One of the labs in my textbook Applied Analog Electronics asks students to look at the inductive spikes created by switching a nFET on and off with a loudspeaker as a load:
My students were very confused when they tried the experiment, because they got a different result:
Of course, I lied to you a little about what their circuit was—they were working with half-H-bridge boards that they had soldered:
The pFET was left unconnected, so the circuit was really the following:
So what difference does the pFET make? Well, with the gate floating and staying near 0V, the pFET turns on when the pFET source voltage gets high enough, allowing the capacitor to charge.
So how did I miss this problem when I did my testing before including the lab in the book? One possibility is that I left out the bypass capacitor—without it you get the expected spike. But I know I had included the capacitor on my half-H-bridge boards—I had to solder up a board without the bypass capacitor specially last night, in order to get the “expected” plot in the first plot of this post. I think what happened is that when I had done my tests, I had always connected the pFET gate to the pFET source, to ensure that the pFET stayed off, but when I wrote the book, I forgot that in the instructions. Here are the plots of the board with the pFET gate and source tied together (both floating), both floating separately, and with the them both tied to 5V:
The pFET source voltage gets quite high when the pFET gate and source are tied together to keep the FET off, but they are not tied to the power rail:
This summer and fall, when I’ll be working on the next edition of the book, I’ll be sure to improve the instructions for the FET lab!
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