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RC4156/RC4157
PRODUCT SPECIFICATION
7
Active Filters
The introduction of low-cost quad op amps has had a strong
impact on active filter design. The complex multiple-
feedback, single op amp filter circuits have been rendered
obsolete for most applications. State-variable active-filter
circuits using three to four op amps per section offer many
advantages over the single op amp circuits. They are rela-
tively insensitive to the passive-component tolerances and
variations. The Q, gain, and natural frequency can be inde-
pendently adjusted. Hybrid construction is very practical
because resistor and capacitor values are relatively low and
the filter parameters are determined by resistance ratios
rather than by single resistors. A generalized circuit diagram
of the 2-pole state-variable active filter is shown in Figure
15. The particular input connections and component-values
can be calculated for specific applications. An important fea-
ture of the state-variable filter is that it can be inverting or
non-inverting and can simultaneously provide three outputs:
lowpass, bandpass, and highpass. A notch filter can be real-
ized by adding one summing op amp.
The RC4156 was designed and characterized for use in
active filter circuits. Frequency response is fully specified
with minimum values for unity-gain bandwidth, slew-rate,
and full-power response. Maximum noise is specified.
Output swing is excellent with no distortion or clipping. The
RC4156 provides full, undistorted response up to 20 kHz
and is ideal for use in high-performance audio and telecom-
munication equipment.
In the state-variable filter circuit, one amplifier performs a
summing function and the other two act as integrators. The
choice of passive component values is arbitrary, but must be
consistent with the amplifier operating range and input signal
Figure 13. Triangle and Square Wave Generator
Figure 14. Triangle Generator—Symmetrical Output Option
+15V
30K
1K
V 0.12V
~
Square Wave
Output
R0
4156/57
A
R1
4156/57
D
V3
Output
Offset
1K
Integrator
5K
5K
R3
20K
R4
1K
20K
+15V
-15V
V4
Triangle
Wave
Output
(+)
-12V
+12V
C0
V2
10K
100K
20K
5K
R2
20K
V1
Comparator
65-0750
Amplitude
Adjust
+15V
-15V
* Optional – asymmetric ramp slopes
10
9
4
8
11
7
6
5
1
3
2
12
13
14
4156/57
C
4156/57
B
*
R1
10K
65-2051