Common Collector (Emitter Follower) Calculator

Calculate small-signal parameters for an emitter follower.

Assumptions: Thermal voltage \(V_T\) is 0.025 V, and values are in SI units.

Step 1: Enter Parameters

e.g., 1 mA (will be converted to A)

e.g., 1000 Ω

e.g., 100

Formulas:
Transconductance: \( g_m = \frac{I_E}{V_T} \) (with \(V_T = 0.025\) V)
Voltage Gain: \( A_v = \frac{R_E}{R_E + \frac{1}{g_m}} \)
Input Resistance: \( r_{in} \approx (\beta + 1)\left(R_E + \frac{1}{g_m}\right) \)
Output Resistance: \( r_{out} \approx \frac{1/g_m}{\beta + 1} \)

Comprehensive Guide to the Common Collector (Emitter Follower) Circuit

Comprehensive Guide to the Common Collector (Emitter Follower) Circuit

The Common Collector circuit, also known as the Emitter Follower, is a popular transistor amplifier configuration used for buffering, impedance matching, and providing a low output impedance while preserving the input signal's voltage. This guide explains how the emitter follower works, its key characteristics, and its common applications.

Table of Contents

  1. Overview of the Common Collector (Emitter Follower)
  2. Circuit Configuration and Operation
  3. Key Characteristics
  4. Mathematical Analysis
  5. Practical Examples
  6. Common Applications
  7. Conclusion

1. Overview of the Common Collector (Emitter Follower)

The emitter follower is one of the three basic transistor amplifier configurations (the other two being the common emitter and common base). In the common collector configuration, the collector is connected directly to the supply voltage, and the input signal is applied to the base, while the output is taken from the emitter. The circuit is called an "emitter follower" because the output voltage at the emitter closely "follows" the input voltage at the base, minus the base-emitter voltage drop.

2. Circuit Configuration and Operation

In a typical NPN emitter follower, the transistor’s collector is tied to a fixed voltage supply (\(V_{CC}\)). The input signal \(v_{in}\) is fed into the base through a biasing network, and the emitter is connected to ground through a resistor \(R_E\). The output voltage \(v_{out}\) is taken from the emitter.

The key operational points are:

  • The emitter voltage (\(v_E\)) is approximately the base voltage (\(v_B\)) minus the base-emitter voltage drop (\(V_{BE}\)), typically about 0.6–0.7 V for silicon transistors.
  • The voltage gain is nearly unity, meaning \(v_{out} \approx v_{in} - V_{BE}\).
  • The circuit offers high input impedance and low output impedance, making it ideal as a buffer.

3. Key Characteristics

The emitter follower exhibits several important features:

  • Voltage Gain ≈ 1: The output voltage follows the input voltage with only a slight offset (the \(V_{BE}\) drop).
  • High Input Impedance: Minimal loading of the preceding stage, ideal for signal buffering.
  • Low Output Impedance: Capable of driving low-impedance loads effectively.
  • Stable Biasing: The configuration provides good thermal stability and linearity.

4. Mathematical Analysis

The fundamental relationship in an emitter follower can be approximated as:

\( v_E \approx v_B - V_{BE} \)

Where:

  • \(v_E\) is the emitter voltage (output).
  • \(v_B\) is the base voltage (input).
  • \(V_{BE}\) is the base-emitter voltage drop (typically 0.6–0.7 V for silicon transistors).

Voltage Gain: Since the emitter voltage closely follows the base voltage, the voltage gain \(A_v\) is approximately:

\( A_v = \frac{v_E}{v_B} \approx 1 \)

Impedance Considerations:

  • The high input impedance ensures that the preceding circuit is not significantly loaded.
  • The low output impedance allows the circuit to drive heavy loads without significant voltage drop.

5. Practical Examples

Example: Emitter Follower Voltage Output

Consider an emitter follower where the input (base) voltage is 5 V and the base-emitter voltage drop is 0.7 V.

\( v_E \approx 5\,V - 0.7\,V = 4.3\,V \)

Thus, the output voltage at the emitter will be approximately 4.3 V.

Example: Buffering a Signal

Suppose you have a sensor that outputs a high-impedance signal at 5 V. You need to drive a low-impedance load without distorting the signal.

An emitter follower is ideal for this purpose. With an input of 5 V and a voltage drop of 0.7 V, the output is 4.3 V. However, the low output impedance allows the load to be driven effectively without drawing excessive current from the sensor.

6. Common Applications

  • Impedance Matching: Acts as a buffer between a high-impedance source and a low-impedance load.
  • Signal Amplification: Although the voltage gain is near unity, it can provide current amplification.
  • Level Shifting: Can be used to shift signal levels, accounting for the base-emitter drop.
  • Power Amplification: In audio and RF circuits, emitter followers are used to drive speakers or antennas.

7. Conclusion

The Common Collector (Emitter Follower) is a versatile and fundamental circuit configuration used extensively for buffering, impedance matching, and providing stable, low-impedance outputs. Its near-unity voltage gain, combined with high input impedance and low output impedance, makes it indispensable in both analog circuit design and real-world applications. Whether you’re designing audio circuits, sensor interfaces, or power amplifiers, understanding the principles behind the emitter follower can significantly enhance your circuit performance.