Figure 3.2 shows one possible low-pass filter. The circuit is essentially a frequency-sensitive voltage divider. At high frequencies the output behaves as if it is shorted while at low frequencies the output appears as an open circuit.

**Figure 3.2:** *RC* low-pass filter

Mathematically we have

The approximations are

At the corner

Therefore

is the *corner frequency* of the filter.
At the corner frequency

We say that the output is down by at the corner frequency.

Tue Jul 13 16:55:15 EDT 1999