High Pass Filter- Explained

A high pass filter is a filter which passes high-frequency signals and blocks, or impedes, low-frequency signals.

In other words, high-frequency signals go through much easier and low-frequency signals have a much harder getting through, which is why it's a high pass filter.

High pass filters can be constructed using resistors with either capacitors or inductors. A high pass filter composed of a resistor and a capacitor is called a high pass RC filter. And a high pass filter with a resistor and an inductor is called a high pass RL filter.

We will go through both of these type of circuits on this page and show how both RC and LC high pass filters are constructed. Both circuits have the effect of passing through high frequency signals while impeding low-frequency ones.

High Pass RC Filter

A high pass RC filter, again, is a filter which passes through high-frequency signals, composed of a resistor and capacitor.

To create a high pass RC filter, the capacitor is placed in series with the power signal entering the circuit, such as shown in the circuit below:

 

The above is a high pass filter. As a capacitor is a reactive device, it offers differing resistance to signals of different frequencies entering through it. A capacitor is a reactive device which offers very high resistance to low-frequency, or DC, signals. And low resistance to high-frequency signals. As it offers very high resistance to DC signals, it blocks them from entering through, as you can see in the circuit diagram above. So this type of filter only allows high-frequency signals to pass through and not DC. This type of capacitor also functions as acoupling capacitor because it couples the AC signal from one part of a circuit to another, while blocking the DC.

High pass filters are very common and are used in many type of circuit setups. One such circuit in which they are used are in microphone circuits. Microphones are devices which need both DC and AC signals in order to work. Microphones need DC power in order to have the power to turn on and be able to operate. And they record AC signals, such as human speech, music, noises. Thus, they deal with both type of signals. The DC is only needed to power the microphone on and should not appear on the output with the AC signal. The DC is for power only and should not be mixed with the audio signal. So to pass the AC signal through and block the DC, we use a high pass filter, so that only the AC signal appears on the output.

How to Build a RC High Pass Filter

Now that we've gone through what a high pass RC filter is, let's go over a practical example of building one.

To build a high pass filter, the components we will use are a function generator, a 0.1µF ceramic capacitor, and a 200Ω Resistor.

This is the schematic of the circuit we will build, shown below:

We input an AC signal into the circuit from the function generator. If you place the signal to a very low frequency signal such as 0.1Hz (100 mHz), the capacitor will block out this voltage signal and it will not go through to output. You can check this if you have an oscilloscope. If you now increase the frequency of the signal to 1KHz, the signal will go through unimpeded, not being blocked at all. The capacitor will have no effect and all high frequency signals pass through completely unimpeded without any attenuation.

This is a diagram below showing what happens to low and high frequencies when fed into this high pass filter:

The low frequency signals (near 0 Hz) are blocked and do not go past the capacitor. Therefore, these signals do not show up on output. The high frequency signals go through unimpeded and pass to output. This is why it's a high pass filter. It passes through high frequencies but block low frequencies.

Note that this low pass filter only blocks very low frequencies. The frequency of the signal has to be very close to 0Hz for the capacitor to have effect (for there to be capacitive effect). This means that the frequency should only be a few millihertz, at most 1Hz. Anything above 1Hz and the capacitor blocks less and less of the signal. Even at 4Hz, there is no capacitive effect, so a 4Hz signal will not be blocked at all but passes through unimpeded and unattenuated. This high pass filter only blocks very low frequencies, near 0Hz.