Main Difference – Active vs. Passive Filters
A filter is an electronic device that can remove specific ranges of frequencies from a signal. A filter could be active or passive. The main difference between active and passive filters is that passive filters cannot cause a power gain (i.e. they cannot bring energy into the circuit). Nor can passive filters regulate the current. An active filter can add energy into the circuit and also control current.
What is a Passive Filter
A passive filter is made up entirely of passive components such as resistors, capacitors and inductors. For instance, a high pass filter (a filter that allows higher frequencies to pass through it while attenuating lower frequencies) can be constructed using a capacitor and a resistor:
A passive high pass filter made using a capacitor and a resistor
A low pass filter allows lower frequency signals to pass through but attenuates higher frequency signals. A low pass filter can be made by switching the resistor and capacitor in the previous circuit:
A passive low pass filter made using a capacitor and a resistor
Note that there are many other ways of constructing these filters, some of them involving inductors.
What is an Active Filter
An active filter uses active components (components with a power gain) such as operational amplifiers in order to filter the signal. For instance, the circuit below is an active high pass filter:
An active high pass filter made using a capacitor, resistors and an operational amplifier
An active low pass filter is shown below:
An active low pass filter made using a capacitor, resistors and an operational amplifier
Difference Between Active and Passive Filters
Type of Components Used
Passive filters are constructed using only passive components (resistors, capacitors and inductors).
Active filters may contain passive as well as active components.
Use of Inductors
Passive filters may consist of inductors.
Active filters do not contain inductors.
Frequency Range
Passive filters can be used at high frequencies by using inductors.
For Active filters, the frequency range is dependent on the bandwidth of the amplifier. Typically, to filter high-frequency signals, passive filters are used.
Effect of Changing the Load
Compared to active filters, properties of passive filters may change significantly when the load is changed.
In comparison, properties of active filters do not change a great deal when the load is changed, since power can be added to the circuit by the active components.
