Op Amps for Everyone - The Repeater Builder's Technical ...

Practical Design Hints
+V CC
+V CC
R 2
C R B
C 1 IN VMID
VMID R 1
R 3
V IN R B
C 2
V OUT
R B
+V CC
V MID
V MID
C B
R B
to further filter stages
Figure 16–49.
Biasing a Second-Order MFB Low-Pass Filter
The input buffer decouples the filter from the signal source. The filter itself is biased via
the noninverting amplifier input. For that purpose, the bias voltage is taken from the output
of a V MID generator with low output impedance. The op amp operates as a difference amplifier
and subtracts the bias voltage of the input buffer from the bias voltage of the V MID
generator, thus yielding a dc potential of V MID at zero input signal.
A low-cost alternative is to remove the op amp and to use a passive biasing network
instead. However, to keep loading effects at a minimum, the values for R B must be significantly
higher than without the op amp.
The biasing of a Sallen-Key and an MFB high-pass filter is shown in Figure 16–50.
The input capacitors of high-pass filters already provide the ac-coupling between filter and
signal source. Both circuits use the V MID generator from Figure 16–50 for biasing. While
the MFB circuit is biased at the noninverting amplifier input, the Sallen-Key high-pass is
biased via the only dc path available, which is R 1 . In the ac circuit, the input signals travel
via the low output impedance of the op amp to ground.
Active Filter Design Techniques
16-49