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Differences between Active and Passive Filter

Electrical Technology 5 minutes read

What is the Difference Between Active and Passive Filter?

Active filter and passive filter are both types of electronics filters that filter a specific frequency or a range of frequencies from a signal. Apart from frequency filtering, they are quite different in design as well as performance.

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Before going into the differences between active and passive filters, let’s discuss their basics first.

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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.

• Related Post: Filters, Types of Filters and Their Applications

What is Active Filter?

Active filter is a type of filter that is designed using active components such as op-amp and transistor. They are used in combination with resistor and capacitor but not inductor. Such filters are capable of providing a power gain. However, their design is complex as compared to passive filters.

Since active filters use active components, they require an external power supply to operate. The filtered signal does not require amplification. Whereas the gain can be modified by varying a certain parameter during operation.

Op-Amp has very high input impedance and very low output impedance. Therefore, the active filter does not have loading effect problem at its source and load. Also, varying the load does not affect the performance of the filter because the load is isolated from the source.

The drawback of an active filter is the requirement of an external source. The filter’s performance depends on the external source’s health. The active components used in active filters have limited bandwidth. Therefore, they are not suitable for filtering high-frequency signals. They are also more susceptible to damage from large current.

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.

Related Posts:

• Types of Active High Pass Filter
• Types of Active Low Pass Filters

What is Passive Filter?

Passive filter only uses passive components such as resistors, capacitors and inductors, etc. it has a very simple design and is very cheap. This filter does not require an external power source to operate and that is why they do not provide any power gain. However, they do use an inductor that makes them able to withstand high current.

The passive components can handle very high frequency. As a result, passive filters are used for high-frequency applications. but they are not suitable for very low-frequency applications. The value of inductance must be increased which requires more bulkier inductor that also increases the size as well as the cost of the filter.

The load resistance is directly connected and affects the overall resistance of the filter. Therefore any change in the load resistor affects the electrical characteristics of the filter. It has limited input and output impedance, their performance is affected by loading affect.

Since passive components consume power from the input signal, the passive filter does not provide power gain and the output signal has a lower magnitude than the input. It requires amplification post-filtering.

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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.

Related Posts:

• Types of Passive High Pass Filters
• Types of Passive Low Pass Filters – RL and RC Passive Filters

Key Differences between Active and passive Filter

Active Filter Passive Filter Active filter uses active components such as Op-amp and transistor Passive filter uses passive components such as resistor, capacitor and inductor. It requires external power source to operate It does not require an external power source. It provides a high power gain It does not provide power gain. It has a very high input impedance It has a medium input impedance. It has a low output impedance It has a medium output impedance Its characteristics do not change with change in load Its electrical characteristics vary significantly with changes in load. It includes active as well as passive components except for the inductor. It is only made from passive components It has frequency limitation due to limited bandwidth of op-amp It has no frequency limitations. It does not have resonance problems They are affected by resonance It has a high Q factor It has a low Q factor They are expensive They are relatively cheaper It cannot handle large current. It is stable and can withstand large current. It has a complex design  It has a simple design They are compact and have small size. Due to the use of inductor they are bulkier and heavy.

Comparison between Active and passive Filters

Power Source

• Active filter require an external power source to operate.
• Passive filter does not require an external power source to operate.

Gain

• Active filter provide a high power gain and amplify the output signal.
• Passive filter does not provide any power gain. In fact, the output signal has a lower magnitude than its input signal.

Design

• Active filter has a complex design.
• Passive filter has a simple design.

Loading Effect

• Active filter does not have a loading effect problem because it has low output and high input impedance.
• Passive filter has a loading effect issue.

Bandwidth

• Active filters have frequency limitations and cannot filter high-frequency signals due to the limited bandwidth of active components.
• Passive filter does not have any frequency limitations.

Size

• Active filter are compact and they have a small size and weight.
• Passive filters are bulkier due to inductor size and heavy especially at a low frequency that requires large inductors..

Cost

• Active filters are expensive and costly due to active components and the requirement of a power source.
• Passive filters are made from cheap passive components.

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Electrical Technology 5 minutes read

Active filters use active components, such as transistors, op-amps, or comparators, to amplify or modify the input signal. This allows them to achieve higher gain, lower distortion, and better stability than passive filters. Active filters can also implement complex filter functions, such as band-pass, band-stop, or notch filters, with fewer components and less space. However, active filters also have some drawbacks. They require a power supply, which may introduce noise or interference. They also have limited bandwidth and dynamic range, as they are affected by the characteristics and limitations of the active devices.

Passive filters use only passive components, such as resistors, capacitors, inductors, or transformers, to shape the input signal. They do not need a power supply, which makes them simpler, cheaper, and more reliable than active filters. They also have higher bandwidth and dynamic range, as they are not limited by the active devices. However, passive filters also have some disadvantages. They have lower gain, higher distortion, and poorer stability than active filters. They also require more components and space to implement complex filter functions, such as band-pass, band-stop, or notch filters.

When choosing or designing a filter, you need to consider some important characteristics, such as frequency response, phase response, attenuation, and ripple. Frequency response describes how the filter affects the amplitude of the input signal at different frequencies. Phase response describes how the filter affects the phase of the input signal at different frequencies. Attenuation describes how much the filter reduces the unwanted frequencies or noise. Ripple describes how much the filter varies the amplitude of the desired frequencies. Different filter types and applications may require different trade-offs between these characteristics.

Designing a filter requires different methods depending on the type, function, and specifications. The transfer function method uses mathematical equations to derive the filter coefficients and components from the desired frequency response and phase response. Additionally, the frequency domain method uses graphical tools, like Bode plots or Nyquist plots, to design the filter by adjusting the gain and phase margins. The time domain method employs graphical tools, such as impulse response or step response, to design the filter by adjusting the rise time and overshoot. Lastly, the prototype method uses standard filter circuits like Butterworth, Chebyshev, or Bessel filters as a starting point before modifying them to meet specific requirements.

Before implementing a filter in a real circuit, it is advisable to simulate and test it using software tools, such as SPICE, MATLAB, or LTspice. These tools can help you verify the filter performance, identify any errors or issues, and optimize the filter parameters and components. You can also use these tools to compare different filter options and select the best one for your application. However, simulation and testing are not enough to guarantee the filter functionality and quality. You also need to consider the practical aspects, such as component tolerances, parasitics, temperature effects, and noise sources, that may affect the filter behavior in a real environment.

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