Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

How Do I Change Gain on a 4 Pole LP Filter?

Status
Not open for further replies.

Hertzilla

Newbie
Joined
Jun 15, 2015
Messages
4
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
47
I used TI's "WEBENCH Filter Designer" online to design a 4 pole low pass filter. The cutoff freq. is 10.5 kHz.

The pass band gain of the circuit is unity, but what I actually need is a gain of -6.5 dB @ 1 kHz. The software won't let me enter negative numbers, so I can't determine what I need to change.

I'm guessing that I may need to add a resistor in the negative feedback loop of one or both stages to reduce the gain(?) ...I don't want to change the specs of the existing circuit other than the gain.

-Thanks

lp.filter.jpg
 
Last edited:

Warpspeed

Advanced Member level 5
Joined
May 23, 2015
Messages
2,213
Helped
754
Reputation
1,510
Reaction score
734
Trophy points
113
Location
Melbourne, Australia
Activity points
17,766
You cannot change the gain within the filter itself, without seriously upsetting its operation.

If you need to change the overall gain, some outside gain or attenuation will be needed either ahead of the filter or after the filter.
As you need to reduce the amplitude by about half, a simple resistive voltage divider at the output would probably be the simplest solution.
 

crutschow

Advanced Member level 5
Joined
Feb 22, 2012
Messages
3,838
Helped
915
Reputation
1,828
Reaction score
899
Trophy points
1,393
Location
L.A. USA Zulu -8
Activity points
21,672
That circuit cannot readily give a gain of less than one in the passband so using an attenuator at the input or output as Warpspeed suggested would likely be the best way to go.

Why do you need a gain of less than 1?
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
48,266
Helped
14,224
Reputation
28,709
Reaction score
12,917
Trophy points
1,393
Location
Bochum, Germany
Activity points
279,523
The Sallen Key topology doesn't support gain below unity, adding a voltage divider will of course work, as suggested.

A multiple-feedback (MFB) topology gives the option of setting a lower gain, but I fear it's not supported by the Webbench designer.
 

godfreyl

Advanced Member level 5
Joined
Apr 18, 2012
Messages
1,981
Helped
632
Reputation
1,266
Reaction score
624
Trophy points
1,393
Activity points
12,776
A voltage divider at the output would compromise the output impedance. I'd replace either R1_S1 or R1_S2 with a voltage divider.
 

Warpspeed

Advanced Member level 5
Joined
May 23, 2015
Messages
2,213
Helped
754
Reputation
1,510
Reaction score
734
Trophy points
113
Location
Melbourne, Australia
Activity points
17,766
You cannot change R1_S2 that will screw up the relative gains within the filter.

But R1_S1 could be turned into an input attenuator, as long as the first stage is being fed from a source impedance of 6K8.

Closest I could quickly get with very common values would be with 14K and 13K in series.
The impedance at the junction works out to 6K74.
Attenuation of that would be 2.0769:1 or 6.35db

You could probably fudge that a bit with a third resistor, or even a potentiometer.
 
Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top