Schematic Symbols for Electronic Filters

Lecture

Electronic filters are used to control the frequency spectrum of a signal. The following schematic symbols are used in circuits for various types of filters:

Low-Pass Filter (LPF):
- Symbol: usually depicted as a triangle indicating the frequency range that is passed. The input and output can be indicated by lines, and inside the triangle there may be labels for the filter type (for example, RC, RL, active).
High-Pass Filter (HPF):
- Symbol: similar to the LPF symbol, but with a different indication of the direction of the frequency range. Usually the triangle indicates that high frequencies are passed.
Band-Pass Filter (BPF):
- Symbol: a combination of LPF and HPF. Usually depicted as two triangles, one indicating low frequencies and the other high frequencies, connected to each other.
Band-Stop Filter (or Band-Reject Filter, BSF):
- Symbol: the opposite of BPF. It includes two triangles indicating the frequencies that are not passed, with a gap for the pass band.
Active Filters:
- Symbols for active filters, such as those using operational amplifiers (op-amps), can be added to the basic LPF, HPF, BPF, and BSF symbols. They usually include op-amp symbols with the corresponding inputs and outputs.
Filters with Adjustable Frequency Response:
- Symbols may include control elements, such as potentiometers, for adjusting the frequency range of the filter.

Universal Frequency Filter

Schematic Symbols for Electronic Filters

This is a general symbol representing a frequency filter. A frequency filter is an electronic circuit that rejects or attenuates certain frequencies and passes others. This symbol represents only a frequency filter in general, not a specific type.

High-Pass Filter

Schematic Symbols for Electronic Filters

This type of filter passes only high-frequency signals and blocks or attenuates low-frequency signals; it is called a high-pass filter.

The symbol depicts two waves (signals), i.e. a lower and an upper signal, corresponding to low and high frequency respectively. The lower dashed signal represents the blocked low frequency.

Low-Pass Filter

Schematic Symbols for Electronic Filters

A low-pass filter blocks or attenuates any high-frequency signal and passes a low-frequency signal; it is called a low-pass filter. The symbol denotes the blocked high frequency.

Band-Pass Filter

Schematic Symbols for Electronic Filters

A band-pass filter blocks both low and high frequencies but passes frequencies between the low and high frequency. The allowed frequency range forms a band, hence the name band-pass filter. The symbol shows that the upper and lower signals are shaded (blocked), while the signal between them passes through.

Band-Stop Filter

Schematic Symbols for Electronic Filters

A band-stop filter, or notch filter, is also known as a rejection filter. As its name suggests, this filter blocks or rejects a specific band of frequencies. Frequencies above or below this specific band pass through without any attenuation.

Variable Notch Filter

Schematic Symbols for Electronic Filters

This is a type of notch filter with a variable frequency range. The rejected frequency band can be changed using a special feature, usually a knob, that alters the values of the components within the circuit.

Band-Pass Filter with Variable Center Frequency

Schematic Symbols for Electronic Filters

This is a type of band-pass filter with a variable center frequency. The center frequency is the midpoint between the lower and upper corner frequencies. Changing the center frequency changes the filter's band.

Band-Pass Filter with Adjustable Selectivity

Schematic Symbols for Electronic Filters

This type of band-pass filter has variable selectivity, which allows the filter to change the allowed frequency band by changing the corner frequencies.

Below is a list of the electronic filter symbols explained above.