RC4156/RC4157

PRODUCT SPECIFICATION

REV. 1.0.1 6/13/01

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

~

~

R

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