This document discusses different types of filter circuits: low pass, high pass, band pass, and band stop filters. It also describes a 3-way speaker crossover network. Low pass and high pass filters have one pass band and one cut-off frequency. Band pass filters have one pass band between two cut-off frequencies, while band stop filters have two pass bands separated by two cut-off frequencies. The input impedance of a filter, called the characteristic impedance, must match the source.

1. Filter Circuits – Low Pass
A low pass filter is one designed to pass signals at frequencies below a specified
cut-off point, (fc ).
frequency
gain
fC
0.707Gm
~~
symbol
T-network
L/2 L/2
C
π-network
C/2 C/2
L
fc =
1
π LC
Cut-off frequency,
Characteristic (Design)
Impedance,
Z0 = R0 =
L
C
L =
R0
π fc
Inductance,
C =
1
π R0 fc
Capacitance,

2. Filter Circuits – High Pass
frequency
gain
fC
0.707Gm
A high pass filter is one designed to pass signals at frequencies above a specified
cut-off point, (fc ).
~~
symbol
T-network
2C
L
2C
π-network
2L 2L
C
fc =
1
4π LC
Cut-off frequency,
Characteristic (Design)
Impedance,
Z0 = R0 =
L
C
L =
R0
4π fc
Inductance,
C =
1
4π R0 fc
Capacitance,

3. Filter Circuits – Band Pass and Band Stop
frequency
gain
f1
f2
0.707Gm
symbol
~
~
~
These filters pass or attenuate and specified band of frequencies and thus have
two cut-off frequencies. The pass/attenuation band is the difference between f1
and f2.
Band Pass
L
L/ 2
2C
L/ 2
2C
C
Band Stop
L
L/ 2
2C
L/ 2
2C
C
symbol
~
~
~
frequency
gain
f1
f2
0.707Gm

4. Filter Circuits – 3-way Speaker Crossover Network
This arrangement of filters can be used to separate the bass, middle and treble
signals from a power amplifier to drive three speakers in a hi-fi system.
Power
Amplifier
Bass
Treble
Midrange

5. Filter Circuits
Activity
1. A high pass filter is required to pass all frequencies above 5kHz and to have
a design impedance of 600Ω. Calculate the circuit values for both T- and π-
sections and sketch filters.
2. A low pass filter is required having a characteristic impedance at zero
frequency of 400Ω and a cut-off frequency of 600Hz. Calculate the circuit
values for both T- and π- sections and sketch the filters.

6. Filter Networks – Summary
• Low-pass and high-pass filters have one pass band and one cut-off
frequency.
• Band pass filters have one pass band and two cut-off frequencies.
• Band stop filters have two pass bands and two cut-off frequencies.
• The input impedance of the filter is called the characteristic impedance and
must be matched to the source.

7. Filter Networks – Summary
• Low-pass and high-pass filters have one pass band and one cut-off
frequency.
• Band pass filters have one pass band and two cut-off frequencies.
• Band stop filters have two pass bands and two cut-off frequencies.
• The input impedance of the filter is called the characteristic impedance and
must be matched to the source.