What is a common-drain FET amplifier and its characteristics?
1 Answer
A common-drain field-effect transistor (FET) amplifier, also known as a source follower or voltage follower, is a type of amplifier circuit that utilizes a field-effect transistor in a common-drain configuration. This configuration offers several unique characteristics and advantages.
Basic Configuration:
In a common-drain amplifier, the FET's drain terminal is the common terminal for both the input and output signals. The input signal is applied to the gate terminal, and the output is taken from the source terminal. The source is usually connected to a fixed voltage supply, acting as a voltage reference.
Characteristics:
High Input Impedance: The input impedance of a common-drain amplifier is relatively high. This means that it doesn't draw much current from the preceding stage, making it suitable for driving circuits with high output impedance, such as sensors.
Low Output Impedance: The output impedance of a common-drain amplifier is relatively low compared to the input impedance. This low output impedance helps to minimize signal loss when driving low-impedance loads, such as other amplifier stages or transmission lines.
Voltage Gain Less Than Unity: The voltage gain of a common-drain amplifier is typically less than one (unity). This is due to the fact that the output voltage closely follows the input voltage, resulting in minimal amplification. The voltage gain can be roughly estimated as (1 - gm * Rd), where gm is the transconductance of the FET and Rd is the drain resistor.
High Current Gain: Despite the low voltage gain, a common-drain amplifier can provide a significant current gain. This makes it useful as a buffer or impedance matching stage, as it can supply relatively high currents without significantly affecting the input signal.
Unity Gain Frequency Response: The common-drain configuration offers a relatively wide bandwidth and good frequency response characteristics. It has a flat frequency response up to a certain frequency, making it suitable for applications where maintaining signal fidelity across a broad frequency range is important.
Voltage Level Shifting: The output voltage of the common-drain amplifier is typically slightly lower than the input voltage due to the voltage drop across the FET's channel. This can serve as a level-shifting function, useful when interfacing signals between different voltage levels.
Input-Output Phase Relationship: The common-drain amplifier has a 0-degree phase shift between the input and output signals. This phase relationship can be advantageous in certain applications where maintaining the input-output phase alignment is crucial.
Common-drain amplifiers are often used in scenarios where voltage buffering, impedance matching, and maintaining a relatively high input impedance are more important than achieving high voltage gain. Examples of such applications include voltage level shifting, signal buffering, and interfacing signals between different parts of a circuit.
Basic Configuration:
In a common-drain amplifier, the FET's drain terminal is the common terminal for both the input and output signals. The input signal is applied to the gate terminal, and the output is taken from the source terminal. The source is usually connected to a fixed voltage supply, acting as a voltage reference.
Characteristics:
High Input Impedance: The input impedance of a common-drain amplifier is relatively high. This means that it doesn't draw much current from the preceding stage, making it suitable for driving circuits with high output impedance, such as sensors.
Low Output Impedance: The output impedance of a common-drain amplifier is relatively low compared to the input impedance. This low output impedance helps to minimize signal loss when driving low-impedance loads, such as other amplifier stages or transmission lines.
Voltage Gain Less Than Unity: The voltage gain of a common-drain amplifier is typically less than one (unity). This is due to the fact that the output voltage closely follows the input voltage, resulting in minimal amplification. The voltage gain can be roughly estimated as (1 - gm * Rd), where gm is the transconductance of the FET and Rd is the drain resistor.
High Current Gain: Despite the low voltage gain, a common-drain amplifier can provide a significant current gain. This makes it useful as a buffer or impedance matching stage, as it can supply relatively high currents without significantly affecting the input signal.
Unity Gain Frequency Response: The common-drain configuration offers a relatively wide bandwidth and good frequency response characteristics. It has a flat frequency response up to a certain frequency, making it suitable for applications where maintaining signal fidelity across a broad frequency range is important.
Voltage Level Shifting: The output voltage of the common-drain amplifier is typically slightly lower than the input voltage due to the voltage drop across the FET's channel. This can serve as a level-shifting function, useful when interfacing signals between different voltage levels.
Input-Output Phase Relationship: The common-drain amplifier has a 0-degree phase shift between the input and output signals. This phase relationship can be advantageous in certain applications where maintaining the input-output phase alignment is crucial.
Common-drain amplifiers are often used in scenarios where voltage buffering, impedance matching, and maintaining a relatively high input impedance are more important than achieving high voltage gain. Examples of such applications include voltage level shifting, signal buffering, and interfacing signals between different parts of a circuit.