Choosing the Feedback Circuit

TS615

31/36

10.2 Active filtering

amplification configuration:

We assume the following expression as the response of the system:

The cutoff frequency is not gain-dependent and so becomes:

The damping factor is calculated by the following expression:

The higher the gain the more sensitive the damping factor is. When the gain is higher than 1, it is

preferable to use some very stable resistor and capacitor values. In the case of R1=R2:

Figure 72. Low-pass active filtering. Sallen-Key

+

_

IN

910

Ω

TS615

25

Ω

OUT

+

_

IN

910

Ω

TS615

25

Ω

OUT

A

V

g1

R

fb

R

g

----------

+

==

T

j

ω

Vout

j

ω

Vin

j

ω

-------------------

g

12

ζ

j

ω

ω

c

-------

j

ω

()

2

ω

c

2

--------------

++

---------------------------------------------

==

ω

c

1

R1R2C 1C2

--------------------------------------

=

ζ

1

2

---

ω

c

C

1

R

1

C

1

R

2

C

2

R

1

C

1

R

1

g

–

++

()

=

ζ

2C

2

C

1

R

fb

R

g

----------

–

2C

1

C

2

------------------------------------

=