# MOSFET Circuit: Bypass capacitor

Does anyone know how a Bypass Capacitor work in it's physical operation? How does a bypass capacitor filter out the ac signal in the device structure and physical operation?

• What has this question got to do with MOSFETs? Do you know how capacitors work? Apr 22, 2015 at 7:29
• Are you talking about a particular circuit? A diagram would be helpful in that case. Apr 22, 2015 at 7:45
• A simple research on google would have given you the answer. Apr 22, 2015 at 8:23

It is the same as any other capacitor. If you put DC through it, current will flow, but not for long; charge accumulates on its plates until no more can flow. At the very beginning, it acts as a short circuit. But as the charge on it increases, the voltage across it increases, to the point where no more current flows.

So when using a capacitor as a bypass capacitor, it is connected as in the diagram on the left. And when used as a coupling capacitor, it's done so as in the diagram on the right:

simulate this circuit – Schematic created using CircuitLab

To understand the circuits, I would suggest you read about RC filter circuits (Resistance-Capacitance filter circuits). The maths is simpler than other types, such as LRC or LC (Inductor-Resistance-Capacitance or Inductance-Capacitance), although you will still need some maths to appreciate how the capacitor works.

You can simulate this circuit below - it will show you how it works in practice. Click the "simulate this circuit" link below it.

simulate this circuit

To run the simulation, go through these steps:

1. Click the "Simulate this circuit" link just below the diagram.
2. Click on the "Simulate" button on the bottom left
3. Click on the "Time domain" option
4. Click "Run Time-Domain Simulation" button.

You should see a graph like this:

What you are seeing here, is the input and the output. The input is a 1V, 10Hz AC signal, with a 1V DC offsest. (So the AC signal goes from 0V to 2V peak-to-peak). After the bypass capacitor, the AC signal is largely removed, leaving just the 1V DC signal. The larger the capacitor value, the less the ripple.