- The amplitude of signals outside this range of frequencies (called stop band) is reduced (ideally reduced to zero).
- Filters are widely used to give circuits such as amplifiers, oscillators and power supply circuits the required frequency characteristic.
- Passive filters are made up of passive components such as resistors, capacitors and inductors and have no amplifying elements (transistors, op-amps, etc) so have no signal gain, therefore their output level is always less than the input.
- The inductors and the capacitors work as opposites. The inductors will block high frequency signals and conduct low frequency signals. The capacitors are going to do just the opposite.
Types of Passive Filters
- Low Pass Filters
- High Pass Filters
- Band Pass Filters
- Band Stop/Rejecct Filters
- Power Line Filters
- SAW Filters
- Signal Filters
- Sinusoidal Filters
Low Pass Filters
- A low-pass filter is a filter that passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency.
- The frequency between the pass- and-stop bands is called the cut-off frequency.
- Low pass filters are used to remove high frequency signals and allow through low frequency signals.
- Low-pass RL and RC filters:
- The frequency at which the low pass filter starts to reduce the amplitude of a signal can be made adjustable. This technique can be used in an audio amplifier as a "TONE" or "TREBLE CUT" control.
High Pass Filters:
- A high-pass filter is an electronic filter that passes signals with a frequency higher than a certain cut-off frequency and attenuates signals with frequencies lower than the cut-off frequency.
- High pass filters are circuits used to remove low frequency signals and allow high frequency signals.
- High-pass RL and RC filters:
- High pass filters are used to remove or attenuate the lower frequencies in amplifiers, especially audio amplifiers where it may be called a "BASS CUT" circuit. In some cases this also may be made adjustable.
Band Pass Filters:
- Band pass filters allow only a required band of frequencies to pass, while rejecting signals at all frequencies above and below this band.
- It is also called as a T filter.
- The T filter consists of three elements, two series−connected LC circuits between input and output, which form a low impedance path to signals of the required frequency, but have a high impedance to all other frequencies.
Band Stop Filters:
- These filters have the opposite effect to band pass filters.
- There are two parallel LC circuits in the signal path to form high impedance at the unwanted signal frequency, and a series circuit forming a low impedance path to ground at the same frequency, to add to the rejection.