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 appears on both inputs. The primary purpose of a difference amplifier is to accurately amplify the voltage difference between its two input terminals, making it particularly useful in applications where you want to subtract one signal from another.
The basic concept of a difference amplifier involves two inputs, labeled as inverting (-) and non-inverting (+), and an output. The output voltage is proportional to the voltage difference between the two inputs, multiplied by the circuit's gain factor. The common-mode rejection property ensures that any signal that is present on both inputs, but is of the same magnitude and phase, will be canceled out or greatly reduced.
Key features of a difference amplifier circuit:
Differential Gain (Ad): This is the amplification factor applied to the voltage difference between the two inputs. It determines how much the output voltage changes for a given change in the input voltage difference.
Common-Mode Rejection Ratio (CMRR): CMRR is a measure of the circuit's ability to reject common-mode signals. It is the ratio of the differential gain to the common-mode gain. A higher CMRR indicates better rejection of unwanted common-mode signals.
Input Impedance: The difference amplifier usually has a high input impedance, which helps prevent loading of the signal sources.
Output Offset: Ideally, the output should be zero when the inputs are equal. However, due to component tolerances and other factors, a small offset voltage may appear at the output when both inputs are at the same voltage.
Uses of a difference amplifier in subtracting signals:
Signal Subtraction: One of the primary uses of a difference amplifier is to subtract two input signals. By connecting the signal to be subtracted to the inverting input and the reference signal to the non-inverting input, the output will be an amplified version of the difference between the two signals. This is particularly useful in applications such as instrumentation, audio processing, and control systems.
Instrumentation Amplification: In measurement systems, a difference amplifier can be used to amplify the difference between two sensor outputs, while rejecting any common noise or interference that affects both sensors.
Biomedical Applications: Difference amplifiers are used in biomedical instrumentation to subtract baseline signals or interference from biological signals, allowing researchers and clinicians to focus on the specific information of interest.
Signal Conditioning: When dealing with sensor outputs, a difference amplifier can be employed to condition and preprocess signals before they are further processed by other circuits.
In summary, a difference amplifier circuit is a versatile tool in electronics that finds its most notable use in accurately subtracting one signal from another while rejecting unwanted common-mode signals, making it valuable in various applications where precise signal manipulation and conditioning are necessary.