Chapter 9

Designing Highpass Filters

First-Order Highpass Filters

First-order filters are the simplest possible; they are completely defined by one parameter: the cutoff frequency.

The usual 1st-order highpass filter is just a capacitor and a resistor. Figure 9.1a shows the normal (non-inverting) highpass version in all its beautiful simplicity. To give the calculated 1st-order response it must be driven from a very low impedance and see a very high impedance looking into the next stage. It is frequently driven from an opamp output, which provides the low driving impedance, but often requires a unity-gain stage A1 to buffer its output, as shown here.

Sometimes it is necessary to invert the phase of the audio to correct an inversion in a previous ...

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