Active Low Pass Filter Simulation help needed

Thread Starter

mishra87

Joined Jan 17, 2016
947
Hi Guys,

I was simulating the LPF using below circuit.
Calculated corner frequency = 1.59KHz
When i run AC analysis i do not find -3db point at 1.59Khz.
I also do not understand why initial magnitude is 6db.

Could anyone figure what i am missing here ?
1635967476437.png
1635967537946.png
 

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Papabravo

Joined Feb 24, 2006
17,646
It is less important in active filters than passive ones but you should also model the soyurce and load impedance of the filter.
 

crutschow

Joined Mar 14, 2008
28,580
When i run AC analysis i do not find -3db point at 1.59Khz.
But it is.
It's not where the absolute gain is -3dB, it's where the gain drops 3dB from the in-band gain (of 6dB).
If you look at the response at the plus input to the op amp, you will see the -3dB point from the in-band gain of 0dB.
 
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eetech00

Joined Jun 8, 2013
2,825
Hi Guys,

I was simulating the LPF using below circuit.
Calculated corner frequency = 1.59KHz
When i run AC analysis i do not find -3db point at 1.59Khz.
I also do not understand why initial magnitude is 6db.

Could anyone figure what i am missing here ?
Its a passive RC filter with its output connected to an opamp gain stage.

1635999679238.png
 

Thread Starter

mishra87

Joined Jan 17, 2016
947
But it is.
It's not where the absolute gain is -3dB, it's where the gain drops 3dB from the in-band gain (of 6dB).
If you look at the response at the plus input to the op amp, you will see the -3dB point from the in-band gain of 0dB.
Hi,

Is it possible to get the unity gain by using this circuit ?

Regards,
M
 

crutschow

Joined Mar 14, 2008
28,580
Is it possible to get the unity gain by using this circuit ?
Sure, with a slight modification.
The gain of a non-inverting op amp circuit (which you have) is 1+(R2/R3).
[You may be confusing the gain of this with an inverting op amp, which does have a gain of R2/R3 (with the input signal going to R3 and R3 not grounded)].
So if you make R2 zero ohms (replace with a short), the op amp will become a unity-gain follower (gain of 1).
Since R3 now does nothing except act as a load on the output, you can remove it also.
 

LvW

Joined Jun 13, 2013
1,365
Hi,
Is it possible to get the unity gain by using this circuit ?
Regards,
M
Yes, of course. However, if the filter transfer function must be the same, the components require a recalculation (modified design formulas for unity-gain feedback).
 

Ian0

Joined Aug 7, 2020
4,169
Yes, of course. However, if the filter transfer function must be the same, the components require a recalculation (modified design formulas for unity-gain feedback).
Not in this case - it is a first order filter with a buffer. All that is required is to change the gain of the buffer.
 

LvW

Joined Jun 13, 2013
1,365
Not in this case - it is a first order filter with a buffer. All that is required is to change the gain of the buffer.
Yes, of course. You are right. My answer applies to the Sallen-Key-structure.
(I could not imagine that somebody asks such a question for a simple buffer).
 
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