Difference between Active and Passive Filters

What Is The Difference between Active and Passive Filters? . Filters are fundamental building blocks in various electronic systems, performing the crucial task of selectively passing desired frequencies in a signal while attenuating unwanted ones. This selective processing allows for extracting specific information or protecting sensitive circuits from harmful noise. There exist two main categories of filters: active and passive!

Difference-between-Active-Passive-Filter

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Curious to know the difference between Active and Passive filters? Passive filters, built with resistors, capacitors, and inductors, leverage the inherent properties of these components to attenuate or pass specific frequencies. Conversely, active filters combine passive components with active elements like operational amplifiers, offering greater control and design flexibility.

While passive filters excel in simplicity and linearity, active filters provide superior performance, improved signal isolation, and the ability to realize more complex transfer functions. However, this enhanced functionality comes at the cost of increased circuit complexity and power consumption. Still confused about the difference between active and passive filters? Let’s explore these and make informed decisions when incorporating filters into our electronic designs.

What is a Filter

A filter is an electronic circuit that has the ability to pass or amplify a certain range of frequencies and block or attenuate other frequencies. They are used for filtering specific frequencies from a signal. Filters are used in signal processing such as removal of noise from a signal and extraction of a message from a signal etc.

There are different types of filters such as digital and analog filters, active and passive filters, linear and non-linear, low-pass, high pass, bandpass filters, etc. We are going to discuss the differences between the active and passive filters.

What is Active Filter?

Active filters are those filter circuits that are designed using transistor and op-amp as their basic components. Along with these elements circuits of active filters also contain resistor and capacitor, but not inductors.

We know filter exhibits the property of frequency selectivity. Thus active filter circuits use transistor and op-amp to pass only a selective band of frequency while attenuating rest of the frequency.

The figure below shows the example of a circuit of an active filter:

In case of active filters in order to generate the required filter characteristic, the interconnection of the op-amp, integrator, invertor etc. with resistor and capacitor are formed.

Usually, the op-amp in the circuit is used in an integrated manner. Thus ensure small size and less bulky. We know op-amp offers high input impedance and low output impedance. Thus such active filters eliminate the loading effect at source and load.

But active components offer finite bandwidth thus it sometimes leads to cause difficulty in operation of the high-frequency signal. Also, the need for an external dc source is present in case of the active filtering unit because it cannot take the driving power from the signal at its input.

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Features of Active Filter

Active filters use active components such as op-amp and transistor.
It also includes passive components such as resistor and capacitor but not inductor.
It requires an external power source to operate.
It has very high input impedance and very low output impedance.
It can be easily cascaded to increase the order of the filter without worrying about the loss in magnitude.
It provides a high power gain at its output.
Change in the load resistance does not affect its electrical characteristics.
It has complex circuitry.
It has a compact design and has a small size.
It is expensive as compared to passive filters.
It has a limited frequency range due to the limited bandwidth of active components. It cannot operate at high frequency.
It cannot handle large currents.

Components Used in Active Filters

Active filters incorporate active components alongside passive ones. They combine passive components with active elements, primarily operational amplifiers (Op-amp) and transistors. Here’s a breakdown of the key players in active filters:

Operational Amplifiers (Op-amps) are high-gain differential amplifiers that amplify the difference between two input voltages. They provide features like high input impedance, low output impedance, and controllable gain.

Transistors can achieve a limited filtering function in basic active filter circuits, requiring fewer components than an op-amp-based design. Some transistor types may operate effectively at lower voltages than readily available op-amps, making them suitable for battery-powered or low-power circuits.

What is Passive Filter?

Passive filters are the filter circuits that are formed using only resistor, inductor and capacitor as their major components. As no amplifying element is present in it thus passive filters offer low signal gain. This leads to the reception of the comparatively low signal at the output of the filter circuit than the applied input signal.

Let us have a look at the circuit of a passive filter:

For radio frequency range, passive filters offer a good response. But the presence of inductor in the circuit creates problem in low-frequency applications. As in case of low frequencies, the inductance of the inductor must be increased, that ultimately need more number of turns in the coil.

Below the RF range, both input, as well as the output impedance of passive filters, creates an issue. Thus these are not much suitable for low-frequency operations. Basically, the band of frequency which is allowed and restricted generates the classification of filters.

So, in case the RLC network is passing only lower band of frequency then it is known to be a low pass filter. Similarly, if the filter is attenuating lower frequency band and passing higher band of frequency then it is a high pass filter.

Features of Passive Filter

Passive filter use only passive components such as resistor, capacitor and inductor.
It does not require an external power source.
It does not provide any power gain.
The change in load resistance affects the characteristics of the passive filter.
It has no frequency limitation and is mostly used for high-frequency applications due to the use of inductor.
For low-frequency applications, it requires a large inductor which increases the size and cost of the passive filter.
It has a simple and easy design.
It is cheaper than an active filter.
It has a larger size and heavy due to bulkier inductors.

Components Used in Passive Filters

Passive filters are constructed using three main components: resistors, capacitors, and inductors. Each component plays a crucial role in shaping the frequency response of the filter.

Resistors are passive components that oppose the flow of electric current. The specific values of resistors, in conjunction with capacitors or inductors, determine the filter’s cutoff frequency. They determine the shape of the frequency response curve. Resistors dissipate electrical energy as heat during their operation. This energy loss can contribute to a reduction in the overall signal strength, especially in high-power applications.

Capacitors are passive components that store energy in an electric field. In passive filters, capacitors block low-frequency signals and pass high-frequency signals. They are used with other components to create high-pass and band-pass filters. Capacitors exhibit decreasing reactance with increasing frequency, allowing high-frequency signals to pass through while attenuating low-frequency signals.

Inductors are passive components that store energy in a magnetic field. In passive filters, inductors block high-frequency signals and pass low-frequency signals. They are used with other components to create low-pass and band-stop filters. Inductors exhibit increasing reactance with increasing frequency, allowing low-frequency signals to pass through while attenuating high-frequency signals