What is a Bypass Capacitor?

0.1uf Ceramic Capacitor to Filter DC

A bypass capacitor is a capacitor that shorts AC signals to ground, so that any AC noise that may be present on a DC signal is removed, producing a much cleaner and pure DC signal.

A bypass capacitor essentially bypasses AC noise that may be on a DC signal, filtering out the AC, so that a clean, pure DC signal goes through without any AC ripple. s

For example, you may want a pure DC signal from a power source.

Below is a transistor circuit. A transistor is an active device, so in order to work, it needs DC power. This power source is VCC. In this case, VCC equals 15 volts.

DC Power Source for Transistor

This 15 volts provides power to the transistor so that the transistor can amplify signals. We want this signal to be as purely DC as possible. Although we obtain our DC voltage, VCC, from a DC power source such as a power supply, the Voltage isn't always purely DC. In fact, many times the voltage is very noisy and contains a lot of AC ripple on it, especially at the 60Hz frequency because this is the frequency at which AC signals run in many countries.

So although we want a pure DC signal, such as below:

Pure DC Voltage

Many times, we get a noisy signal that looks like:

Noisy DC Voltage Signal

A DC signal such as this is actually very common. This is undesired because it adds noise to the transistor circuit. Therefore, this noisy DC signal will be imposed on the AC signal. So the AC signal which may have music or some type of recording will now have much more noise.

This noise which is on the signal is AC ripple. Many times when using a DC Power supply connected to an AC power outlet, it will have some of the AC noise transfer to the DC power voltage. AC ripple can also appear from other sources, so even batteries can have AC noise imposed on it.

To eliminate this AC ripple, we use a bypass capacitor. So our transistor circuit above will have a bypass capacitor added to it:

Bypass Capacitor for a Transistor Circuit

The capacitor blocks DC voltages, so that none of the DC voltage goes through the capacitor. A capacitor has very high reactance at low frequencies, so all DC is blocked. However, capacitors have much less resistance at higher frequencies (AC signals). So AC signals go through the capacitor and then to ground. Therefore, AC signals get shunted to ground.

How a Bypass Capacitor Works

So a capacitor blocks the DC from entering it but accepts the AC noise that may be on the DC line and shunts or bypasses it to ground. This is how bypass capacitors work.

Related Resources

What is a Coupling Capacitor?

What is a Smoothing Capacitor?

How to Test a Capacitor

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