What is an emitter follower?

An emitter follower, also known as a common collector amplifier, is a type of transistor amplifier circuit commonly used in electronics. It is designed to provide a high input impedance and a low output impedance while maintaining the same phase as the input signal. This makes it useful for various applications in electronics, including signal buffering and impedance matching.

Here's how an emitter follower works:

1. Transistor Configuration: The emitter follower circuit typically employs an NPN bipolar junction transistor (BJT). This transistor has three terminals: the emitter, the base, and the collector.
2. Input and Output: The input signal is applied to the base of the transistor, and the output is taken from the emitter terminal.
3. Amplification and Phase: The emitter follower provides unity voltage gain, which means that the output voltage follows the input voltage closely. In other words, it doesn't amplify the voltage significantly; it mainly provides current gain. Importantly, the phase of the output signal is the same as that of the input signal.
4. Impedance Transformation: One of the primary purposes of an emitter follower is to transform the input and output impedances. The input impedance seen by the source (connected to the base) is relatively high, making it a good buffer for devices with high output impedance. The output impedance seen by the load (connected to the emitter) is quite low, which allows it to drive loads with lower impedance effectively.
5. Current Amplification: The emitter follower provides current amplification. When a small input current flows into the base terminal, a larger current flows out of the emitter terminal, but the voltage remains approximately the same. This property makes it suitable for driving loads that require more current than the source can provide.
6. Stability: The emitter follower is known for its high input impedance, which means it doesn't load the preceding stage significantly. This property helps maintain signal integrity and minimizes the loading effect on the source.
7. Common Collector Configuration: The name "common collector" arises from the fact that the collector terminal is common between the input and output sides of the transistor.

Emitter followers are commonly used in various electronic applications, such as audio amplifiers, impedance matching circuits, and as a buffer stage between different parts of a circuit. They are particularly useful when you need to interface circuits with different impedance levels, ensuring efficient signal transfer while maintaining phase integrity.