Common gate amplifier

Definition

A common gate amplifier is a type of FET amplifier configuration where the gate terminal is held at a constant voltage while the input signal is applied to the source terminal. This configuration allows for high input impedance and low output impedance, making it suitable for specific applications like RF amplification and impedance matching. The common gate setup provides a unique way to amplify signals, particularly beneficial in scenarios where phase inversion is not desired.

5 Must Know Facts For Your Next Test

1. In a common gate amplifier, the input is applied at the source terminal while the gate is grounded or held at a reference voltage, resulting in a non-inverting configuration.
2. Common gate amplifiers are known for their high frequency response due to their ability to reduce capacitance effects at high frequencies.
3. This amplifier configuration typically has low input impedance and high output impedance, making it ideal for certain types of applications.
4. They are particularly useful in RF circuits where maintaining phase relationships between signals is crucial.
5. Common gate amplifiers can achieve a voltage gain greater than one, allowing them to effectively amplify weak signals.

Review Questions

• How does the configuration of a common gate amplifier affect its input and output characteristics?
  • In a common gate amplifier, the configuration leads to low input impedance because the input is applied at the source terminal. This means it can handle higher current levels but may not be suitable for all signal sources. Conversely, it has high output impedance, which allows it to drive loads effectively without significant loss of voltage, making it advantageous for specific applications.
• Compare and contrast the common gate amplifier with other FET configurations such as common source and common drain amplifiers.
  • The common gate amplifier differs from common source and common drain configurations primarily in terms of input and output impedances. While common source amplifiers have high input impedance and moderate output impedance, the common gate has low input impedance but high output impedance. In contrast, the common drain configuration (also known as source follower) exhibits high input and low output impedance. Each configuration serves different purposes based on these characteristics.
• Evaluate the role of common gate amplifiers in RF applications and explain why they are preferred in those scenarios.
  • Common gate amplifiers play a crucial role in RF applications due to their ability to maintain phase integrity while amplifying high-frequency signals. Their low input impedance allows them to interface effectively with antennas or other low-impedance sources. Moreover, their capability to deliver substantial voltage gain without phase inversion makes them ideal for use in RF circuits where signal fidelity is essential for effective transmission and reception.

Related terms

FET: A Field Effect Transistor, which is a type of transistor that relies on an electric field to control the flow of current, commonly used in amplifying or switching applications.

input impedance:

The measure of how much resistance an amplifier presents to an input signal, important for ensuring minimal signal loss when connecting different stages of a circuit.

output impedance:

The measure of how much resistance an amplifier presents to its output signal, impacting how well it can drive connected loads.