The common collector amplifier, also known as an emitter follower, is a fundamental transistor amplifier configuration. In this circuit, the collector terminal serves as the common connection point between input and output circuits. The collector is directly connected to the power supply VCC, making it common to both input and output signals. The input signal is applied to the base terminal, while the output is taken from the emitter terminal. This configuration creates a unique amplifier with specific characteristics that we'll explore.
Now let's analyze the specific configuration characteristics that distinguish the common collector amplifier from other types. In the common collector configuration, the collector terminal is directly connected to the positive power supply VCC, making it the common reference point. The input signal is applied between the base and collector terminals, while the output is taken between the emitter and collector. Notice that the load resistor RE is placed in the emitter circuit, not the collector circuit. This is fundamentally different from the common emitter configuration, where the load resistor RC is in the collector circuit and the emitter is grounded. In the common base configuration, the base is grounded and serves as the common terminal. These connection patterns are the key to identifying each amplifier type.
Let's establish concrete criteria for identifying common collector circuits through a systematic checklist. First, examine the collector terminal connection - it should be connected directly to VCC for NPN transistors or to ground for PNP transistors, making it the common reference point. Second, verify that the input signal is applied to the base terminal. Third, confirm that the output signal is taken from the emitter terminal. Fourth, check that the load resistor RE is positioned in the emitter circuit, not the collector circuit. Finally, observe the biasing arrangement which maintains proper DC operating conditions. These identification features apply to both NPN and PNP transistor configurations, though the power supply connections are reversed. The numbered callouts highlight these key identification points in both circuit examples.
Now let's apply our identification knowledge to real-world circuit examples. First, we have the basic emitter follower - the simplest common collector configuration with collector connected to VCC, input at base, output at emitter, and load resistor RE in the emitter circuit. Second is the Darlington pair configuration, which uses two transistors in a common collector arrangement to achieve very high current gain while maintaining the same identification features. Third, we see a voltage divider bias circuit, which uses resistors R1 and R2 for biasing but still maintains the common collector characteristics with RE in the emitter circuit. Finally, as a counter-example, we show a common emitter circuit where the load resistor RC is in the collector circuit and the emitter is grounded - this clearly fails our identification criteria. Notice how the first three circuits all have their load resistors in the emitter path, confirming they are common collector amplifiers.
Finally, let's verify our identification by examining the distinctive electrical characteristics of common collector amplifiers. The input impedance is very high, approximately beta times RE, making it excellent for buffering high-impedance sources. The output impedance is very low, approximately RE divided by beta, allowing it to drive low-impedance loads effectively. The voltage gain is approximately unity, meaning the output voltage closely follows the input voltage with minimal attenuation. The current gain is high, approximately beta plus one, providing significant current amplification. Most importantly, there is no phase inversion - the output signal is in phase with the input signal, which is why it's called an emitter follower. These characteristic properties directly result from the circuit configuration we identified in previous scenes, where the collector is common, input is at the base, and output is taken from the emitter with the load resistor in the emitter circuit.