What is the significance of common-mode voltage in differential amplifiers and its rejection ratio?
1 Answer
In differential amplifiers, the common-mode voltage and its rejection ratio are important concepts that help to understand the device's performance and its ability to reject unwanted noise.
Common-mode voltage (Vcm):
In a differential amplifier, two inputs are provided: one for the non-inverting signal (Vp) and one for the inverting signal (Vm). The common-mode voltage (Vcm) is the average voltage level at both inputs, and it represents the voltage that is present equally at both inputs. Mathematically, it is the average of Vp and Vm:
Vcm = (Vp + Vm) / 2
The differential amplifier amplifies the voltage difference between Vp and Vm while attempting to reject any signal that appears as a common-mode voltage.
Common-mode rejection ratio (CMRR):
The common-mode rejection ratio (CMRR) is a measure of the ability of a differential amplifier to reject common-mode signals and amplify only the differential signals. It is defined as the ratio of the differential-mode gain (Ad) to the common-mode gain (Ac). Mathematically, it is expressed as:
CMRR = Ad / Ac
A high CMRR value is desirable because it indicates that the differential amplifier is more effective at amplifying the desired signal (differential signal) while rejecting any noise or interference that appears as a common-mode voltage.
The CMRR is typically expressed in decibels (dB) and is a crucial parameter for assessing the quality and performance of differential amplifiers. Higher CMRR values imply better common-mode rejection capabilities, which are essential in many applications where noise and interference are common.
In practical scenarios, it is challenging to achieve infinite CMRR, so engineers design differential amplifiers to have the highest possible CMRR for their specific application requirements. Factors such as component matching, balanced circuit design, and careful layout play a significant role in achieving good common-mode rejection in differential amplifiers.
Common-mode voltage (Vcm):
In a differential amplifier, two inputs are provided: one for the non-inverting signal (Vp) and one for the inverting signal (Vm). The common-mode voltage (Vcm) is the average voltage level at both inputs, and it represents the voltage that is present equally at both inputs. Mathematically, it is the average of Vp and Vm:
Vcm = (Vp + Vm) / 2
The differential amplifier amplifies the voltage difference between Vp and Vm while attempting to reject any signal that appears as a common-mode voltage.
Common-mode rejection ratio (CMRR):
The common-mode rejection ratio (CMRR) is a measure of the ability of a differential amplifier to reject common-mode signals and amplify only the differential signals. It is defined as the ratio of the differential-mode gain (Ad) to the common-mode gain (Ac). Mathematically, it is expressed as:
CMRR = Ad / Ac
A high CMRR value is desirable because it indicates that the differential amplifier is more effective at amplifying the desired signal (differential signal) while rejecting any noise or interference that appears as a common-mode voltage.
The CMRR is typically expressed in decibels (dB) and is a crucial parameter for assessing the quality and performance of differential amplifiers. Higher CMRR values imply better common-mode rejection capabilities, which are essential in many applications where noise and interference are common.
In practical scenarios, it is challenging to achieve infinite CMRR, so engineers design differential amplifiers to have the highest possible CMRR for their specific application requirements. Factors such as component matching, balanced circuit design, and careful layout play a significant role in achieving good common-mode rejection in differential amplifiers.