What Is a Differential Amplifier?
A differential amplifier is a basic analog circuit that amplifies the voltage difference between its two inputs while rejecting the signal that appears common to both inputs.
How Does It Work?
The output is proportional to the difference between the two inputs: (V+ − V−). Signals appearing on both inputs at once (common-mode) are largely canceled; this is the basis of noise rejection.
Common-Mode Rejection
The same noise that couples externally onto both input lines is common-mode, and the differential amplifier rejects it. Only the useful difference signal is amplified; this ability is measured by the CMRR.
| Signal | Behavior |
|---|---|
| Difference (useful) | Amplified |
| Common-mode (noise) | Rejected |
Relationship to the Op-Amp and In-Amp
A simple differential amplifier can be built with a single op-amp and four resistors. When higher accuracy and input impedance are needed, this structure is enhanced into an instrumentation amplifier.
Where Is It Used?
Sensor-signal reading, measurement in noisy environments, ADC input stages, differential line receivers and communication circuits.
Gain Is Set by Resistors
In an op-amp-based differential amplifier the gain is set by the ratio of the input and feedback resistors. The exact matching of these four resistors is essential for the common-mode rejection (CMRR) to stay high; so in precise applications matched resistor networks or ready differential-amplifier ICs are preferred.
Common Mistakes
- Resistor matching: if the resistors in an op-amp circuit don't match, the CMRR drops.
- Low input impedance: a simple circuit can load the sensor; an in-amp may be needed.
- Out-of-range signal: the common-mode voltage must not exceed the supply limit.
Frequently Asked Questions
What's the difference between a differential amplifier and an in-amp?+
Why does it reject noise?+
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