Output Impedance Emitter Follower Calculator
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Reviewed by Calculator Editorial Team
An emitter follower is a common amplifier configuration that provides voltage gain while maintaining a low output impedance. Calculating the output impedance of an emitter follower circuit is essential for proper circuit design and optimization. This guide explains how to determine the output impedance of an emitter follower using our calculator tool.
What is Output Impedance?
Output impedance is a measure of how well a circuit can deliver current to a load. It represents the opposition to current flow at the output of a circuit. A low output impedance means the circuit can deliver more current to a load, while a high output impedance indicates the circuit struggles to drive current.
In amplifier circuits, output impedance affects how well the amplifier can drive a load. A low output impedance is desirable for driving heavy loads or connecting multiple stages of amplification.
Emitter Follower Circuit
An emitter follower is a common-emitter amplifier configuration with the output taken from the emitter rather than the collector. This configuration provides voltage gain while maintaining a low output impedance, making it useful for buffering and impedance matching.
The emitter follower is also known as a common-collector amplifier because the collector is common to both input and output.
The basic emitter follower circuit consists of a transistor with the emitter connected to ground, the base connected to the input signal, and the collector connected to a positive voltage supply through a resistor.
Calculating Output Impedance
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 VT by the emitter current IE. The output impedance is then the parallel combination of re and the load resistance RL.
Example Calculation
Let's calculate the output impedance for an emitter follower with the following parameters:
- Emitter current (IE) = 10mA
- Load resistance (RL) = 1kΩ
First, calculate the emitter resistance re:
re = VT/IE = 26mV/10mA = 2.6Ω
Next, calculate the output impedance Zout:
Zout = re || RL = 2.6Ω || 1kΩ = (2.6 × 1000)/(2.6 + 1000) ≈ 2.57Ω
The output impedance of this emitter follower circuit is approximately 2.57Ω.
Practical Applications
Understanding and calculating the output impedance of an emitter follower is crucial for several practical applications:
- Buffering: Emitter followers are often used to buffer high-impedance sources from low-impedance loads.
- Impedance Matching: They can match the output impedance of one stage to the input impedance of the next stage.
- Signal Isolation: The emitter follower provides electrical isolation between stages while maintaining voltage gain.
By accurately calculating the output impedance, engineers can optimize circuit performance and ensure proper signal transmission.
Limitations
While emitter followers offer several advantages, they also have some limitations:
- Current Gain: The emitter follower does not provide current gain, which can be a limitation in some applications.
- Voltage Swing: The output voltage swing is limited by the power supply voltage and the transistor characteristics.
- Temperature Sensitivity: The emitter resistance re is temperature-dependent, which can affect the output impedance.
These limitations should be considered when designing circuits that use emitter follower configurations.
FAQ
- What is the difference between output impedance and input impedance?
- Output impedance refers to the opposition to current flow at the output of a circuit, while input impedance refers to the opposition to current flow at the input of a circuit. They represent different aspects of a circuit's behavior.
- How does load resistance affect output impedance?
- The load resistance is combined in parallel with the emitter resistance to determine the total output impedance. A lower load resistance will result in a lower output impedance.
- Can the output impedance of an emitter follower be zero?
- No, the output impedance of an emitter follower cannot be zero. It will always have a finite value determined by the parallel combination of the emitter resistance and the load resistance.
- What factors influence the emitter resistance?
- The emitter resistance is primarily influenced by the emitter current and the thermal voltage. Higher emitter currents result in lower emitter resistances.
- How can I verify the output impedance of an emitter follower circuit?
- You can verify the output impedance by measuring the voltage across the output terminals with and without a load connected. The ratio of the voltage change to the current change gives the output impedance.