load resistance (RL) = 1kΩ
- Thermal voltage (VT) = 26mV
- Calculate re: re = 26mV / 10mA = 2.6Ω
- Calculate Zout: Zout = (2.6Ω × 1kΩ) / (2.6Ω + 1kΩ) ≈ 2.35Ω
The output impedance of this emitter follower is approximately 2.35Ω.
FAQ
What is the difference between emitter follower and common emitter?
An emitter follower provides voltage buffering while a common emitter provides current gain. The emitter follower has higher input impedance and lower output impedance than a common emitter stage.
Why is output impedance important in emitter followers?
Output impedance determines how well the circuit maintains its output voltage when connected to a load. Lower output impedance means better voltage buffering and more stable operation.
What factors affect emitter follower output impedance?
The emitter current, load resistance, and transistor characteristics all affect the output impedance. Higher emitter current reduces re, while higher load resistance increases the parallel combination.