Short note: Diode clamper circuit

A diode clamper (dc restorer) is a circuit that shifts the dc level of an ac signal up or down without changing its shape. It consists of a capacitor, a diode, a resistor and sometimes a dc battery. It works because the capacitor stores a dc voltage through the diode and the time constant RC is made much larger than the signal period, so the stored voltage does not leak away.

Types

• Positive clamper: shifts the waveform upward. The negative peak is clamped at 0 V (or at +V if a battery is used).
• Negative clamper: shifts the waveform downward. The positive peak is clamped at 0 V.
• Biased clamper: a dc battery in series with the diode moves the clamp level to ±V.

Positive clamper working

 Vin o---| |---+---o Vout
          C    |
              D (anode to ground)   R across output
 GND o---------+---o

• During the first negative half cycle, the diode conducts and charges the capacitor to Vm.
• Afterwards, the capacitor works like a battery of Vm in series with the source, so Vout = Vin + Vm.
• The output swings from 0 to 2Vm.

 Vin  /\  /\        Vout  /\  /\ 2Vm
 ----/--\/--\-- 0       --/  \/  \--- 0

Uses: TV receivers (restoring the dc level of the video signal), radar, voltage multipliers, and level shifting in test circuits.