clamper

A clamper is an electronic circuit that shifts the DC operating point of an AC signal without changing its shape or amplitude. It does this by forcing the waveform to swing symmetrically around a new voltage level, typically ground or some other reference potential. The circuit accomplishes this using a diode and capacitor, sometimes with a resistor, arranged so that the diode conducts on one half-cycle and blocks on the other, allowing charge to accumulate on the capacitor and establish a new baseline.

The simplest clamper consists of a series capacitor feeding into a diode connected to ground (or a bias voltage), with the capacitor charging until the diode's forward voltage is reached. On the next cycle, the capacitor voltage adds to the input signal, pushing the waveform so that its negative peak just touches the reference level. A resistor in parallel with the diode provides a discharge path between cycles, allowing the circuit to adapt if the input frequency or shape changes.

Common configurations and applications

Positive clampers force the negative peak of the waveform to a reference voltage; negative clampers force the positive peak. In analog television receivers, clampers restored the DC level of video signals after AC coupling through the transmission chain. RF and microwave test equipment uses clampers to set the baseline of pulsed signals for accurate measurement. Oscilloscopes with clamping circuits can lock a waveform's position relative to a trigger or measurement reference without distortion.

The performance of a clamper depends heavily on diode forward voltage (typically 0.6 to 0.7 volts for silicon), capacitor leakage, and the time constant formed by the resistor and capacitor. If the time constant is too short, the capacitor discharges between cycles and the clamping voltage drifts. If it is too long, the circuit responds slowly to changes in input frequency or duty cycle. For fast video signals or pulse trains with variable duty cycles, precision clampers use active components such as operational amplifiers to regulate the reference voltage and reduce diode forward-voltage error.

The term 'clamper' reflects the circuit's action of clamping, or holding, one peak of the waveform to a fixed point. It is sometimes called a 'DC restorer' in video applications, because it restores the DC level lost during AC coupling. This contrasts with a limiter or clipper, which cuts off the signal above or below a threshold, and with a peak detector, which measures the peak but does not restore it to subsequent cycles.