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 a bootstrap technique, is a configuration used to enhance the input and output impedance characteristics of the amplifier. This configuration is commonly employed in analog circuits and integrated circuits (ICs) to achieve certain design objectives. Let's break down how it provides high input impedance and low output impedance:
High Input Impedance:
The key component that enables a bootstrapped source follower to have high input impedance is the bootstrapping capacitor. In a typical common-drain (source follower) configuration, the gate of the transistor is connected directly to the input signal, and the gate-source capacitance (C_gs) presents a significant impedance to the input.
In contrast, in a bootstrapped source follower, the gate is connected to a bootstrapping capacitor (C_boot), which is in turn connected to a high-impedance node, typically the drain of the transistor or a node with a large impedance. The bootstrapping capacitor is charged to a voltage close to the drain voltage, effectively "following" the changes in the output voltage. This technique creates a positive feedback effect on the gate voltage, reducing the effective gate-source voltage variation and thus reducing the effective input capacitance.
As a result, the input impedance is significantly increased compared to a regular source follower configuration. The bootstrapping technique minimizes the loading effect on the input signal, making the amplifier more sensitive to weak input signals and maintaining a high input impedance.
Low Output Impedance:
The output impedance of a bootstrapped source follower is inherently low due to the common-drain (source follower) configuration of the transistor itself. In a common-drain amplifier, the output voltage is taken from the drain terminal, which is directly connected to the power supply voltage through a resistor (load resistor).
Since the drain terminal voltage follows the input signal with a voltage gain close to unity (small signal voltage gain), the output impedance at the drain is essentially determined by the drain resistor (load resistor) and the on-resistance of the transistor.
The transistor's output impedance in the common-drain configuration (Z_out = R_d + r_on) is inherently low because the on-resistance (r_on) of a MOSFET (metal-oxide-semiconductor field-effect transistor) is typically quite small compared to other types of transistors like bipolar junction transistors (BJTs). Therefore, the output impedance is mainly determined by the drain resistor (R_d).
The bootstrap technique doesn't directly impact the output impedance, but it enhances the input impedance, which is beneficial for signal sources with relatively high output impedance. By maintaining a high input impedance and low output impedance, the bootstrapped source follower becomes a useful buffer or impedance matching stage in various electronic circuits.
High Input Impedance:
The key component that enables a bootstrapped source follower to have high input impedance is the bootstrapping capacitor. In a typical common-drain (source follower) configuration, the gate of the transistor is connected directly to the input signal, and the gate-source capacitance (C_gs) presents a significant impedance to the input.
In contrast, in a bootstrapped source follower, the gate is connected to a bootstrapping capacitor (C_boot), which is in turn connected to a high-impedance node, typically the drain of the transistor or a node with a large impedance. The bootstrapping capacitor is charged to a voltage close to the drain voltage, effectively "following" the changes in the output voltage. This technique creates a positive feedback effect on the gate voltage, reducing the effective gate-source voltage variation and thus reducing the effective input capacitance.
As a result, the input impedance is significantly increased compared to a regular source follower configuration. The bootstrapping technique minimizes the loading effect on the input signal, making the amplifier more sensitive to weak input signals and maintaining a high input impedance.
Low Output Impedance:
The output impedance of a bootstrapped source follower is inherently low due to the common-drain (source follower) configuration of the transistor itself. In a common-drain amplifier, the output voltage is taken from the drain terminal, which is directly connected to the power supply voltage through a resistor (load resistor).
Since the drain terminal voltage follows the input signal with a voltage gain close to unity (small signal voltage gain), the output impedance at the drain is essentially determined by the drain resistor (load resistor) and the on-resistance of the transistor.
The transistor's output impedance in the common-drain configuration (Z_out = R_d + r_on) is inherently low because the on-resistance (r_on) of a MOSFET (metal-oxide-semiconductor field-effect transistor) is typically quite small compared to other types of transistors like bipolar junction transistors (BJTs). Therefore, the output impedance is mainly determined by the drain resistor (R_d).
The bootstrap technique doesn't directly impact the output impedance, but it enhances the input impedance, which is beneficial for signal sources with relatively high output impedance. By maintaining a high input impedance and low output impedance, the bootstrapped source follower becomes a useful buffer or impedance matching stage in various electronic circuits.