Low Pass Filter for an H-Bridge

Analog signals are easily corrupted. Unless the line they are on is relatively short, their fidelity will be compromised, and the signal will be lost. This is most apparent for sound signals to speakers. Professional engineers spend years fine tuning the system, so noise will be kept at a minimum. Amateur systems keep all of the components close together and lines relatively short, so that noise will not be an issue. What if you want to place speakers clear on the other side of a room? No analog solution exists, or is prohibitively expensive.

A digital solution might exist, granted that it is fast enough. Some more information on what that solution might entail exists here. It consists of either a class D amplifier or an H bridge, driven by PWM, and using a carrier signal. To filter out the carrier signal after the class D amplifier requires either an RC, RLC, or active filter. The problem with all of the class D amplifier is it requires the use of two power supplies.

The H bridge doesn't require two power supplies, but the choice of filters is much more limited. Here is one implementation.

It has two inductors, in series with the load (resistor), and a capacitor in parallel with the load. Here is a close up of the filter.

The resistance and capacitance form an impedance, and all the currents are equal.

The currents, being equal, is just the voltage across the filter divided by the impedance.

The voltage of interest is that across the resistance.

What results is a second order low pass filter, where omega not is the cut off frequency, and Q is the amplitude at that frequency. To obtain a maximally flat response, Q must equal the square root of one over two. The cut off frequency is this.

Q is this.

Leading to this.

Again, as with the other passive filters, to obtain a small cut off frequency, the inductor and capacitor will have to be very large. The carrier signal will have to be as large as possible, to keep the components economical and small. Clearly, this is no small engineering feat.


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Electrical Design Engineer, Minneapolis, Minnesota