# How can we control Low pass filter.

#### apollo321j

Joined Dec 30, 2020
28
Yes you're right ,thank you for adjustment.

#### bertus

Joined Apr 5, 2008
22,305
Hello,

Did you have a look at the PDF I posted in post #4?

Bertus

#### ericgibbs

Joined Jan 29, 2010
19,115
hi 321,
The PDF in post #4, explains the Sallen Key filters.

I am not sure what you are asking: Hi ericgibbs what do you think about Sallen key ?

As you may know there a about 4 common types of filter design, with a different number of Stages/Poles.

The choice of filter is determined by the result you are trying to achieve.

E

Update:

#### apollo321j

Joined Dec 30, 2020
28
hi ericgibbs ,
Thank you so much for helping.

#### bertus

Joined Apr 5, 2008
22,305
Hello,

As @ericgibbs said, there are several types of filters available.
The attached PDf will show you the different types and responses.

Bertus

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#### apollo321j

Joined Dec 30, 2020
28
Thank you ever so much Bertus .

#### MrAl

Joined Jun 17, 2014
11,722
Hi ,I am studying about Filter Circuits and I cannot find " How to control (adjust) the gain in active low pass filter circuits? " could you explain me ? thank you .
Hello there,

I got here a little late (he he) but maybe i can still help a little even though there was a lot from other members already. Here i will outline a very general method.

First, this falls into the category of "AC Circuit Analysis" and there are ways to handle this kind of analysis that make it possible to analyze ANY circuit that is purely AC driven and contains only lumped linear components. This circuit fits that description too but even much, much more complicated circuits can be analyzed using a general solution that is not hard to learn or to perform. The only prerequisite is DC circuit analysis with linear components and complex numbers and complex algebra, and complex numbers are just a little more invovled than regular numbers. You do need to know a general DC circuit analysis method though but you can always pick up Nodal analysis (if you havent already) and that would get you very far.

The only real changes to the DC analysis is that you first transform the capacitors and resistors into their complex forms, then everything else is much the same except you are now working with complex quantities instead of just real numbers. Real numbers are like 3.4, 5.91, 78, etc., and complex numbers are like 4.3+2.77j, 1.234+943.1j, etc.

In the end, you solve for the real and imaginary parts of the solution and that tells you the output(s) AC amplitude and phase shift. In a filter application you do this several times over and that gives you the entire frequency plot and also the phase plot.
There is also what is known as a "Bode" plot. A true Bode plot is an approximate plot of the response. This is simple for first order filters but as you go up in order it gets more tricky you need a table or graph reference to get the curve close enough to exact for many applications. This and the fact that Bode analysis was invented some 70 years ago before computers were widespread like today makes it a little obsolete except in school classes or for simple circuits because with computer software you can generate frequency graphs in the blink of an eye. Back in the 1930's they did not have this luxury so Bode came up with this idea of an approximation. I am sure it helped many an engineer over the years but in the modern age we have computes and associated software, so if you calculate the transfer function you can graph the response quite easily. Of course you can also use a simulator but then you dont learn as much

#### apollo321j

Joined Dec 30, 2020
28

#### MrAl

Joined Jun 17, 2014
11,722
You are welcome. BTW i meant to say "you first transform all capacitors and inductors into their impedances" but you also might transform the voltage and current sources also and none of this is exceptionally hard to do.