# Why are T-Filters better than L-Filters?

I was watching a beginners video about electronic filters. In that video, the presenter shows the same filter (same inductance and capacitance values) configured as an "L-Filter" and a "T-Filter" as follows:

During the video, the presenter mentions that the T-Filter provides better overall filtering over the L-Filter.

Could someone please help me understand how splitting the inductors as shown in the T-Filter makes for a better filter? What is occurring on the filter that makes one better than the other?

Thanks.

• I think the correct terminology is second order and third order. The top is the second order filter, bellow is the third order filter. – ArchHaskeller Sep 23 '15 at 4:25

With a high impedance measurement device on the output, both filters will be identical in performance. However, if the output of the T filter were loaded with maybe a resistor, the filter would tend to change from a 2nd order low pass filter to a 3rd order low pass filter. If the resistor were swapped for another capacitor then the filter would tend to become a 4th order filter.

As to exact frequency response this cannot be determined without values.

On the top diagram the two solenoids act together as one. This makes this a second order low pass filter.

On the bottom diagram, since the solenoids are separated. It is a third order low pass filter.

There are many unknown parameters in your circuit. However it should look like this. Third order attenuates the signal's amplitude better.

In most situations where passive filters are used, both the input and output will have finite impedance. If the input and output impedance are resistive, their behavior may be modeled as a series or parallel resistors. For a voltage-based input or current-based output, impedance would appear as a resistor in series with the input or output; for a current-based input or voltage-based output, impedance would appear as resistance in parallel.

Adding such resistors to the LC circuit should help clarify the differences among different LC filter topologies.