What is the significance of common-mode rejection in differential amplifiers and its measurement?
1 Answer
Common-mode rejection (CMR) is a critical parameter in differential amplifiers, which are widely used in electronic circuits to amplify the difference between two input signals while rejecting any common-mode signal. The common-mode signal refers to any signal that appears in phase and with the same magnitude on both inputs of the differential amplifier.
The significance of common-mode rejection in differential amplifiers lies in its ability to suppress noise and interference that is common to both inputs, while amplifying the desired differential signal. In real-world scenarios, electronic circuits are often exposed to various sources of common-mode noise, such as electromagnetic interference (EMI), power supply ripple, and ground loop voltages. Without adequate common-mode rejection, this noise can be amplified along with the desired signal, leading to inaccuracies and reduced signal-to-noise ratio.
The common-mode rejection ratio (CMRR) is used to quantify the effectiveness of a differential amplifier in rejecting common-mode signals. It is defined as the ratio of the differential-mode gain (the amplification of the desired signal) to the common-mode gain (the amplification of the common-mode signal). A high CMRR indicates a strong ability to reject common-mode signals.
CMRR is usually expressed in decibels (dB) and calculated using the following formula:
CMRR (dB) = 20 * log10 (|Adm| / |Acm|)
Where:
Adm is the differential-mode gain (amplification of the desired signal).
Acm is the common-mode gain (amplification of the common-mode signal).
Typically, a higher CMRR value is desirable, as it indicates better common-mode rejection. In practical terms, CMRR values of 60 dB or higher are considered good for many applications, although this can vary depending on specific requirements.
To measure the common-mode rejection of a differential amplifier, the following steps are typically taken:
Apply a known common-mode signal: A test signal with a specific amplitude and frequency is applied to both inputs of the differential amplifier. This signal represents the common-mode input.
Measure the output voltage: The output voltage of the differential amplifier is measured while the common-mode signal is applied.
Calculate CMRR: Using the measured output voltage and the known amplitude of the common-mode input signal, the CMRR can be calculated using the formula mentioned earlier.
Repeat for different frequencies: The common-mode rejection may vary with frequency due to the amplifier's response characteristics. Therefore, it's essential to measure CMRR at various frequencies if the differential amplifier is expected to operate over a range of frequencies.
By measuring and optimizing the common-mode rejection of a differential amplifier, engineers can design more robust and accurate electronic circuits, especially in applications where noise rejection is crucial, such as in instrumentation, audio equipment, and communication systems.
The significance of common-mode rejection in differential amplifiers lies in its ability to suppress noise and interference that is common to both inputs, while amplifying the desired differential signal. In real-world scenarios, electronic circuits are often exposed to various sources of common-mode noise, such as electromagnetic interference (EMI), power supply ripple, and ground loop voltages. Without adequate common-mode rejection, this noise can be amplified along with the desired signal, leading to inaccuracies and reduced signal-to-noise ratio.
The common-mode rejection ratio (CMRR) is used to quantify the effectiveness of a differential amplifier in rejecting common-mode signals. It is defined as the ratio of the differential-mode gain (the amplification of the desired signal) to the common-mode gain (the amplification of the common-mode signal). A high CMRR indicates a strong ability to reject common-mode signals.
CMRR is usually expressed in decibels (dB) and calculated using the following formula:
CMRR (dB) = 20 * log10 (|Adm| / |Acm|)
Where:
Adm is the differential-mode gain (amplification of the desired signal).
Acm is the common-mode gain (amplification of the common-mode signal).
Typically, a higher CMRR value is desirable, as it indicates better common-mode rejection. In practical terms, CMRR values of 60 dB or higher are considered good for many applications, although this can vary depending on specific requirements.
To measure the common-mode rejection of a differential amplifier, the following steps are typically taken:
Apply a known common-mode signal: A test signal with a specific amplitude and frequency is applied to both inputs of the differential amplifier. This signal represents the common-mode input.
Measure the output voltage: The output voltage of the differential amplifier is measured while the common-mode signal is applied.
Calculate CMRR: Using the measured output voltage and the known amplitude of the common-mode input signal, the CMRR can be calculated using the formula mentioned earlier.
Repeat for different frequencies: The common-mode rejection may vary with frequency due to the amplifier's response characteristics. Therefore, it's essential to measure CMRR at various frequencies if the differential amplifier is expected to operate over a range of frequencies.
By measuring and optimizing the common-mode rejection of a differential amplifier, engineers can design more robust and accurate electronic circuits, especially in applications where noise rejection is crucial, such as in instrumentation, audio equipment, and communication systems.