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Right now I'm using an N-Channel MOSFET. I am inputting a 2V pk-pk square wave into the gate, the drain is connected to a bare wire (that is connected to nothing) and the source is connected to ground. For some reason there's a voltage reading in the drain of my MOSFET. Is this normal?

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  • \$\begingroup\$ What device and how are you doing the measurement? \$\endgroup\$ – Peter Smith Jul 26 '16 at 9:14
  • \$\begingroup\$ Whenever you connect a high impedance voltmeter/oscilloscope etc. to anything you will have some "voltage reading". Especially if there is a signal nearby. \$\endgroup\$ – Curd Jul 26 '16 at 9:17
  • \$\begingroup\$ I was using an oscilloscope to do the measurements. Sorry first time with MOSFETs, so I only know how they work based on what I learned from Youtube \$\endgroup\$ – Kurst Jul 26 '16 at 9:21
  • \$\begingroup\$ A graph of the measured signal would be helpful. \$\endgroup\$ – Curd Jul 26 '16 at 9:37
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If you see just the "edges" (positive/negative peaks at the times when there are rising/falling edges at the input signal) it can be explained by the capacitance between gate and drain.

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  • \$\begingroup\$ How does the capacitance create these sharp edges? What should I be googling to learn more about this phenomenon? \$\endgroup\$ – Kurst Aug 22 '16 at 3:05
  • \$\begingroup\$ @Kurst: The capacitance doesn't create the sharp edges; they are there already in your square wave signal at the gate. DC can not pass a capacitor; quick voltage changes can. You should learn how a capacitor works. \$\endgroup\$ – Curd Aug 22 '16 at 15:35
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There is a capacitance between gate and drain. So some signal may get to drain.

UPDATE:

When you leave drain floating, then that circuit behaves "almost" like this: enter image description here It is very simplified, but I think it is OK for explanation.

I wired it on breadboard, and this is the response: enter image description here yellow = input signal (1MHz square, 5Vpp)

blue = output signal (on floating drain)

So, yes, it is normal, that signal gets from gate to floating drain.

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