A clamp or clamp circuit is an electrical circuit used to prevent another circuit from exceeding a certain predetermined voltage level. It operates by sensing the output voltage of the monitored circuit and then as the output voltage approaches the preset limit, applies an electric load which draws greater and greater current from the output in a regulated manner in order to prevent the output voltage from exceeding the predetermined voltage level. The clamp circuit works only if it has a lower output impedance than the monitored circuit thereby overpowering that circuit. It is the circuit which places either the positive or negative peak of a signal at the required direct current level.

A clamp circuit has no memory -- when the voltage is significantly below the limit, the clamp circuit always draws almost no current. (In this way it differs from a crowbar circuit).

Alternatively a clamping circuit may also be defined as a circuit which inserts a DC component into a signal. Perhaps the most common such clamping circuit is the DC restorer circuit in analog television receiver, which uses reference levels in the sync pulse in the horizontal blank (inserted during video modulation).

The network must have a capacitor, a diode and a resistive element, but it can also employ an independent DC supply to introduce an additional shift. The magnitude of R and C must be chosen so that t = RC is large enough to ensure that the voltage across the capacitor does not discharge significantly during the diode's "Unconducting" interval.

The term voltage clamp is often used to refer to the clamp circuit.

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