Define common-drain configuration in field-effect transistors (FETs).
1 Answer
The common-drain (CD) configuration is one of the three basic configurations used in operating field-effect transistors (FETs). In this configuration, the FET's gate terminal is connected to a signal source, the drain terminal is connected to a load resistor, and the source terminal is grounded. It's also referred to as the "source follower" configuration.
Here's a breakdown of the key characteristics and behavior of the common-drain configuration:
Gate Connection: The gate terminal of the FET is connected to the input signal source. This controls the voltage at the gate and, consequently, the current flowing between the source and drain terminals.
Source Connection: The source terminal is directly connected to the ground reference, providing a fixed reference potential.
Drain Connection: The drain terminal is connected to a load resistor, which is typically in series with the FET. The other end of the load resistor is connected to a power supply voltage (VDD), creating a voltage bias across the load.
Voltage Gain: The common-drain configuration has a voltage gain of less than 1. This means that the output voltage (across the load resistor) will generally be smaller in magnitude than the input voltage applied to the gate. The voltage gain can be calculated as the ratio of the change in output voltage to the change in input voltage.
Current Gain: The common-drain configuration offers a high current gain. The current flowing through the load resistor (and the FET itself) is approximately equal to the current flowing into the gate terminal.
Phase Relationship: The output signal is in phase with the input signal. This is advantageous for applications where impedance matching or voltage buffering is needed.
Input-Output Relationship: The output voltage follows the input voltage quite closely, minus the voltage drop across the FET due to its internal resistance.
Voltage Biasing: Since the source terminal is grounded, the FET's gate-source voltage is constant (VGS = 0) during operation. This means the FET operates in the "pinch-off" region, where the drain current is relatively constant.
Input Impedance: The input impedance of the common-drain configuration is relatively high, making it suitable for interfacing with high-impedance signal sources.
Output Impedance: The output impedance of this configuration is relatively low, which can help drive low-impedance loads.
The common-drain configuration is often used as a voltage buffer or impedance matching stage in electronic circuits. It provides a high input impedance and a low output impedance, making it useful for isolating stages with different impedance characteristics while maintaining a consistent signal level.
Here's a breakdown of the key characteristics and behavior of the common-drain configuration:
Gate Connection: The gate terminal of the FET is connected to the input signal source. This controls the voltage at the gate and, consequently, the current flowing between the source and drain terminals.
Source Connection: The source terminal is directly connected to the ground reference, providing a fixed reference potential.
Drain Connection: The drain terminal is connected to a load resistor, which is typically in series with the FET. The other end of the load resistor is connected to a power supply voltage (VDD), creating a voltage bias across the load.
Voltage Gain: The common-drain configuration has a voltage gain of less than 1. This means that the output voltage (across the load resistor) will generally be smaller in magnitude than the input voltage applied to the gate. The voltage gain can be calculated as the ratio of the change in output voltage to the change in input voltage.
Current Gain: The common-drain configuration offers a high current gain. The current flowing through the load resistor (and the FET itself) is approximately equal to the current flowing into the gate terminal.
Phase Relationship: The output signal is in phase with the input signal. This is advantageous for applications where impedance matching or voltage buffering is needed.
Input-Output Relationship: The output voltage follows the input voltage quite closely, minus the voltage drop across the FET due to its internal resistance.
Voltage Biasing: Since the source terminal is grounded, the FET's gate-source voltage is constant (VGS = 0) during operation. This means the FET operates in the "pinch-off" region, where the drain current is relatively constant.
Input Impedance: The input impedance of the common-drain configuration is relatively high, making it suitable for interfacing with high-impedance signal sources.
Output Impedance: The output impedance of this configuration is relatively low, which can help drive low-impedance loads.
The common-drain configuration is often used as a voltage buffer or impedance matching stage in electronic circuits. It provides a high input impedance and a low output impedance, making it useful for isolating stages with different impedance characteristics while maintaining a consistent signal level.