ACTIVE_FILTERS_Theory_and_Design

Sallen–Key Filters 41
3.5 n
10 K
C 2
10 K
+
−
24 n
C 1
20 K
LM741
R 1 R 2
R b
R a
2.2 K
(a)
30.00
0.0
20.00
–36.0
Gain dB
10.00
0.00
–72.0
–108.0
Phase Deg
–10.00
–144.0
–20.00
100 1 K
Frequency in Hz
(b)
–180.0
10 K
FIGURE 2.14 (a) Second-order LP Bessel filter, f 1 = 1 kHz, K = 10; (b) its phase response.
and Equation (2.29), becomes
3− K
= a ∴
a
K
= 3−
a
b
(2.39)
We have an “equal component” VCVS low-pass filter, for a normalized cutoff
frequency of w 1 = 1 rad/s. However, we pay a premium for the convenience of having
equal resistors and capacitors. The passband gain will be fixed (Equation 2.39).