Common drain amplifier

Definition

A common drain amplifier, also known as a source follower, is a type of field-effect transistor (FET) amplifier configuration that provides high input impedance and low output impedance. This configuration allows the amplifier to serve as a buffer between circuits, transferring signals while preserving their voltage levels with minimal distortion. It is particularly useful in applications where impedance matching is crucial, as it effectively isolates the input from the output.

5 Must Know Facts For Your Next Test

1. In a common drain amplifier, the output is taken from the source terminal of the FET, which leads to a direct relationship between input and output voltage.
2. The common drain configuration typically offers a voltage gain of approximately 1, making it ideal for buffering applications.
3. One of the main advantages of this configuration is its high input impedance, often in the megaohm range, which minimizes loading effects on preceding stages.
4. Common drain amplifiers are less sensitive to variations in supply voltage compared to other configurations, contributing to their stability in operation.
5. This amplifier type is widely used in RF applications, signal conditioning, and interfacing with high-impedance sources.

Review Questions

• How does a common drain amplifier affect the voltage level of an incoming signal?
  • A common drain amplifier has a voltage gain close to 1, meaning it does not significantly alter the voltage level of an incoming signal. Instead, it acts as a buffer, providing isolation while maintaining the integrity of the original signal. This characteristic makes it suitable for applications where signal fidelity is essential and prevents loading effects that might occur if the source was directly connected to a load.
• What role does input impedance play in the effectiveness of a common drain amplifier?
  • Input impedance is crucial in determining how well a common drain amplifier can interface with previous circuit stages. The high input impedance ensures that the amplifier does not draw significant current from its source, allowing it to accurately reproduce signals without distorting them. This property enables effective impedance matching, which is especially important when connecting sensitive components or high-impedance sources to lower impedance loads.
• Evaluate the significance of using a common drain amplifier in RF applications compared to other FET configurations.
  • The use of common drain amplifiers in RF applications is significant due to their ability to maintain high signal integrity while providing excellent impedance matching. Unlike other FET configurations that may introduce more distortion or have lower input impedances, common drain amplifiers minimize loading effects on high-impedance sources and deliver stable output levels. This makes them particularly effective for signal conditioning in RF circuits where clarity and fidelity are paramount, allowing for optimal performance across various frequencies.

Related terms

Field-Effect Transistor (FET): A type of transistor that controls current flow through an electric field, commonly used in amplifiers and switching devices.

Voltage Gain: The ratio of output voltage to input voltage in an amplifier, indicating how much an amplifier boosts the voltage level of a signal.

Input Impedance: The measure of how much resistance an amplifier presents to the input signal, influencing how easily a signal can be fed into the amplifier.