Page 164 - Analog and Digital Filter Design
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Highpass Filters
The normalized resistor values for highpass Sallen and Key designs are related
and
to lowpass pole locations by. R‘1= o~.~ R’2 = GlP +ULP
Dl I’
Hence the normalized R’I = 0.7071 and the normalized R‘2 = 1/0.7071 = 1.4142.
R’
. where R’ is the
Frequency scaling should now be carried out using R = -
2x4 c
normalized value. Thus RI = 41.374Q and R2 = 82,749R.
State Variable Highpass Filters
The state variable circuit is actually a simple type of biquad. It provides a
second-order stage suitable for use in all-pole filter designs. The state variable
circuit has a lower sensitivity to the op-amp’s gain-bandwidth product limita-
tion, and section Q factors of up to 200 are possible. The penalty for having this
good performance is that it needs three op-amps and associated passive com-
ponents, as shown in Figure 5.15.
R1 R3
R5
R6
Figure 5.15
State Variable Highpass (All-Pole)
