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 signals that appear on both inputs. It is commonly used to subtract one signal from another, which makes it useful in various applications, such as instrumentation, signal conditioning, and communication systems.
The primary function of a difference amplifier is to provide an amplified output that is proportional to the voltage difference between its two input terminals. This is achieved by using active components like operational amplifiers (op-amps) and appropriate resistor networks. The difference amplifier rejects any signals that are present on both input terminals in equal magnitude (common-mode signals), as they ideally do not contribute to the amplified output.
Here's a simplified explanation of how a difference amplifier works and its use in subtracting signals:
Basic Circuit Configuration:
A typical difference amplifier circuit consists of an operational amplifier with two input terminals, often labeled as "+" and "-", and an output terminal. Each input terminal is connected to a separate input signal source, and appropriate resistor networks are used to create the desired amplification and subtraction effect.
Subtracting Signals:
Suppose you have two input signals, Vā and Vā, and you want to subtract Vā from Vā. You would connect Vā to the "+" input terminal and Vā to the "-" input terminal of the difference amplifier circuit.
The operational principle of the difference amplifier ensures that the output voltage (V_out) is proportional to the difference between the input voltages (Vā - Vā). Mathematically:
V_out = Gain Ć (Vā - Vā)
The gain of the difference amplifier determines the amplification factor of the output signal. The output will be positive if Vā is greater than Vā and negative if Vā is greater than Vā.
Common-Mode Rejection:
A key feature of a difference amplifier is its ability to reject common-mode signals. Common-mode signals are those that appear equally on both input terminals (V_common). These can arise due to noise or interference. The difference amplifier's design minimizes the impact of common-mode signals on the output, effectively isolating the desired signal difference.
In practical applications, difference amplifiers are used to process signals accurately, particularly in scenarios where it is crucial to measure the difference between two signals while ignoring any common interference. For example, in medical equipment, instrumentation, audio processing, and communication systems, difference amplifiers play a vital role in extracting meaningful information from complex signal environments.