A CC amplifier, short for Common Collector amplifier, is a fundamental type of transistor circuit used primarily to provide voltage buffering.
Understanding the Common Collector (CC) Amplifier
In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer. It gets its name from the fact that the collector terminal of the transistor is connected to the DC power supply (effectively common to both input and output signals, often via a capacitor in AC analysis), and the input signal is applied to the base, with the output taken from the emitter.
Why is it Called an Emitter Follower?
The alternative name, emitter follower, comes from a key characteristic of this circuit: the voltage at the emitter terminal closely follows the voltage applied to the base terminal. The output voltage is approximately equal to the input voltage minus the base-emitter voltage drop (Vbe), which is relatively constant for a conducting transistor.
The Role of a Voltage Buffer
As highlighted by its typical use, the common collector amplifier functions effectively as a voltage buffer. A voltage buffer is a circuit designed to transfer a voltage from a high-impedance source to a low-impedance load without reducing the voltage significantly.
This buffering capability is crucial when a circuit with high output impedance needs to drive a load with low input impedance. Without a buffer, connecting the low-impedance load directly would "load down" the source, causing the signal voltage to drop drastically. The CC amplifier solves this problem by having a:
- High input impedance: It draws very little current from the source circuit.
- Low output impedance: It can supply a relatively large current to the load circuit without its output voltage dropping significantly.
Key Characteristics of a CC Amplifier
Based on its topology and function as a voltage buffer, the common collector amplifier exhibits several important characteristics:
- Voltage Gain: The voltage gain is approximately unity (very close to 1). It amplifies current and power, but not voltage.
- Input Impedance: Typically high.
- Output Impedance: Typically low.
- Phase Shift: There is no phase inversion between the input and output voltage signals; they are in phase.
- Current Gain: It provides significant current gain.
- Power Gain: It provides power gain.
Comparison to Other BJT Topologies
Alongside the Common Collector configuration, the other two basic single-stage BJT amplifier topologies mentioned are:
- Common Emitter (CE) Amplifier: Known for voltage and current gain, often used for general amplification, and has phase inversion.
- Common Base (CB) Amplifier: Known for voltage gain but no current gain, high input impedance, and no phase inversion. Often used for high-frequency applications or impedance matching.
The CC amplifier distinguishes itself primarily by its unity voltage gain and excellent buffering properties, making it ideal for impedance matching and driving low-impedance loads.
