Why is the Output Voltage of a Positive Voltage Clamper with Resistive Load Different?

Discussion in 'General Electronics Chat' started by Beanz1114, Apr 30, 2016.

  1. Beanz1114

    Thread Starter New Member

    Nov 13, 2014
    Hello there,
    I have a quick question regarding the positive voltage clamper. I am well aware that they shift the AC signal by a certain DC level, which in my case is 1V. That is confirmed through my simulation in Figure 1. Figure 2 shows the output voltage, around 1.77V, taking into account the non-ideal turn on voltage of the diode which is around 0.23V. However, when a resistive load is connected to it, the output voltage is totally different than what I expected, and I can't, for the life of me figure out why. Figure 3 shows the circuit with resistive load and figure 4 shows the output voltage across the resistor. Shouldn't the output voltage still be the same as without the load as suggested by some of the websites (conceptelectronics and circuitstoday)?

    Figure 1: Positive Clamper Circuit

    Figure 2: Output Voltage of Positive Clamper taken across Diode

    Figure 3 : Positive Voltage Clamper with Resistive Load

    Figure 4 : Output Voltage across Rload
  2. Dyslexicbloke

    Well-Known Member

    Sep 4, 2010
    The simple answer is that they are different circuits ...

    look at the current in the diode/resistor and then consider the effect this has on the capacitor voltage, without the resistor how is the capacitor charge and discharge, its AC current, affected?

    The beast way to learn is to play until you understand.
  3. OBW0549

    Distinguished Member

    Mar 2, 2015
    Look carefully at your circuit, in particular the value of capacitor C1. The reactance of a 1 pF capacitor at 1 kHz is Xc = 1/(2πfC) = 159 MΩ. This capacitive reactance, along with R1, sets up a voltage divider with a division ratio of 15900:1, giving an output voltage of 1 V/15900 or 63 μV, which is exactly what your simulation shows.

    Change the value of C1 to 100 μF and see what happens.
    Beanz1114 likes this.
  4. crutschow


    Mar 14, 2008
    Random part values lead to random results. :rolleyes:
  5. RichardO

    Late Member

    May 4, 2013
    Also, you need to pick a part number for the diode -- a 1N914 is a good choice to start with. Using the "default" parts can lead to very strange results in the simulation.
  6. Beanz1114

    Thread Starter New Member

    Nov 13, 2014
    Thanks @OBW0549! I never thought of it as a voltage divider circuit before. That makes perfect sense. Thanks again! :)