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).

 

[ravenprp]

.

Hi Jason,

Thank you for sharing your experiment and question with us. It seems like you have a good understanding of using a high pass filter for pulse shaping. However, I think the issue you are facing is with the polarity of the output spikes from your filter.

Firstly, I would suggest double checking the connections and values of your circuit to make sure there are no errors there. If everything seems correct, then the issue may lie in the type of high pass filter you are using.

The high pass filter you mentioned from Wikipedia is a first-order filter, which means it only has one reactive component (the capacitor). This type of filter is known to produce spikes with opposite polarity to the input signal, as you have observed.

To get positive voltage spikes, you could try using a second-order high pass filter, which has two reactive components (a capacitor and an inductor). This type of filter is known to produce spikes with the same polarity as the input signal.

Alternatively, you could also try using a Schmitt trigger circuit, which is specifically designed for pulse shaping and produces sharp positive spikes. You can find many circuit diagrams and explanations online for this type of circuit.

I hope this helps and good luck with your experiment! Keep exploring and experimenting, that's how we learn and innovate. :)