
Active signal processing in the frequency domain 139
Notice that Q gives the magnitude of the output of
a second-order section (for unity input) at the
corner or resonant frequency, not the maximum
output. This occurs, in the case of the low-pass
output, at zero frequency for Q values of less than
1/2, and at a finite frequency below the corner
frequency for Q>\I2. For higher and higher
values of the frequency of maximum response
moves closer and closer to the corner frequency.
Since the HP and LP outputs are always in
antiphase, if we combine them as in Figure 6.9a
there will be zero output at the resonant fre-
quency: we have a notch circuit ...