"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Practical Design Hints
16-48
low-pass filter, the RC network can affect the filter characteristic. Then it is necessary to
either include the biasing network into the filter calculations, or to insert an input buffer
between biasing network and the actual filter circuit, as shown in Figure 16–48.
V IN
CIN
+V CC
R B
R B
V MID
Figure 16–48. Biasing a Sallen-Key Low-Pass
VMID R1 R2 VMID
VMID C IN ac-couples the filter, blocking any dc level in the signal source. V MID is derived from
V CC via the voltage divider. The op amp operates as a voltage follower and as an impedance
converter. V MID is applied via the dc path, R 1 and R 2, to the noninverting input of the
filter amplifier.
Note that the parallel circuit of the resistors, R B , together with C IN create a high-pass filter.
To avoid any effect on the low-pass characteristic, the corner frequency of the input highpass
must be low versus the corner frequency of the actual low-pass.
The use of an input buffer causes no loading effects on the low-pass filter, thus keeping
the filter calculation simple.
In the case of a higher-order filter, all following filter stages receive their bias level from
the preceding filter amplifier.
Figure 16–49 shows the biasing of an multiple feedback (MFB) low-pass filter.
C 1
C 2
+V CC
V OUT