Common-mode rejection ratio

Definition

The common-mode rejection ratio (CMRR) is a measure of an operational amplifier's ability to reject common-mode signals, which are signals that appear simultaneously and in-phase on both inputs. A high CMRR indicates that the op-amp can effectively differentiate between the desired differential signal and unwanted noise or interference, ensuring better performance in practical applications. It is crucial for amplifiers used in environments with significant noise, allowing them to amplify the intended signal while minimizing the impact of common-mode noise.

5 Must Know Facts For Your Next Test

1. CMRR is typically expressed in decibels (dB), calculated as 20 times the logarithm of the ratio of differential gain to common-mode gain.
2. A CMRR of 100 dB means that the differential gain is 100,000 times greater than the common-mode gain, indicating a strong ability to reject noise.
3. In practical applications, CMRR can be affected by factors like temperature changes and variations in component values, making it important to choose high-quality components.
4. The ideal op-amp has an infinite CMRR, but real op-amps will have finite values typically ranging from 70 dB to over 130 dB.
5. When designing circuits, engineers often use feedback techniques to improve CMRR and enhance overall performance.

Review Questions

• How does a high common-mode rejection ratio benefit practical applications in electronic circuits?
  • A high common-mode rejection ratio enhances practical applications by enabling op-amps to effectively amplify desired signals while suppressing unwanted noise or interference. This is particularly important in environments with significant electrical noise, where differentiating between the true signal and noise is crucial for accurate signal processing. As a result, circuits utilizing high CMRR op-amps can deliver cleaner and more reliable outputs.
• What factors can influence the common-mode rejection ratio in an operational amplifier and how might these be mitigated in circuit design?
  • Factors influencing the common-mode rejection ratio include temperature variations, component tolerances, and mismatches in input characteristics. Engineers can mitigate these effects by selecting high-quality components with tight tolerances and implementing feedback mechanisms that balance input conditions. Additionally, careful circuit layout can reduce noise coupling and improve overall performance.
• Evaluate the implications of having a low common-mode rejection ratio on the performance of an operational amplifier circuit in real-world scenarios.
  • Having a low common-mode rejection ratio can severely impact an operational amplifier's performance by allowing common-mode noise to be amplified along with the desired differential signal. This leads to distorted outputs and unreliable operation, especially in noisy environments. In critical applications such as medical instrumentation or audio processing, this can result in significant errors or degradation of signal quality, underscoring the importance of selecting op-amps with adequate CMRR for specific uses.

Related terms

Differential Amplifier: A type of amplifier that amplifies the difference between two input signals while rejecting any signals that are common to both inputs.

Input Impedance: The resistance faced by an incoming signal at the input of an amplifier, influencing how much of the signal is allowed to pass through.

Op-Amp Gain: The amplification factor of an operational amplifier, which determines how much the input signal will be amplified at the output.