A High-Pass Filter is a circuit that only allows high-frequency signals to pass, and attenuates or reduces signals whose frequencies are below its cut-off frequency.  It is also referred to as a 'low-cut filter' or, when used in audio applications, as a 'bass-cut filter' or 'rumble filter'. One common application of high-pass filters is for driving tweeters (speakers designed for high-pitch sounds), so as to block low-frequency signals that can interfere with or even damage the tweeter. The high-pass filter is the opposite of the low-pass filter.

An ideal high-pass filter is one that completely blocks all frequencies below a given frequency, while allowing all those with higher frequencies to pass unchanged.  Of course, an ideal high-pass filter doesn't exist, so in the real world, the effectiveness and efficiency of a high-pass filter is described is terms of the level of attenuation of signals with frequencies below a cut-off frequency.  The cut-off frequency of a high-pass filter is the frequency at which the output voltage equals 70.7% of the input voltage.

Figure 1 shows some common implementations of high-pass filters. Note that in each of the high-pass filters shown above, the inductors are in shunt with the input while the capacitors are in series with the input.  This is because the reactance XL of an inductor increases with the signal frequency, i.e., XL = 2πfL, while the reactance XC of a capacitor decreases with the signal frequency, i.e., XC = 1 / 2πfC.  Thus in these high-pass filters, the capacitors resist the passing of an ac signal as the frequency decreases, while the inductors shunt them towards the ground as the frequency decreases.  Either way,  the effect is to attenuate the signal as frequency decreases.

Figure 1.  High-Pass Filters

See Also:   Op Amp High Pass Filter;  Low-Pass Filter;  Reactance; ;  More Articles