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 .

 

hi 321,
As you mention Gain, I assume that a OPA is going to be used as part of the LP filter.??

Do you have a staring point circuit diagram and a LP filter specification.?

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Yes thank you for replying ericgibbs there is a sample circuit here and true it has Op-amp .

 

Hello,

Have a look at chapter 3 of the attached PDF.

Bertus

 

hi 321,
That looks OK for a test run.
What frequency ranges did you have in mind, we should have a design spec.
eg: the frequency point at -3dB and Gain figure.
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Hi ericgibbs it's my circuit and bode plot ,I don't understand you clearly .is that what you want ,thank you .

 

hi 321,
Will look over your posted circuit shortly.
As a LP filter, this demo plot is what I would expect to see.
It has a Low Pass bandwidth from 1 thru ~20kHz the -3dB point on the plot
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Added the actual circuit.

 

Then eric is my bode plot wrong ? and can you explain more thank you .

 

hi 321,
This is your post #3 circuit, with the damping resistors at [R1/R2]~0.6 giving ~+4db 1 Hz thru #20kHz

Do you have values for the circuit in post #6.?

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oh . I am so sorry I write them but pc gived me error then delete them Voltage = 10 Volt, R0= R1 = 1K OHM , Capacitors = 1pikoFarad,R3=5.85 KOHM,R2=10K Ohm .Then , I think I find wrong bode plot my transfer function absurd but I don't know (My transfer function = 1.585/(s^2 + 0.002415 s +1).my question " Design second order active low pass filter with 4 dB gain. Determine cut-off frequency (you are free) and use appropriate circuit elements. Compare the results with the desired paramaters. (cut-off) "

 

hi,
This is your circuit in post #6.
What -3dB point design point did you calculate.??

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Update:
Check your 1pF value calculation

 

Okey I will check it then my 3 db and my -3 dB is approximately View attachment 239465like that.Therefore there is another capacitor in my design .

 

hi,
I guess you know that you should expect a gain of ~+4dB from that type of circuit.
The 5.85k/10k are important.
There are ready calculated Tables for these values.
BTW: Note C2....
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No ,I mailed to my teacher he said okey he accept my submission thank you for all helping me ericgibbs.Have a nice day I solve it .

 

hi 321,
Pleased to hear the acceptance, good luck with your studies.
When you have time check the maths.
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hi 747,
In simulators the current direction in components is specified due to the Net list, so some indication of polarity can avoid mistakes in measuring the current direction in a component.

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@anniel747
Added example.
R2 has been rotated 180 deg
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R1 N001 0 1k
R2 0 N001 1k
V1 N001 0 10v
.tran 1
.backanno
.end

 

Yes just simulator thing ,normally I use proteus and there is no polarized resistor in there ,I don't use .

 

I use proteus and there is no polarized resistor in there

All simulated resistors have a current direction polarity, which gives a plus or minus sign when viewing the current in the resistor.
This current polarity direction normally doesn't show on the simulation schematic.

 

How to control (adjust) the gain in active low pass filter circuits?

You do realize that adjusting the gain of the Sallen-Key filter, also affects the filter frequency response for a given set of filter component values(?).

 

You do realize that adjusting the gain of the Sallen-Key filter, also affects the filter frequency response for a given set of filter component values(?).

Changing the gain of a Sallen-Key filter actually changes the pole locations and alters the filter type, changing the group delay and passband ripple. If you want to change the gain of the filter, you can add an adjustable gain stage to the output. In this way the gain setting and the filter characteristics are decoupled from each other. This is the easiest way to do what you seem to want to do.