# RF Wireless World

This page describes Pi filter circuit schematic and its working. It mentions benefits or advantages of Pi filter and drawbacks or disadvantages of Pi filter.

Introduction : There are three basic impedance matching filters viz. L network, T network and π network. L network is easy to implement due to its two components requirement. Moreover L network do not change Q of the circuit. T and π networks are more complex compare to L network but they are preferred as they generate higher output voltages with low ripple factors.

### What is Pi Filter ?

The network which has components (L, C, R) connected in the form of a π shape is called pi network. The filtering device which uses L and C in the π shape is called Pi filter. It can be used both as full wave and half wave rectifiers. The figure-2 depicts use of Pi filter as LPF (Low Pass Filter) and HPF (High Pass Filter). In LPF configuration, two capacitors are connected in parallel of input and output where as a inductor is connected in series. In HPF configuration, two inductors are connected in parallel of input and output where as a capacitor is connected in series. Following are the mathematical equations of a Pi filter as LPF.
• Cut off frequency, Fc = 1/(π*(L*C)1/2)
• C value i.e. C1 and C2 = 1/(Z0*πFc)
• L value i.e. L1 = Z0/(π*Fc)
Where Z0 is characteristic impedance with value of 50 Ohm or 75 Ohm.

Following are the mathematical equations of a Pi filter as HPF.
• Cut off frequency, Fc = 1/(4*π*(L*C)1/2)
• C value i.e. C1 = 1/(4*Z0*π*Fc)
• L value i.e. L1 and L2 = Z0/(4*πFc)
Where Z0 is characteristic impedance with value of 50 Ohm or 75 Ohm.

### Benefits or advantages of Pi filter

Following are the benefits or advantages of Pi filter:
➨It is very easy to implement in RF circuit designs.
➨It is easy to replace these three components due to its easy access for de-soldering and re-soldering.
➨It provides higher output DC voltages.
➨It delivers output with much lower AC ripples.
➨It offers better load regulation compare to L type filter.

### Drawbacks or disadvantages of Pi filter

Following are the drawbacks or disadvantages of Pi filter:
➨Pi filter requires higher value capacitors which make them bulky. This is a major concern for space constraint designs.
➨Pi filters are not suitable when load current varies as they offer poor voltage regulation.
➨Pi filter is not suitable for high current flow through it. This makes series inductors bulky as well as expensive.