# Pros/Cons/differences of High pass inductive vs capacitive filter

A simple high pass filter can consist of a capacitor in series with a resistor or a resistor in series with an inductor. Why would I use one over the other? I also wonder the same thing about low pass filters. Do capacitive high pass filters have less energy usage or increased speed, or clearer filtering capabilities compared to inductive high pass filters?

Thank you!

• A series inductor as high pass filter, really? You might get more insight into looking at actual things, like in a simulator. Build two filters, do an AC analysis and compare results. – PlasmaHH Feb 8 '17 at 10:56
• Good idea! I'll do that. – IgnisImperatrix Feb 9 '17 at 23:33

You appear to be talking about these two variants: - Both are equally good but you will find that for low frequencies of cut-off (such as audio) the parallel output inductor becomes too physically big compared against a series capacitor. Just try calculating a 100 Hz filter with a 1 kohm resistor.

The capacitor should have a reactance of 1000 ohms at 100 Hz hence, it has a value of 1.6 uF. The inductor also has a reactance of 1000 ohms and therefore has a value of 1.6 henries and will be bulky in comparison, have an annoying self resonant frequency in the kHz region and will cost possible 100 times more than the capacitor.

If you are thinking about a much lower impedance then things change; if R is now 10 ohms, L would be 16 mH and C would be 160 uF i.e. a less clear-cut differentiation.

Go do some math and look up inductors at distributors websites.

Exactly the same argument applies to low pass filters because the same components are used but the output for low-pass is taken across the "other" component: - The two circuit configurations have the same transfer function, so can be made to behave identically as filters. However, there is a major difference in the input and output impedances of the two networks. The C-R configuration works by preventing the low frequency energy from a source from entering the filter while the R-L configuration shunts that energy away from the load.

If, for example, you had an power amplifier generating a broadband signal into a load, and you wanted to add a filter to select the high frequency components, the C-R configuration would be preferable. This is because the C-R filter presents a high impedance to the amplifier for the frequencies you don't want at the output. The R-L configuration, in contrast, presents a low impedance to these frequencies, forcing the amplifier to produce a signal that you are then just going to shunt to ground.

• Thanks for your insight, I wish I could accept all these answers. – IgnisImperatrix Feb 9 '17 at 23:34

Exactly they are not in series, because the output is taken from the middle point.

One L + one R high pass filter is mathematically equivalent with one C + one R high pass filter. No difference if the components are ideal. In practice the inductors degrade to unusable much faster than the capacitors as the frequency increases. This is due the losses and stray capacitance in the wire coils. At low frequencies the inductors are bulky.

One C + one R filter is often too unselective. For steeper frequency discrimination more complex circuits are needed. They must have inductors and capacitors. By adding more Rs and Cs one can get no advantage. At low frequencies (sub 1 MHz) the need of inductors can be passed by using an opamp based active filter. It has an opamp+Cs +Rs.

This all is proven in the mathematical circuit synthesis theory.