1

1

1

=

1

-

-

= O

1

jwc

+

-

C

LMV771, LMV772, LMV774

SNOSA04F – MAY 2004 – REVISED SEPTEMBER 2010

www.ti.com

ACTIVE FILTER

Active filters are circuits with amplifiers, resistors, and capacitors. The use of amplifiers instead of inductors,

which are used in passive filters, enhances the circuit performance while reducing the size and complexity of the

filter.

The simplest active filters are designed using an inverting op amp configuration where at least one reactive

element has been added to the configuration. This means that the op amp will provide "frequency-dependent"

amplification, since reactive elements are frequency dependent devices.

LOW PASS FILTER

The following shows a very simple low pass filter.

Figure 7. Lowpass Filter

The transfer function can be expressed as follows:

By KCL:

(9)

Simplifying this further results in:

(10)

or

(11)

Now, substituting

(12)

(13)

Low pass filters are known as lossy integrators because they only behave as an integrator at higher frequencies.

corner, the capacitor will have the dominant impedance and hence the circuit will behave as an integrator and

the signal will be attenuated and eventually cut. The bode plot for this filter is shown in the following picture:

16

Submit Documentation Feedback

Copyright © 2004–2010, Texas Instruments Incorporated

Product Folder Links: LMV771 LMV772 LMV774