Elektor_Mag_-_January-February_2021

74 January & February 2021 http://www.elektormagazine.com

The input impedance of active filters can be made very high by
adding a buffer at the input; a buffer at the output can make the
output impedance very low. As a result, we no longer need to worry

about the correct ‘termination’ or the matching of impedances at

the input and the output. However, passive filters are often still used

at very high frequencies. These are designed for specific input and

Analogue

Filter Design (Par t 3)

Passive Filters

By Alfred Rosenkränzer (Germany)

The third and final part of this series on the design of analogue filters deals with the

subtleties of passive filters. Since only purely passive components such as resistors, coils

and capacitors are used here, amplification is not feasible. Thus high input and low output

impedances cannot be realised. However, high frequencies in the three-digit MHz range and

above are less problematic.

analogue electronics

L1

R 1 C 1

L2 R 2

C 2

L3

C 3

L1

R 1

L2 R 2

C 1

L3

L1 C 2 C 3

C 1

R 1 L2

C 2

L3

C 1 C 3 R 2

R 1 L1

C 2

L2

C 3 R 2

L3

L1

R 1 C 1

L2 R 2

C 2 C 3

L1

R 1

L2 R 2

C 1

L3

L1 C 2

C 1

R 1 L2

C 2

L3

C 1 R 2

R 1 L1

C 2

L2

C 3 R 2

L1

R 1 C 1

L2 R 2

C 2

L1

R 1

L2 R 2

L1 C 1 C 2

C 1

R 1 L2

C 1 C 2 R 2

R 1 L1

C 2

L2

R 2

L1

R 1 C 1

R 2

C 2

L1

R 1

L2 R 2

L1 C 1

C 1

R 1 L2

C 1 R 2

R 1 L1

C 2 R 2

L1

R 1 C 1

L1 R 2

R 1

R 2

L1 C 1

C 1

R 1

C 1 R 2

R 1 L1

R 2

R 1 C 1

L1 R 2

R 1

R 2

R 1 L1

C 1 R 2

R 1

R 2

LP Shunt First LP Series First HP Shunt First HP Series First

1. Order


2. Order


3. Order


4. Order


5. Order


6. Order

200522-001

Figure 1: Basic schematics for passive low-pass and high-pass filters with PI and T structures from first through to sixth order. Both the corner frequency and
the frequency characteristic depends on the component values. The structures shown here are suitable for Bessel, Butterworth and Chebyshev filters.