Clamper (electronics)
A clamper (or clamping circuit or clamp) is an electronic circuit that fixes either the positive or the negative peak excursions of a signal to a defined voltage by adding a variable positive or negative DC voltage to it.
A diode clamp (a simple, common type) consists of a diode, which conducts electric current in only one direction and prevents the signal exceeding the reference value; and a capacitor, which provides a DC offset from the stored charge.
The capacitor forms a time constant with a resistor load, which determines the range of frequencies over which the clamper will be effective.
A clamper will bind the upper or lower extreme of a waveform to a fixed DC voltage level.
The network must have a capacitor, a diode, and optionally a resistive element and/or load, but it can also employ an independent DC supply to introduce an additional shift.
The magnitude of R and C must be chosen such that the time constant RC is large enough to ensure that the voltage across the capacitor does not discharge significantly during the interval the diode is nonconducting.
Clamp circuits are categorised by their operation: negative or positive, and biased or unbiased.
A positive clamp circuit (negative peak clamper) outputs a purely positive waveform from an input signal; it offsets the input signal so that all of the waveform is greater than 0 V. A negative clamp is the opposite of this—this clamp outputs a purely negative waveform from an input signal.
The effect of the diode voltage drop on the circuit output will be divided down by the gain of the amplifier, resulting in an insignificant error.
During the first negative phase of the AC input voltage, the capacitor in a positive clamper circuit charges rapidly.
, is large enough to ensure that the voltage across the capacitor does not discharge significantly during the diode's non-conducting interval.
These sets have a DC restorer circuit, which returns the voltage of the video signal during the "back porch" of the line blanking (retrace) period to 0 V. Low-frequency interference, especially power line hum, induced onto the signal spoils the rendering of the image and, in extreme, cases causes the set to lose synchronization.
