# Question about active low pass filtering

#### Chen Ron

Mar 29, 2018
1
hello trying to analyze a circuit and couldn't understand this part of the circuit, in the description of the circuit it says its an active low pass filter. can anyone help me with analyzing this circuit? THANKS

#### Attachments

• circuit1.pdf
32.1 KB · Views: 51

#### LvW

Apr 12, 2014
604
There are three points to be answered:
* Is there any negative DC feedback (stable bias point)?
* What about the gain for low frequencies?
* What about the gain for rising (that means: high) freqiencies?

#### Audioguru

Sep 24, 2016
3,654
1) The negative DC feedback is obvious.
2) The gain for low frequencies is not affected by the 5n6 and 560p capacitors.
3) "Gain for rising" might be talking about Easter. The mid audio frequencies in this filter are shown to rise only 2dB in my simulation:

#### Attachments

• inverting active lowpass filter.png
25.1 KB · Views: 113

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,102
Ignore the 10μF 100 kΩ RC combination at the input. Its purpose is blocking of DC components only. Then read on e.g. here.

#### Ratch

Mar 10, 2013
1,098
hello trying to analyze a circuit and couldn't understand this part of the circuit, in the description of the circuit it says its an active low pass filter. can anyone help me with analyzing this circuit? THANKS

OK, first I will do an analysis of the circuit using LaPlace transforms, and then I will check the results with a CAD program.

The four simultaneous equations for the frequency analysis are as follows:

The Bode plot of vout/vin is:

And, the Bode plot of the CAD program is:

Notice that the x-axis of the first Bode plot in radians and the x-axis of the CAD program is in hertz. Ask if you have any questions of how the solution was derived.

Ratch

#### Audioguru

Sep 24, 2016
3,654
The Bode plot of Vout/Vin has a peak at 4.3kHz and a slope of 12dB/octave. Why radians?
The CAD plot has a peak at 7kHz and a slope of 12dB/octave.
LTspice has a peak at 1.3kHz and a slope at 6dB/octave.
Which one is correct??

#### Ratch

Mar 10, 2013
1,098
The Bode plot of Vout/Vin has a peak at 4.3kHz and a slope of 12dB/octave. Why radians?

The peak of Vout/Vin has a peak of 44000 radians = 7 kHz. The slope drops 30 dB from 10 kHz to 50 kHz. So, 10 2^x = 50 . x = 2.32 . The slope is 30/2.32 = 12.92 dB/octave. Radians is what my plotting routine does. I have not figured out to make it do Hz yet.

The CAD plot has a peak at 7kHz and a slope of 12dB/octave.

The peak of the CAD is 7 kHz. The slope drops 30 dB from 10 kHz to 50 kHz. So the figures are the same as above for 12,92 dB/octive

LTspice has a peak at 1.3kHz and a slope at 6dB/octave.

The peak of Ltspice is 1.3 kHz. I figure the slope to be 5.45 dB/octave.

Which one is correct??

I think all presentations are correct. I suspect LTspice did not use an ideal op-amp like I did. I will leave it up to you to check and see. I have great confidence in my calculations for an ideal op-amp.

Ratch

#### Audioguru

Sep 24, 2016
3,654
Almost any opamp works fine at 7kHz except a lousy LM324 and LM358. I doubt if LTspice would consider a 50 years old 741 opamp that has trouble above 9kHz.

#### Ratch

Mar 10, 2013
1,098
Almost any opamp works fine at 7kHz except a lousy LM324 and LM358. I doubt if LTspice would consider a 50 years old 741 opamp that has trouble above 9kHz.

I would check LTspice and see what parameters they are using. My manual derivation of the Bode plot and the CAD program seem to agree with each other.

Ratch

#### WHONOES

May 20, 2017
1,215
I have tried it out on my trusty simulator using a TL071 and an Ideal opamp getting very similar results. They vary a little from what others have done.
See attached plot.

#### Attachments

• Filter Bode Plot.pdf
40.4 KB · Views: 42

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