How does a bootstrapped source follower provide high input impedance and low output impedance?
1 Answer
A bootstrapped source follower, also known as a common-drain amplifier with bootstrapping, is a configuration used to achieve high input impedance and low output impedance. Let's break down the operation and see how it accomplishes these characteristics:
Source Follower Configuration:
A source follower, or common-drain amplifier, is a type of FET (Field Effect Transistor) amplifier configuration. It consists of a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) with its gate connected to the input signal, its drain connected to the output, and its source connected to the common ground reference.
Basic Operation:
The input signal is applied to the gate terminal of the MOSFET. The output is taken from the drain terminal, and the source terminal is connected to a fixed reference voltage, typically the ground (0V). The gate-source voltage (Vgs) controls the drain current (Id) flowing through the MOSFET.
High Input Impedance:
The input impedance of a common-drain amplifier is mainly determined by the gate-source impedance (Zgs) of the MOSFET. In an ideal scenario, the gate of a MOSFET draws no current (i.e., infinite input impedance). However, in practice, the gate of a MOSFET has some input capacitance, which results in a finite input impedance.
When the source follower is bootstrapped, a capacitor is used to create a voltage buffer at the gate terminal. The capacitor is connected between the gate terminal and the output terminal (drain terminal). This arrangement ensures that the gate voltage closely follows the output voltage.
During operation, as the output voltage changes, the capacitor charges or discharges accordingly. This change in voltage at the gate keeps the gate-source voltage (Vgs) constant or nearly constant, thus keeping the MOSFET operating in its linear region. As a result, the gate-source impedance (Zgs) remains relatively high, providing the bootstrapped source follower with a high input impedance.
Low Output Impedance:
The output impedance of a common-drain amplifier is relatively low due to the connection of the load to the drain terminal. However, when the source follower is bootstrapped, the output impedance is further improved.
As mentioned earlier, the gate voltage closely follows the output voltage. As a result, when the output voltage changes, the gate voltage also changes accordingly. This change in gate voltage alters the MOSFET's conduction, effectively adjusting the drain current to accommodate for the output voltage change. This self-adjusting behavior helps to reduce the output impedance of the source follower.
In summary, a bootstrapped source follower achieves high input impedance by using a capacitor to keep the gate voltage stable, and it achieves low output impedance by adjusting the drain current based on the output voltage changes. This configuration is commonly used in electronic circuits where a high input impedance and low output impedance are desired, such as impedance matching applications or voltage buffering scenarios.
Source Follower Configuration:
A source follower, or common-drain amplifier, is a type of FET (Field Effect Transistor) amplifier configuration. It consists of a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) with its gate connected to the input signal, its drain connected to the output, and its source connected to the common ground reference.
Basic Operation:
The input signal is applied to the gate terminal of the MOSFET. The output is taken from the drain terminal, and the source terminal is connected to a fixed reference voltage, typically the ground (0V). The gate-source voltage (Vgs) controls the drain current (Id) flowing through the MOSFET.
High Input Impedance:
The input impedance of a common-drain amplifier is mainly determined by the gate-source impedance (Zgs) of the MOSFET. In an ideal scenario, the gate of a MOSFET draws no current (i.e., infinite input impedance). However, in practice, the gate of a MOSFET has some input capacitance, which results in a finite input impedance.
When the source follower is bootstrapped, a capacitor is used to create a voltage buffer at the gate terminal. The capacitor is connected between the gate terminal and the output terminal (drain terminal). This arrangement ensures that the gate voltage closely follows the output voltage.
During operation, as the output voltage changes, the capacitor charges or discharges accordingly. This change in voltage at the gate keeps the gate-source voltage (Vgs) constant or nearly constant, thus keeping the MOSFET operating in its linear region. As a result, the gate-source impedance (Zgs) remains relatively high, providing the bootstrapped source follower with a high input impedance.
Low Output Impedance:
The output impedance of a common-drain amplifier is relatively low due to the connection of the load to the drain terminal. However, when the source follower is bootstrapped, the output impedance is further improved.
As mentioned earlier, the gate voltage closely follows the output voltage. As a result, when the output voltage changes, the gate voltage also changes accordingly. This change in gate voltage alters the MOSFET's conduction, effectively adjusting the drain current to accommodate for the output voltage change. This self-adjusting behavior helps to reduce the output impedance of the source follower.
In summary, a bootstrapped source follower achieves high input impedance by using a capacitor to keep the gate voltage stable, and it achieves low output impedance by adjusting the drain current based on the output voltage changes. This configuration is commonly used in electronic circuits where a high input impedance and low output impedance are desired, such as impedance matching applications or voltage buffering scenarios.