A differential amplifier is a fundamental electronic circuit that amplifies the difference between two input signals while rejecting any common-mode signal (signals that are present equally in both inputs). It is often used in various applications in signal processing and analog circuit design due to its ability to provide high gain and excellent rejection of common-mode noise.
Function of a Differential Amplifier:
A differential amplifier typically consists of two transistors (usually BJTs or FETs) with their emitters (or sources) tied together and connected to a common current source. The inputs are applied to the bases (or gates) of the transistors, and the output is taken from the collectors (or drains). The basic operation of a differential amplifier is as follows:
Differential Mode Operation: When there is a voltage difference between the two input terminals (V+ and V-), the transistors amplify this voltage difference. As a result, the output voltage is proportional to the difference between the voltages applied to the inputs (Vout ∝ V+ - V-).
Common Mode Rejection: When the same voltage is applied to both inputs (common-mode signal), the transistors respond equally, leading to no differential voltage across them. Therefore, the output voltage remains constant, and the common-mode signal is effectively rejected.
Applications in Signal Processing:
Instrumentation Amplifiers: Differential amplifiers are widely used in instrumentation amplifiers, which are specialized amplifiers used to amplify small differential signals, often in the presence of significant common-mode noise. They are commonly used in sensor interfaces and bridge-based measurements.
Balanced Audio Transmission: In audio applications, differential amplifiers are used to transmit audio signals over long distances with improved noise rejection. This is commonly used in professional audio equipment and balanced audio cables.
Differential ADC Drivers: Differential amplifiers are used as drivers for analog-to-digital converters (ADCs) to amplify and condition analog signals before they are converted to digital values.
Differential Op-Amps: Some operational amplifiers are designed as differential input amplifiers to provide better performance in terms of common-mode rejection and noise reduction.
Communication Systems: Differential amplifiers are employed in communication systems to increase the signal-to-noise ratio and to suppress common-mode interference in data transmission.
Biomedical Signal Processing: In biomedical applications, differential amplifiers are used to amplify the small bio-potential signals (e.g., ECG, EEG) while rejecting common-mode noise from the environment.
Overall, differential amplifiers are crucial components in various signal processing applications, where they help in enhancing the quality of signals by providing high gain and reducing common-mode interference, leading to improved accuracy and reliability of the processed signals.