High Pass Filter: Circuit, Transfer Function & Bode Plot
Contents
- High Pass Filter Definition: A high pass filter allows frequencies higher than a certain cutoff and blocks lower ones, useful in electronic filtering.
- Circuit Components: These filters can use simple components like resistors and capacitors or include operational amplifiers for more complex applications.
- High Pass Filter Transfer Function: The transfer function mathematically represents how the filter processes signals, emphasizing frequencies above the cutoff.
- Bode Plot Overview: A Bode plot visualizes the frequency response of the filter, showing changes in signal magnitude and phase with frequency.
- Practical Applications: High pass filters are used in diverse fields like audio technology and image processing to enhance signal quality.
A filter removes unwanted elements, much like a water filter cleans impurities from water. Similarly, an electric filter works to cleanse signals in electronic circuits.
An electric filter, made up of resistors, inductors, capacitors, and amplifiers, passes signals of a certain frequency while attenuating others that are too high or too low.
The frequency at which filter operates, that frequency is known as cut-off frequency. The cut-off frequency is set while designing the filter.
What is a High Pass Filter?
A high pass filter (also known as a low-cut filter or bass-cut filter) is an electronic filter that permits signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than that cutoff frequency.
The inverse of a high-pass filter is a low-pass filter, which allows signals with frequencies lower than the cut-off frequency and blocks all frequencies above this cut-off frequency. There are also band pass filters, which combine the functionality of high pass filters and low pass filters to only allow frequencies within a specific frequency range.
High Pass vs Low Pass Filters
The characteristics of a high pass filter are exactly the opposite characteristics of a low pass filter. The difference include:
| High Pass Filter (HPF) | Low Pass Filter (LPF) | |
| Definition | HPF is an electric filter that allows signals with higher frequency than the cut-off frequency. It is known as the low-cut filter. | LPF is an electric filter that allows signals with a lower frequency than the cut-off frequency. It is known as the high-cut filter. |
| Circuit Diagram | In HPF, the capacitor followed by the resister. | In LPF, the resister followed by the capacitor. |
| RC Filter |  |
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| Operating frequency | Higher than the cut-off frequency | Lower than the cut-off frequency. |
| Importance | It is important to cancel the low-frequency noise from the input signal. | It is important to cancel the aliasing effect. |
| Applications | It is used in amplifiers like an audio amplifier, low-noise amplifier. | It used in communication circuits as an anti-aliasing filter. |
Types of High Pass Filters
There are many types of high-pass filters according to the circuit design and components used to make a filter. The various types of high-pass filters include:
Passive High Pass Filter
The passive filter consists of only passive elements like resistor, inductor, and capacitor. It will not use any external power source or amplification components.
A passive high pass filter consists of a combination of resistor and capacitor (RC) or resistor and inductor (RL).
Active High Pass Filter
The Active filter is a combination of a passive filter with an operational amplifier (OP-AMP) or it includes an amplifier with gain control.
It is made by connecting an inverting or non-inverting component of OP-AMP with a passive filter.
RC High Pass Filter
The RC filter is one type of passive filter because it consists only of a capacitor is in series with the resistor.
The circuit diagram of high pass and low pass filter is the same, just interchange the capacitor and resistor. The circuit diagram of the RC high pass filter is as shown in the below figure.
The capacitor offers very high reactance for the signal with a frequency lower than the cut-off frequency. In this case, the capacitor act as an open switch.
The capacitor offers low reactance for the signal with a frequency higher than the cut-off frequency. In this case, the capacitor act as a close switch.
First Order High Pass Filter
First Order High pass filter consists of only one capacitor or inductor. This type of filter has a transfer function of the first order.
It means if you derive an equation in s-domain, the maximum power of āsā is one. This is only possible if you use only one energy storage element like inductor and capacitor.
The first order filter can be active or passive, depending on the use of elements. If it uses only active elements, it can be a first-order filter. RC high pass filter is a first-order passive high pass filter.
Second Order High Pass Filter
Second-Order high pass filter can derive by cascading two first-order high pass filters. Therefore, it consists of two reactive components and makes a second-order circuit.
The main difference in the first order and second-order filter slope in the stop band. The slop of the second-order filter is twice of the first-order filter.
For example, if we consider a first-order Butterworth filter, the slop is +20 db/decade and for second-order Butterworth filter, the slop is +40 db/decade.
