A difference amplifier circuit, also known as a differential amplifier, is an electronic circuit designed to amplify the difference between two input signals while rejecting any common-mode signal that is present in both inputs. Its primary purpose is to amplify the "difference" signal between two input voltages while suppressing or ignoring any signals that are common to both inputs. This makes it particularly useful in scenarios where the desired signal is carried on top of a common noise or interference.
The key features and purposes of a difference amplifier circuit include:
Signal Amplification: The main purpose of a difference amplifier is to amplify the voltage difference between its two input terminals. This is accomplished by amplifying the voltage at one input and inverting the voltage at the other input, then taking the difference between these two amplified voltages. The amplified difference signal can be used for various purposes, such as further processing, measurement, or control.
Common-Mode Rejection: One of the most important features of a difference amplifier is its ability to reject common-mode signals. Common-mode signals are those present in both input terminals with the same magnitude and phase. These signals could be noise, interference, or any external disturbances affecting both input lines. The difference amplifier circuit is designed in such a way that it amplifies only the difference between the input signals and suppresses the common-mode signals, resulting in improved signal quality.
Balanced Inputs: Difference amplifiers typically have balanced inputs, meaning they have two input terminals that are equal and opposite with respect to each other. This balance helps in rejecting common-mode signals because any signal that is common to both inputs will be canceled out when calculating the voltage difference.
Instrumentation Amplification: Difference amplifiers are commonly used in instrumentation and measurement applications where accurate signal amplification and noise rejection are crucial. For example, in medical equipment like electrocardiograms (ECGs), where the weak electrical signals from the heart need to be accurately measured amidst various types of interference, differential amplifiers play a critical role.
Voltage Subtraction: The circuit can also be configured to perform voltage subtraction, where one input voltage is subtracted from another input voltage, providing a customized output signal that corresponds to the desired mathematical operation.
Operational Amplifier (Op-Amp) Implementation: Difference amplifiers can often be built using operational amplifiers (op-amps) and appropriate resistor networks. Op-amps are widely used in such circuits due to their high input impedance, high gain, and versatile configuration possibilities.
In summary, the purpose of a difference amplifier circuit is to amplify the difference between two input voltages while rejecting any common-mode signals. This makes it a valuable tool in scenarios where accurate signal processing, noise reduction, and rejection of unwanted interference are essential, such as in measurement, communication, and control systems.