Explain the operation of a common-gate MOSFET amplifier and its voltage gain characteristics.
1 Answer
A common-gate MOSFET amplifier is a type of transistor amplifier configuration that uses a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) in a common-gate configuration. In this setup, the gate terminal of the MOSFET is the input, the source is the output, and the drain is connected to the power supply. The common-gate configuration is used to achieve specific characteristics in the amplifier circuit.
Operation of Common-Gate MOSFET Amplifier:
DC Biasing: The first step in designing any MOSFET amplifier is to establish the DC biasing conditions. This involves setting up a DC voltage at the gate terminal and source terminal to ensure that the MOSFET operates in the desired region of its characteristic curves. Usually, a voltage divider network is used to create the appropriate biasing voltage.
AC Signal Input: Once the DC biasing is set, an AC signal is superimposed on top of the DC bias at the gate terminal. This AC input signal is the one we want to amplify. The AC signal can be represented as Vgs, where Vgs = Vg(ac) + Vg(dc).
Signal Amplification: When the AC signal is applied to the gate terminal, it causes variations in the gate-source voltage (Vgs). These variations result in changes in the MOSFET's channel resistance, which in turn lead to variations in the drain current (Id). Since the source is common to both the input and output, the output voltage Vds (drain-source voltage) varies in response to the input signal.
AC Load: A load resistor (Rd) is connected between the drain terminal and the power supply to create the output voltage. The AC variations in Vds cause corresponding AC voltage variations across Rd, which form the amplified output signal.
Output Signal: The amplified output signal is taken from the voltage across the load resistor (Vout = Vds(ac)).
Voltage Gain Characteristics:
The voltage gain of a common-gate MOSFET amplifier is given by the formula:
Av = -gm * Rd
Where:
Av is the voltage gain.
gm is the transconductance of the MOSFET, which is the change in drain current (Id) for a change in gate-source voltage (Vgs).
Rd is the load resistor.
The negative sign in the formula indicates that the output signal is inverted with respect to the input signal. The voltage gain of the common-gate MOSFET amplifier depends on two key parameters:
Transconductance (gm): The transconductance (gm) is a measure of how much the drain current (Id) changes in response to a change in gate-source voltage (Vgs). It depends on the MOSFET's biasing conditions and its physical characteristics.
Load Resistance (Rd): The load resistance (Rd) is the resistor connected to the drain terminal, and it determines the output signal voltage that can be developed across it. A larger value of Rd results in higher voltage gain, but it also affects other aspects of the amplifier's performance.
In summary, the common-gate MOSFET amplifier provides voltage amplification by utilizing the transconductance of the MOSFET to convert input voltage variations into corresponding output current variations, which are then converted back to voltage variations by the load resistor, creating the amplified output signal.
Operation of Common-Gate MOSFET Amplifier:
DC Biasing: The first step in designing any MOSFET amplifier is to establish the DC biasing conditions. This involves setting up a DC voltage at the gate terminal and source terminal to ensure that the MOSFET operates in the desired region of its characteristic curves. Usually, a voltage divider network is used to create the appropriate biasing voltage.
AC Signal Input: Once the DC biasing is set, an AC signal is superimposed on top of the DC bias at the gate terminal. This AC input signal is the one we want to amplify. The AC signal can be represented as Vgs, where Vgs = Vg(ac) + Vg(dc).
Signal Amplification: When the AC signal is applied to the gate terminal, it causes variations in the gate-source voltage (Vgs). These variations result in changes in the MOSFET's channel resistance, which in turn lead to variations in the drain current (Id). Since the source is common to both the input and output, the output voltage Vds (drain-source voltage) varies in response to the input signal.
AC Load: A load resistor (Rd) is connected between the drain terminal and the power supply to create the output voltage. The AC variations in Vds cause corresponding AC voltage variations across Rd, which form the amplified output signal.
Output Signal: The amplified output signal is taken from the voltage across the load resistor (Vout = Vds(ac)).
Voltage Gain Characteristics:
The voltage gain of a common-gate MOSFET amplifier is given by the formula:
Av = -gm * Rd
Where:
Av is the voltage gain.
gm is the transconductance of the MOSFET, which is the change in drain current (Id) for a change in gate-source voltage (Vgs).
Rd is the load resistor.
The negative sign in the formula indicates that the output signal is inverted with respect to the input signal. The voltage gain of the common-gate MOSFET amplifier depends on two key parameters:
Transconductance (gm): The transconductance (gm) is a measure of how much the drain current (Id) changes in response to a change in gate-source voltage (Vgs). It depends on the MOSFET's biasing conditions and its physical characteristics.
Load Resistance (Rd): The load resistance (Rd) is the resistor connected to the drain terminal, and it determines the output signal voltage that can be developed across it. A larger value of Rd results in higher voltage gain, but it also affects other aspects of the amplifier's performance.
In summary, the common-gate MOSFET amplifier provides voltage amplification by utilizing the transconductance of the MOSFET to convert input voltage variations into corresponding output current variations, which are then converted back to voltage variations by the load resistor, creating the amplified output signal.