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Practical Design Hints
16.8 Practical Design Hints
This section introduces dc-biasing techniques for filter designs in single-supply applications,
which are usually not required when operating with dual supplies. It also provides
recommendations on selecting the type and value range of capacitors and resistors as
well as the decision criteria for choosing the correct op amp.
16.8.1 Filter Circuit Biasing
The filter diagrams in this chapter are drawn for dual supply applications. The op amp operates
from a positive and a negative supply, while the input and the output voltage are
referenced to ground (Figure 16–46).
+V CC
C 1
R 1
R 2
V IN
VOUT
– V CC
Figure 16–46.
Dual-Supply Filter Circuit
For the single supply circuit in Figure 16–47, the lowest supply voltage is ground. For a
symmetrical output signal, the potential of the noninverting input is level-shifted to midrail.
+V CC
C IN
R B
R 1
R 2
V IN
R B
V MID
V OUT
Figure 16–47.
Single-Supply Filter Circuit
The coupling capacitor, C IN in Figure 16–47, ac-couples the filter, blocking any unknown
dc level in the signal source. The voltage divider, consisting of the two equal-bias resistors
R B , divides the supply voltage to V MID and applies it to the inverting op amp input.
For simple filter input structures, passive RC networks often provide a low-cost biasing
solution. In the case of more complex input structures, such as the input of a second-order
Active Filter Design Techniques
16-47