How to invert voltage polarity?

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Thread Starter


Joined May 27, 2015
I have a feeling that an Op-amp can get this out of the way pretty easily but I'm not sure exactly how.

I have this...

The oscillator works with some bugs (I'll be asking about that shortly in another question.) Eventually I would like M1 to be the switch for a buck converter. There's a problem with that.

My oscillator output signal is positive. No problem. Just use an NPN channel MOSFET, right? I don't think so.

NPN channel transistors, as far as I understand, can only be used after a voltage drop to ground. I.E. you use them to control whether a certain circuit has a path to ground or not. If I stick a resistor above M1 everything works fine. However, if I stick a buck converter above it, it doesn't workout fine because the buck converter is not a current shunt. (All of the voltage doesn't drop over the buck converter.)

So, I could use a PNP channel MOSFET, the only problem with that is that my output signal is positive. Adding my positive output signal to a PNP channel MOSFET would just increase the depletion region...or increase already infinite resistance.

Regardless of whether or not I really need to invert the signal (although I'm very interested in the solution to my problem) I think it would be a good thing to know how to invert a voltage.
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