Diode square rate detection circuit

The diode square rate detector circuit is shown in the figure. Because it is a small signal input,
a static bias voltage VQ needs to be applied to make its static operating point located at the Q point of the depressed part of the diode characteristic curve
to ensure that the diode is always conducting and the quadratic term coefficient is large. The function of capacitor c is to form a high-frequency
path and filter out high-frequency signals and high-order harmonics.
    Since the diode is always conducting, the equivalent input resistance of the small signal detector is approximately equal to the on-resistance rd of the diode .     Small signal square rate detectors are commonly used in instrumentation.