THS4031, THS4032

100-MHz LOW-NOISE HIGH-SPEED AMPLIFIERS

SLOS224C – JULY 1999 – REVISED APRIL 2000

27

POST OFFICE BOX 655303

APPLICATION INFORMATION

general configurations

When receiving low-level signals, limiting the bandwidth of the incoming signals into the system is often

required. The simplest way to accomplish this is to place an RC filter at the noninverting terminal of the amplifer

(see Figure 52).

VI

VO

C1

+

–

RG

RF

R1

f

–3dB +

1

2

pR1C1

V

O

V

I

+ 1 )

R

F

R

G

1

1

) sR1C1

Figure 52. Single-Pole Low-Pass Filter

If even more attenuation is needed, a multiple-pole filter is required. The Sallen-Key filter can be used for this

task. For best results, the amplifier should have a bandwidth that is 8 to 10 times the filter frequency bandwidth.

Otherwise, phase shift of the amplifier can occur.

VI

C2

R2

R1

C1

RF

RG

R1 = R2 = R

C1 = C2 = C

Q = Peaking Factor

(Butterworth Q = 0.707)

(

=

1

Q

2 –

)

RG

RF

_

+

f

–3dB +

1

2

pRC

Figure 53. 2-Pole Low-Pass Sallen-Key Filter