Do you know what passive filters are and what are they for? In this article, the World of Electrical explains what filters are, what a passive high-pass filter is, how a high-pass filter works and what are the characteristics of a high-pass filter. Come on!
What are the filters?
Filters play a fundamental role in electricity, for example, in signal processing, video, audio, and data, also being used in power, telecommunications, control, and other applications.
In electrical, filters have the ability to filter a given signal, that is, electrical filters have the function of selecting or rejecting one or several frequency ranges of an electrical signal.
There are several types of filters, for example, high-pass filters. A low-pass filter, band-pass filter, and filters reject range. All of these filters can be passive filters and active filters, where passive filters are those that are made essentially of passive components such as resistors, capacitors, and inductors.
All filters have again, which is the relationship between the output signal and the input signal, and this gain is different between passive and active filters. The gain of a passive filter will always be less than 1 and the gain of an active filter can be greater than 1, that is, passive filters have the characteristic of attenuating the signal and active filters have the characteristic of amplifying the signal.
Passive high-pass filter
Passive high-pass filters are circuits that allow the passage of high-frequency signals and reduce the intensity of low-frequency signals. That is, from a reference frequency it allows higher frequencies to pass freely and lower frequencies to be attenuated.
Passive high-pass filter – Operation
The circuit of a passive high-pass filter is composed of a capacitor in series with a resistance. The output signal of the high-pass filter is paralleled with a resistor, like a voltage divider. See the image below the passive high-pass filter.
To understand how a passive high-pass filter works, you need to know about some concepts of electricity, and especially how a capacitor behaves in alternating current (AC) circuits and direct current (DC) circuits.
To understand the operation of the high-pass filter, we must take into account the capacitive reactance. When the capacitor is subjected to alternating signals it has a resistance, which is defined as capacitive reactance.
If we consider the value of the constant capacitive reactance and we have the formula of the capacitive reactance, it is enough to understand how a high-pass filter works. The capacitive reactance formula is shown in the image below.
We can observe that if the frequency value is low, the capacitive reactance will be high and if the frequency is high, its capacitive reactance will below. That is, low-frequency signals the capacitor tends to increase its capacitive reactance and block it, and high-frequency signals the capacitor tends to lower its capacitive reactance and allow the signal to pass.
Now that we know how a capacitor works in certain circuits, and what a passive high-pass filter circuit looks like, it is easier to understand how the high-pass filter works.
If a load is connected in parallel with the resistor, the high-frequency signals will pass, as the capacitive reactance will below, and if a low-frequency signal is connected to the filter input, the capacitor offers a capacitive reactance, not allowing the passage of the signal. These characteristics are that of a passive high-pass filter, because high-frequency signals pass, while lower-frequency signals are blocked by the filter.
Passive high-pass filter – Cutoff frequency
In order to make a more complete analysis of this filter, we need to understand and define what the cutoff frequency is, because in this analysis that we have just done we did not have any defined reference to know what is high or low frequency. The cut-off formula for the passive high-pass filter is shown in the image below.
By the analysis we just made, only with the capacitor it is possible to cut the lower frequency signals and let the higher frequency signals pass, the however we need to define the filter cutoff frequency, so we need the resistor because without it we could not define the frequency values that would be allowed to pass or be blocked.
In passive filters, the cutoff frequency is also defined as the frequency at which the signal is attenuated, where this attenuation is approximately 30%, that is, the output signal has approximately 70% of the input signal strength.v