# Active Low Pass Filter Simulation help needed

#### mishra87

Joined Jan 17, 2016
1,034
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 ?

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#### Ian0

Joined Aug 7, 2020
9,802
Because the op-amp has a gain of 2.
Gain of 2 = 6dB

#### BobTPH

Joined Jun 5, 2013
8,938
And when you start at 6db the -3db point is at 3db, not -3db.

Bob

#### Papabravo

Joined Feb 24, 2006
21,225
It is less important in active filters than passive ones but you should also model the soyurce and load impedance of the filter.

#### Audioguru again

Joined Oct 21, 2019
6,688
It is not an active filter. It is simply a 1st-order RC passive filter feeding a buffer amplifier.

#### Papabravo

Joined Feb 24, 2006
21,225
It is not an active filter. It is simply a 1st-order RC passive filter feeding a buffer amplifier.
It is more active than inactive. Just sayin'.

#### bertus

Joined Apr 5, 2008
22,276
Hello,

You might want to have a look at the attached PDF.

Bertus

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#### crutschow

Joined Mar 14, 2008
34,407
It is more active than inactive. Just sayin'.
Typically if it's called an active filter, there is feedback from the output back to the filter elements, such as a Salen-Key. Just sayn'.

#### crutschow

Joined Mar 14, 2008
34,407
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
3,934
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.

#### mishra87

Joined Jan 17, 2016
1,034
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
34,407
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.

#### Ian0

Joined Aug 7, 2020
9,802
Hi,

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

Regards,
M
Leave out R3

#### LvW

Joined Jun 13, 2013
1,754
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
9,802
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
1,754
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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