Page 140 - Analog and Digital Filter Design

137
Analog Lowpass Filters

Note that the output is in-phase with the input (subject to phase shifts due to
the filter's response). The output could have been taken from A3, but would have
been inverted.

The equations for this filter allow the arbitrary choice of capacitor, C.

1
R1=-
20 C
1 1
R2 = R3= R4 = --
C&Zz - o,,
c'
A circuit gain of greater than unity can be achieved if the value of R4 is reduced.
Dividing the value of R4 given in the last equation by a factor K gives the circuit
a gain. The gain is equal to K.

Cauer and Inverse Chebyshev Active Filters

To design a Cauer or Inverse Chebyshev filter a different circuit topology is
required. The Cauer response has zeroes outside the passband, so a notch circuit
is required. This can be achieved using a circuit that is an extension of the state
variable filter and is known as a biquad. This circuit is illustrated in Figure 4.13.

R1 R3
C

Odd Order Only
/

R6

Figure 4.13
The Biauad Filter