What is a bootstrapped source follower (Common Drain) amplifier?
1 Answer
A bootstrapped source follower, also known as a common drain amplifier, is a type of electronic amplifier circuit used in analog electronics. It is commonly used to buffer a high-impedance input signal while providing a low-impedance output signal. The term "bootstrapped" refers to a technique used to improve the input impedance of the amplifier circuit.
The basic configuration of a bootstrapped source follower (common drain) amplifier consists of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) configured in a common drain arrangement, with the gate terminal connected to the input signal and the source terminal serving as the output. The drain terminal is connected to a power supply.
The key feature of this amplifier is the use of a bootstrapping capacitor, connected between the gate and the drain of the MOSFET. The capacitor serves as an AC coupling between these two terminals and plays a crucial role in boosting the input impedance of the amplifier.
Here's how it works:
Input signal: The input signal is applied to the gate terminal of the MOSFET. As the gate is a high-impedance node, it can be susceptible to loading effects. However, the presence of the bootstrapping capacitor mitigates this issue.
Bootstrapping capacitor: During the amplifier's operation, when the output voltage changes, the bootstrapping capacitor charges and discharges accordingly. This creates a voltage across the capacitor that adds to the input voltage at the gate terminal. This effect essentially "boosts" the voltage at the gate, providing a larger voltage swing at the input than what is directly applied.
Output signal: The source terminal of the MOSFET is the amplifier's output, and it follows the voltage at the gate (input) with very little attenuation. The output impedance is relatively low, making it suitable for driving low-impedance loads.
Benefits of bootstrapped source follower:
High input impedance: The bootstrapping capacitor increases the effective input impedance of the amplifier, making it less sensitive to the loading effects of subsequent stages.
Low output impedance: The source follower configuration provides a low output impedance, making it an excellent buffer for driving other circuits or loads.
Voltage buffering: The voltage gain of a source follower is less than unity (less than one), typically slightly below 1, so it doesn't amplify voltage but effectively buffers and isolates the input from the output.
Bootstrapped source followers find applications in audio amplifiers, impedance matching circuits, and other scenarios where high input impedance and low output impedance are desired.
The basic configuration of a bootstrapped source follower (common drain) amplifier consists of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) configured in a common drain arrangement, with the gate terminal connected to the input signal and the source terminal serving as the output. The drain terminal is connected to a power supply.
The key feature of this amplifier is the use of a bootstrapping capacitor, connected between the gate and the drain of the MOSFET. The capacitor serves as an AC coupling between these two terminals and plays a crucial role in boosting the input impedance of the amplifier.
Here's how it works:
Input signal: The input signal is applied to the gate terminal of the MOSFET. As the gate is a high-impedance node, it can be susceptible to loading effects. However, the presence of the bootstrapping capacitor mitigates this issue.
Bootstrapping capacitor: During the amplifier's operation, when the output voltage changes, the bootstrapping capacitor charges and discharges accordingly. This creates a voltage across the capacitor that adds to the input voltage at the gate terminal. This effect essentially "boosts" the voltage at the gate, providing a larger voltage swing at the input than what is directly applied.
Output signal: The source terminal of the MOSFET is the amplifier's output, and it follows the voltage at the gate (input) with very little attenuation. The output impedance is relatively low, making it suitable for driving low-impedance loads.
Benefits of bootstrapped source follower:
High input impedance: The bootstrapping capacitor increases the effective input impedance of the amplifier, making it less sensitive to the loading effects of subsequent stages.
Low output impedance: The source follower configuration provides a low output impedance, making it an excellent buffer for driving other circuits or loads.
Voltage buffering: The voltage gain of a source follower is less than unity (less than one), typically slightly below 1, so it doesn't amplify voltage but effectively buffers and isolates the input from the output.
Bootstrapped source followers find applications in audio amplifiers, impedance matching circuits, and other scenarios where high input impedance and low output impedance are desired.