Calculate Output Impedance Emitter Follower

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 is essential for understanding its performance characteristics and ensuring proper circuit operation.

What is Output Impedance?

Output impedance is a measure of how much the output voltage of a circuit changes when a load is connected. It represents the internal resistance of the circuit and is crucial for determining how well the circuit can drive a load without significant voltage drop.

In an emitter follower circuit, the output impedance is typically much lower than the input impedance, which is one of the key advantages of this configuration. A low output impedance means the circuit can drive heavy loads without significant voltage loss.

Emitter Follower Circuit

An emitter follower is a common-emitter amplifier with a resistor connected between the emitter and ground. The resistor provides negative feedback, which stabilizes the circuit and reduces distortion.

The basic emitter follower circuit consists of:

A transistor (usually NPN or PNP)
A resistor connected to the emitter
A load resistor connected to the collector
Input and output coupling capacitors

The emitter resistor (RE) plays a crucial role in determining the output impedance of the circuit. The value of RE affects the voltage gain and the output impedance of the emitter follower.

Calculating Output Impedance

The output impedance of an emitter follower can be calculated using the following formula:

Z_out = RE * (1 + β)

Where:

Z_out is the output impedance
RE is the emitter resistor value
β (beta) is the current gain of the transistor

This formula shows that the output impedance is directly proportional to both the emitter resistor value and the current gain of the transistor. A higher β value results in a higher output impedance, while a larger RE value increases the output impedance as well.

Note: The current gain β is typically provided in the transistor datasheet. For most general-purpose transistors, β ranges from 50 to 300.

Example Calculation

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

Emitter resistor (RE) = 1 kΩ (1000 Ω)
Current gain (β) = 100

Using the formula:

Z_out = 1000 Ω * (1 + 100) = 1000 Ω * 101 = 101,000 Ω = 101 kΩ

Therefore, the output impedance of this emitter follower circuit is 101 kΩ.

This low output impedance makes the emitter follower well-suited for driving heavy loads without significant voltage drop.

FAQ

What is the difference between input and output impedance in an emitter follower?

In an emitter follower, the input impedance is typically high (on the order of megohms), while the output impedance is low (on the order of kilohms). This configuration allows the emitter follower to provide voltage gain while maintaining a low output impedance.

How does the emitter resistor affect the output impedance?

The emitter resistor (RE) has a direct impact on the output impedance. As RE increases, the output impedance also increases. This is because RE forms a voltage divider with the transistor's internal resistance, effectively increasing the overall output impedance.

What happens to the output impedance if the transistor's current gain (β) changes?

The output impedance is directly proportional to the transistor's current gain (β). As β increases, the output impedance also increases. This is because a higher β means the transistor can amplify more current, which in turn increases the effective output impedance.