Diode clamper circuit operation

A diode clamper (dc restorer) is a circuit that adds a dc level to an ac signal, shifting the whole waveform up or down without changing its shape. It consists of a capacitor, a diode and a resistor (and a battery if an extra shift is needed).

Positive clamper (shifts the waveform up)

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

Take Vin = Vm sin ωt, an ideal diode, and an RC time constant much larger than the signal period.

Operation

• First negative half cycle: the output tries to go negative, so the diode is forward biased and conducts. The capacitor quickly charges through the diode to Vm (input-side plate negative), and Vout is held near 0 V.
• After that: the capacitor stays charged to about Vm and cannot discharge (the diode is off and R is large). It acts as a battery of Vm in series with the input.
• Output: Vout = Vin + Vm. The waveform now swings from 0 V to +2Vm; its negative peak is clamped at 0 V (−0.7 V for a real diode).

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

Negative clamper

If the diode is reversed, the capacitor charges to Vm in the opposite polarity during the positive peak, and Vout = Vin − Vm. The positive peak is clamped at 0 V and the output swings from −2Vm to 0.

Biased clamper

A battery V in series with the diode moves the clamping level to ±V. The output is then Vin + (Vm ± V).

Applications: restoring the dc level in TV receivers, test equipment, and voltage multiplier circuits.