Filtros Con Múltiples Realimentaciones

Mini Tutorial

MT-220

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OUT

Multiple Feedback Filters
C4

byHank Zumbahlen,
Analog Devices, Inc.

R5

C1

C3

IN
10425-002

R2

IN THIS MINI TUTORIAL
Three sample multiple feedback filters are designed in this
mini tutorial, one in a series ofmini tutorials describing
discrete circuits for precision op amps.

Figure 2. Multiple Feedback High-Pass Filter

The design equations for the multiple feedback band-pass filter
are given in theMultiple Feedback Band-Pass Design Equations
section

The multiple feedback filter, a popular configuration, uses an
op amp as an integrator as shown in Figure 1. Therefore, the
dependence of thetransfer function on the op amp parameters
is greater than in the Sallen-Key realization.

(1)

where H is the gain of the circuit. The multiple feedback filter
inverts the phase of the signal.This is equivalent to adding the
resulting 180° phase shift to the phase shift of the filter itself.
OUT

R4

C5

R1

R3
C2

10425-001

IN

R5
C3

IN
R2

Figure 3. MultipleFeedback Band-Pass Filter

This circuit is widely used in low Q (< 20) applications. It
allows some tuning of the resonant frequency, F0, by making
R2 variable. Q can be adjusted (with R5) as well,but this also
changes F0.
One way to tune the filter F0 is by monitoring the output of the
filter with the horizontal channel of an oscilloscope, with the input
to the filter connected to thevertical channel. The display will be
a Lissajous pattern. This pattern is an ellipse that collapses to a
straight line at resonance, since the phase shift is 180°. In addition,
adjust the output formaximum output, which occurs at resonance;
however, this is usually not as precise, especially at lower values of
Q where there is a less pronounced peak.

MULTIPLE FEEDBACK LOW-PASS DESIGN...