High pass filter for pulse shaping

[Jdo300]

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:

http://en.wikipedia.org/wiki/High_pass_filter

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

Thanks,
Jason O

 

[berkeman]

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).