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High pass filter for pulse shaping

  1. May 15, 2007 #1
    Hello All,

    I have been doing some experiments where I need to make a very short pulse to switch on and off a MOSFET I am using (IRF840) but all I have as a pulse source is a small function generator which outputs square waves fixed at 50% duty cycle.

    So I had an idea to use a simple high pass filter like this one mentioned on Wikipedia:


    So I decided to duplicate the simpe circuit to shorten the width of the pulses coming out of my function generator. The output square wave was about 4V with a +2V DC Offset. I put the output in series with a 3.4nF cap and a potentiometer to adjust the width. Then I fed that output directly to the gate of the MOSFET. But It didn't switch the FET on.

    So I took off the FET and replaced it with a 2KOhm resistor and scoped across that to see what the output looked like. It appeared that I was getting nice spikes but they were going negative rather than positive.

    So I'm wondering if someone could give me a tip on how to adapt the high pass filter to make positive voltage spikes that I can use to switch on my MOSFET. Any help/advice appreciated :smile:

    Jason O
  2. jcsd
  3. May 15, 2007 #2


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    Staff: Mentor

    If you have a 0-4V square wave and follow it with a capacitor and then a resistor to ground as you describe, then you should be seeing +/-2Vp spikes at the square wave edges, centered on 0V. In order to protect the MOSFET gate, you should probably put in a clamping diode from ground to your gate (clamps the negative spikes at -0.6V instead of allowing -2V).
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