Clamper Circuit Trivia Questions & FAQ

A clamper is an electronic circuit that prevents a signal from exceeding a certain defined magnitude by shifting its DC value. The clamper does not restrict the peak-to-peak excursion of the signal, but moves it up or down by a fixed value.

A circuit that places either the positive or negative peak of a signal at a desired level is known as a clamping circuit. It simply adds or subtracts the dc component to the input signal. These circuits are also known as DC voltage restorers. A diode clamp (a simple, common type) relies on a diode, which conducts electric current in only one direction; resistors and capacitors in the circuit are used to maintain an altered dc level at the clamper output.

For a clamping circuit at least three components: a diode, a capacitor and a resistor are required. Sometimes an independent dc supply is also required to cause an additional shift.

The necessity to establish the extremity of the positive or negative signal excursion at some reference level (that is, to introduce a dc component to the input signal) arises in connection with a signal that has passed through a capacitive coupling network and lost its dc component, as in case of television receivers. However, normally the dc component introduced is not identical with the dc component lost in transmission.

Clamp circuits are categorized by their operation; negative or positive and biased and unbiased.

A clamp circuit adds dc component (positive or negative) to the input signal so as to push it either on the positive side, or on the negative side. When the circuit pushes the signal on the positive side or upward, the negative peak of the signal coincides with the zero level and the circuit is called the positive clamper. On the other hand, when the signal is pushed on the negative side or downward, the positive peak of the input signal coincides with the zero level and the circuit is called the negative clamper.

Clamping can be used to adapt an input signal to a device that cannot make use of or may be damaged by the signal range of the original input.

The two components creating the clamping effect are a capacitor, followed by a diode in parallel with the load. The clamper circuit relies on a change in the capacitor?s time constant; this is the result of the diode changing current path with the changing input voltage. The magnitude of R and C are chosen so that is large enough to ensure that the voltage across the capacitor does not discharge significantly during the diode's "Non conducting" interval. During the first negative phase of the AC input voltage, the capacitor in the positive clamper charges rapidly. As Vin becomes positive, the capacitor serves as a voltage doubler; since it has stored the equivalent of Vin during the negative cycle, it provides nearly that voltage during the positive cycle; this essentially doubles the voltage seen by the load. As Vin becomes negative, the capacitor acts as a battery of the same voltage of Vin. The voltage source and the capacitor counteract each other, resulting in a net voltage of zero as seen by the load.

Clamping circuits are often employed in television receivers as dc restorers. The incoming composite video signal normally processes through capacitive coupled amplifiers which eliminate the dc component thereby losing the black and white reference levels and the blanking level. These reference levels have to be restored before applying the video signal to the picture tube. This is accomplished by employing clamping circuits. Clamping circuits are also used in storage counter, analog frequency meter, capacitance meter, divider and stair-case waveform generator.

For good clamping action, the circuit time constant t = RC should be at least ten times the time period of the input signal voltage , it is large enough to ensure that the voltage across the capacitor C does not change significantly during the interval the diode is non-conducting.