Discuss the operation of a common-drain MOSFET amplifier and its voltage gain characteristics.
1 Answer
A common-drain MOSFET amplifier, also known as a source follower or voltage follower, is a specific configuration of a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) used for amplification and impedance matching in electronic circuits. In this configuration, the MOSFET's gate is the input terminal, the drain is the output terminal, and the source is the common terminal, meaning it is connected to the reference potential (usually ground).
Operation:
When a voltage signal is applied to the gate of the MOSFET, it modulates the width of the channel between the source and the drain. The channel acts as a resistor whose value depends on the voltage at the gate. As the gate voltage increases, the channel resistance decreases, allowing more current to flow from the drain to the source. Conversely, as the gate voltage decreases, the channel resistance increases, reducing the current flow.
The source terminal is directly connected to the gate terminal (no resistance between them), creating negative feedback in the circuit. As a result, the source voltage closely follows the voltage applied to the gate, and this is why it's called a "voltage follower." The drain current, however, is not affected by this feedback, as the gate-to-source voltage is always near zero due to the feedback effect, making the drain current approximately constant.
Voltage Gain Characteristics:
The voltage gain (Av) of a common-drain MOSFET amplifier is approximately equal to 1. In theory, it should be 1, but there is a small voltage drop across the MOSFET due to its finite output resistance (r_ds) and the load resistance (R_load). This leads to a slightly reduced output voltage compared to the input voltage.
The voltage gain of a common-drain MOSFET amplifier can be calculated using the following formula:
Av = 1 / (1 + (R_load / r_ds))
Where:
R_load = Load resistance connected to the drain terminal of the MOSFET.
r_ds = Output resistance of the MOSFET (drain-to-source resistance).
For practical MOSFETs, r_ds is typically quite low, and as a result, the voltage gain of the common-drain amplifier is still very close to 1. This low voltage gain is often desired in specific applications where impedance matching between different stages of a circuit is necessary.
Advantages and Applications:
The common-drain MOSFET amplifier has several advantages, including:
High input impedance: Due to the gate being the input terminal, it has very high input impedance, making it suitable for interfacing with high-impedance sources without loading them significantly.
Low output impedance: The source follower exhibits low output impedance, making it capable of driving low-impedance loads efficiently.
Unity voltage gain: It provides a near-unity voltage gain, ensuring that the output voltage closely tracks the input voltage.
Applications of common-drain MOSFET amplifiers include impedance matching, buffering, level shifting, and DC voltage coupling between circuit stages. Its ability to provide a high input impedance and low output impedance makes it valuable in various analog signal processing applications.
Operation:
When a voltage signal is applied to the gate of the MOSFET, it modulates the width of the channel between the source and the drain. The channel acts as a resistor whose value depends on the voltage at the gate. As the gate voltage increases, the channel resistance decreases, allowing more current to flow from the drain to the source. Conversely, as the gate voltage decreases, the channel resistance increases, reducing the current flow.
The source terminal is directly connected to the gate terminal (no resistance between them), creating negative feedback in the circuit. As a result, the source voltage closely follows the voltage applied to the gate, and this is why it's called a "voltage follower." The drain current, however, is not affected by this feedback, as the gate-to-source voltage is always near zero due to the feedback effect, making the drain current approximately constant.
Voltage Gain Characteristics:
The voltage gain (Av) of a common-drain MOSFET amplifier is approximately equal to 1. In theory, it should be 1, but there is a small voltage drop across the MOSFET due to its finite output resistance (r_ds) and the load resistance (R_load). This leads to a slightly reduced output voltage compared to the input voltage.
The voltage gain of a common-drain MOSFET amplifier can be calculated using the following formula:
Av = 1 / (1 + (R_load / r_ds))
Where:
R_load = Load resistance connected to the drain terminal of the MOSFET.
r_ds = Output resistance of the MOSFET (drain-to-source resistance).
For practical MOSFETs, r_ds is typically quite low, and as a result, the voltage gain of the common-drain amplifier is still very close to 1. This low voltage gain is often desired in specific applications where impedance matching between different stages of a circuit is necessary.
Advantages and Applications:
The common-drain MOSFET amplifier has several advantages, including:
High input impedance: Due to the gate being the input terminal, it has very high input impedance, making it suitable for interfacing with high-impedance sources without loading them significantly.
Low output impedance: The source follower exhibits low output impedance, making it capable of driving low-impedance loads efficiently.
Unity voltage gain: It provides a near-unity voltage gain, ensuring that the output voltage closely tracks the input voltage.
Applications of common-drain MOSFET amplifiers include impedance matching, buffering, level shifting, and DC voltage coupling between circuit stages. Its ability to provide a high input impedance and low output impedance makes it valuable in various analog signal processing applications.