The Common Drain amplifier, also known as a Source Follower, is a type of field-effect transistor (FET) amplifier configuration commonly used in electronic circuits.
Common Drain (CD) Amplifier or Source Follower:
Configuration:
- The Common Drain amplifier uses an FET (Field Effect Transistor) or MOSFET (Metal-Oxide-Semiconductor FET) in a specific configuration.
- In this setup, the signal is applied to the gate, the output is taken from the source, and the drain is connected to the power supply.
Working Principle:
- It's called a Source Follower because the output voltage follows the input voltage minus the threshold voltage of the FET. There is no phase inversion between input and output.
- When a signal is applied to the gate, a voltage variation is created between the gate and the source, controlling the flow of current through the FET.
- The output voltage follows the input voltage, less the threshold voltage, hence offering high input impedance and low output impedance.
Characteristics and Applications:
- High Input Impedance: The input impedance is high, making it suitable for applications where the load shouldn't affect the input circuit.
- Low Voltage Gain: Typically, the voltage gain is less than unity due to the voltage drop across the FET.
- Buffering: It works as a buffer between high impedance sources and low impedance loads, preventing the load from affecting the source.
Applications:
- Buffering: Used as a buffer between high-impedance sources (like sensors or pickups) and low-impedance loads (like amplifiers or ADC inputs).
- Matching: Often employed in impedance matching circuits.
- Signal Isolation: Used to isolate a high-impedance input from a low-impedance output.
Advantages and Disadvantages:
- Advantages: High input impedance, low output impedance, low distortion, and good current drive capability.
- Disadvantages: Limited voltage gain, affected by the threshold voltage, and lower output compared to the input.
The Source Follower configuration is an important circuit arrangement in electronics, commonly used in various applications where impedance matching and signal buffering are essential without requiring significant voltage amplification.
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