Fundamentals of Passive Filters and Passive Low-Pass Filters in Circuit Design

What is a Filter?

Filters are a critical element of electronic systems that allow desired signal frequencies to pass through while suppressing unwanted ones, thus influencing signal integrity and guaranteeing proper functioning in numerous applications. The amplitudes of these waves can be determined by analysing the signal periodically.

The main purpose of filters is to eliminate undesired noise or interference while enhancing the desired signal components. Filters enhance signal quality and avoid aliasing. Filters help maintain signal integrity and extract relevant information from complex signals in various electronic domains, including telecommunications, audio systems, and data acquisition.  

Types of Filters

Active Filters

An active filter is one of the filters that utilizes active components like operational amplifiers (Op-amp) and transistors for its design. Active filters have greater selectivity, better isolation of signal, and the capability to implement more accurate transfer functions.

Passive Filters

Passive filters depend only on the passive elements – resistors (R), capacitors (C), and inductors (L) – to manipulate signal frequencies. As in the case of low frequencies, the inductance of the inductor must be increased, which ultimately requires a greater number of turns in the coil. It requires amplification post-filtering.

Features of Passive Filter

● It does not require an external power source.

● It does not provide any power gain.

● It has a simple and easy design.

● It is cheaper than an active filter.

● It has a large size and a heavy weight due to the bulky inductors.

Key Differences Between Active and Passive Filter

● Due to the involvement of active elements, active filters are costly. But the cost is low due to the involvement of passive components in it for passive filters.

● The active filter's circuit orientation is relatively complicated. In contrast, compared to passive filters, comparatively simpler circuitry has passive filters.

● Active filters require an external power supply for the operation of the circuit. However, passive filters don't need an external source of energy since it derives the energy for its operation from the input signal applied.

● Inductor is the fundamental component of passive filters and it creates issues at low frequencies. Therefore, passive filters are best suited for operation in the RF range. Whereas active filters give a better response at low frequency.

● Active filters have a large quality factor value compared to passive filters.

● The active filter weight is less in comparison to passive filters.

● Active devices are more sensitive to changes in temperature. Nonetheless, passive components are relatively insensitive to the same.

Passive Filters: Circuit Configurations and Transfer Functions

Low-Pass Filters

Low-pass filters allow low-frequency signals to pass through and reduce high-frequency signals. The simplest low-pass filter is a resistor (R) in series with a capacitor (C). The frequency at which the transition takes place is referred to as the "cut-off" or "corner" frequency.

High-Pass Filters

High-pass filters allow high-frequency signals to pass through and reject low-frequency signals. The simplest high-pass filter is a capacitor (C) in series with a resistor (R). The high pass filter permits only high frequency signals from its cut-off frequency, ƒc point and above to infinity to pass through but rejects those any lower.

Band-Pass Filters

Band-pass filters pass a defined group of frequencies but attenuate frequencies outside the range. It is possible to produce a simple band-pass filter by cascading a high-pass and a low-pass filter. The bandwidth of the filter is established by the quality factor, such that increased values of Q reduce the passband.

Band-Stop Filters (Notch Filters)

Band-stop filters or notch filters reduce a particular range of frequencies while passing frequencies beyond that range. A basic band-stop filter can be designed by having a series LC circuit in parallel with a resistor(R).

Passive Low Pass Filter

A Low Pass Filter is a circuit which is designed to change, reshape, or block all unwanted high frequency of an electric signal and only accept or allow those desired signals of the designer of the circuit. That is, they "filter out" unwanted signals and an ideal filter will divide and pass sinusoidal input signals according to their frequency. In low-frequency applications (to 100kHz), passive filters are typically built as straightforward RC (Resistor-Capacitor) networks, but higher frequency filters (over 100kHz) tend to be made up of RLC (Resistor-Inductor-Capacitor) elements.

Frequency Response: 1st-order Low Pass Filter

The Bode Plot shows the Frequency Response of the filter to be nearly flat for low frequencies and all the input signal is passed directly to the output, having a gain of nearly 1, known as unity, until it reaches its Cut-off Frequency point (ƒc). This is because the reactance of the capacitor is high at low frequencies and blocks any passage of current via the capacitor. Beyond this cut-off frequency point the circuit's output falls to zero with a -20dB/ Decade or (-6dB/Octave) "roll-off". Notice the slope angle, this -20dB/ Decade roll-off will be identical for any RC pair.

Passive Filter: Applications

● Passive Low Pass Filter applications include audio amplifiers and speaker sets to route lower frequency bass sound to the big bass speakers or to attenuate any high frequency noise or "hiss" type of distortion. When implemented in such fashion in audio these low pass filters are sometimes referred to as being a "high-cut", or "treble cut" filter.

● In addition, passive filters are used in sensor signal conditioning, which eliminate noise from sensor outputs prior to processing or display. By eliminating high-frequency noise, passive filters enhance the precision and reliability of sensor measurements, critical for accurate monitoring and control in industrial and scientific applications.

● Used in air filtration systems to filter out particulate matter from the atmosphere. Used in dialysis machines and other medical devices where high filtration levels are essential.

● Other fields where passive filters are widely utilized include automobile electronics and alternative energy systems, further reinforcing their extensive practicality and relevance.

unikeyic Electronics possesses a large stock of filters and filter accessories.

It can be purchased from their website. Passive Filters are also available for purchase from Unikeyic Electronics.