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. In other words, it amplifies the voltage difference between its two input terminals while ignoring any voltage that appears simultaneously on both inputs. This makes it particularly useful in applications where the signal of interest is carried in the voltage difference between two signals, while common noise or interference affects both signals equally.
The primary purpose of a difference amplifier circuit is to extract useful information from the input signals while eliminating or minimizing unwanted noise or interference that affects both inputs equally. This is especially important in various measurement, instrumentation, and communication systems, where the signal of interest might be relatively small compared to background noise.
Here are some key points about the purpose and functionality of a difference amplifier circuit:
Signal Amplification: The circuit amplifies the voltage difference between the two input signals by a certain factor, known as the gain. The amplified output can then be further processed or used for various purposes.
Common-Mode Rejection: One of the main advantages of a difference amplifier is its ability to reject common-mode signals. Common-mode signals are voltages that appear on both input terminals simultaneously, which might be due to interference or noise. Since the difference amplifier amplifies only the voltage difference between the inputs, it effectively cancels out common-mode signals, resulting in a clean output signal that primarily reflects the desired signal.
Instrumentation: Difference amplifiers are commonly used in instrumentation applications where accurate measurement of small signals is crucial. For example, in medical equipment like electrocardiographs (ECGs) or in strain gauge-based sensors, where the signal is typically very weak compared to the noise, a difference amplifier helps enhance the signal of interest.
Signal Conditioning: In some cases, the difference amplifier is used to condition or prepare a signal for further processing or conversion. It can be used to adjust the signal's level, offset, or range before it's fed into subsequent stages of a system.
Balanced Communication: In communication systems, especially in differential signaling used in balanced audio lines, Ethernet connections, or some digital interfaces, difference amplifiers help transmit data with better noise immunity, as any common-mode interference gets rejected.
Operational Amplifiers (Op-Amps): Difference amplifiers are often built using operational amplifiers (op-amps) as the basic building blocks. Op-amps are high-gain amplifiers with differential input terminals, making them well-suited for creating differential amplifier circuits.
In summary, the purpose of a difference amplifier circuit is to amplify the difference between two input signals while effectively rejecting common-mode interference or noise. This makes it a valuable tool in applications requiring accurate signal extraction and noise reduction.