I'm trying to create a project but I'm stuck somewhere. I decided to put a small portion into a simulator and it seems like the MOSFET is able to turn on and electrons can flow from the second diode. How is that possible in real life? Or maybe does it only work on a simulator?

The circuit: The circuit

Thanks a lot!

  • 1
    \$\begingroup\$ and electrons can flow Whan discussing circuits we talk about current flow, not electron flow. The arrows in the symbols of the diodes and MOSFET indicate the direction of forward current flow. \$\endgroup\$ Apr 7, 2021 at 7:47

2 Answers 2


If I did not oversee anything, it should work as follows:

Assume that the peak voltage of the source is 10V.

phase 1 (Upper terminal of the source is 10V)

Capacitor is charged to:

$$V_C = 10V - V_D $$

phase 2 (Lower terminal of the source is 10V)

Gate of the Mosfet "sees" the supply voltage + the charged voltage across capacitor, hence

$$V_G=V_i + V_C$$

At the same time, the lower diode is foward biased, hence current flows through the MOSFET, now active.


Here is some Illustration of the voltage levels:


NOTE: The above assumptions do not consider the time required for charging and the sinusoidal shaped source, therefore the peak voltage levels might differ.

  • \$\begingroup\$ So can a MOSFET work when it's gate and source are on a different voltage potential? It's threshold voltage is +2V. \$\endgroup\$ Apr 7, 2021 at 8:06
  • \$\begingroup\$ What do you mean exactly? During phase 2 in my example, VG=V_i + V_C and the source of the mosfet is in principle 0V, hence V_GS=VG > +2V \$\endgroup\$
    – vtolentino
    Apr 7, 2021 at 8:11
  • \$\begingroup\$ So once the MOSFET's gate is charged up (like a capacitor) the MOSFET will then conduct regardless of which potential the gate is connected to? \$\endgroup\$ Apr 7, 2021 at 8:12
  • \$\begingroup\$ But for the gate to be charged, +2V needs to be applied at gate. How is that even possible when MOSFET's source is at the same potential as the gate? \$\endgroup\$ Apr 7, 2021 at 8:13
  • \$\begingroup\$ When the MOSFET'S built-in capacitance is being charged up, the source end of the mosfet is being held a 0V by the source.itself. \$\endgroup\$
    – vtolentino
    Apr 7, 2021 at 8:49

Your voltage source is AC so in the negative cycle current can flow to the Mosfet drain. on the positive cycle the capacitor is charged and can keep the voltage at the gate during the negative cycle and therefore keep the Mosfet ON

  • \$\begingroup\$ Appreciate your answer a lot! Can you please explain me how a MOSFET with a threshold voltage of +2V can be turned on like that? Because the MOSFET's gate and source aren't on the same potential. How does it work? \$\endgroup\$ Apr 7, 2021 at 8:06

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