Input impedance

Definition

Input impedance is the measure of how much a device resists the flow of current at its input terminal, expressed in ohms. It is an important characteristic in amplifier configurations, as it affects how the amplifier interacts with the source signal and determines the overall performance of the circuit. High input impedance is generally preferred as it minimizes loading effects on the preceding stage, allowing for better signal transfer and fidelity.

5 Must Know Facts For Your Next Test

1. In FET amplifiers, the input impedance is typically very high, often in the megohm range, allowing them to connect to high-impedance sources without significant signal loss.
2. For BJT amplifiers, input impedance can be lower than that of FETs, usually in the range of hundreds to thousands of ohms, depending on configuration.
3. The design of an amplifier can be influenced by input impedance, as it dictates how well an amplifier will interface with other components in a circuit.
4. In comparing BJT and FET characteristics, FETs offer better input impedance, which can lead to improved performance in certain applications like audio and sensor interfacing.
5. Mismatch between source and input impedances can lead to reflections and distortion, which is crucial to consider in designing effective amplifier circuits.

Review Questions

• How does input impedance affect the performance of FET amplifiers compared to BJT amplifiers?
  • Input impedance significantly influences the performance of both FET and BJT amplifiers. FET amplifiers typically have very high input impedance, which allows them to interface well with high-impedance sources without drawing much current. In contrast, BJT amplifiers generally have lower input impedance, which may cause more loading on the source. This difference affects how each type of amplifier is used in practical applications, with FETs being preferred in situations where minimal loading is critical.
• Discuss the implications of loading effects caused by mismatched input impedance in amplifier circuits.
  • Loading effects from mismatched input impedance can lead to significant distortion and loss of signal fidelity in amplifier circuits. When an amplifier's input impedance is low compared to the source's output impedance, it draws more current than desired, causing a drop in voltage across the source. This not only diminishes signal strength but also alters frequency response characteristics. Designers must carefully match impedances to avoid these issues and ensure optimal performance.
• Evaluate the importance of selecting appropriate input impedance values in the design of amplifier circuits for specific applications.
  • Choosing appropriate input impedance values is critical in amplifier circuit design as it directly affects signal integrity and overall system performance. For example, high input impedance is ideal for interfacing with sensors or other high-impedance sources to prevent loading effects that could degrade performance. On the other hand, certain applications may require lower input impedance to achieve specific gain characteristics. Evaluating these requirements allows engineers to optimize circuit behavior based on application needs while ensuring efficient power usage and reliable operation.

Related terms

Output Impedance: Output impedance is the resistance to current flow at the output terminal of a device, influencing how the device can drive loads connected to it.

Voltage Divider: A voltage divider is a simple circuit that reduces voltage by dividing it between two resistors, illustrating the impact of input and output impedance in circuits.

Loading Effect: Loading effect occurs when a circuit's performance is affected by the connection of another circuit, often due to mismatched impedances.