A circuit that performs the mathematical function of differentiating an input voltage with respect to time. In plain language, a differentiator produces an output voltage that tells how quickly its input voltage is changing. If the input voltage is increasing, the output voltage will be positive; if the input is decreasing, the output is negative. The magnitude of the output tells how quickly the input is changing. When the input is not changing, the output is zero.
An interesting characteristic of a differentiator is that if a sine wave is input, the output will also be a sine wave, but phase shifted 90 degrees ahead of the input. Because of this, differentiators are sometimes used in quadrature oscillator circuits. A few modular synthesizer manufacturers offer a differentiator as a stand-alone circuit.