Which amplifier to be selected?

Hi,
I'm going to design an active low pass filter for the audio before feeding the signal into the PIC's analog pin. Sallen and key circuit is used.
I'm not sure what type of amplifier should I use. In the simulation, I just simply take one op amp in LTSpice to see the frequency response of the circuit.
Audio source: MP3 player, <3 Vpp

Is LMV716 suitable?
Or any amp is recommended for this application?

Thanks

Hi Banana,
Your lowpass filter is a gradual rolloff Bessel type instead of a sharp cutoff Butterworth.

Both low voltage opamps are in tiny surface-mount packages only.
Not many ordinary opamps work with a supply voltage as low as 3V. An old LM358 dual opamp works with a supply voltage of 3V but then its wax output voltage is only about +1.5V to +1.8V.

This is not Butterworth filter? But I learn designing this with equal R and equal C.
I've just read a reference book, with different method. The mathematical calculation shows that, R to be equal, so I used 2.2 khm:. For the cut off frequency of ~22 kHz, C is 3.3 nF. If equal C is used, the cut off frequency is not at 22 kHz but much lower.

If C2 is 0.707C and C3 is 1.414C, the cut off frequency is at 22 kHz.

Is that the correct way?

I've seen another book using equal R and equal C, but the simulation doesn't meet the desired cut off frequency.

Thanks

If the opamp has a gain of 1.0 then for a Butterworth 2nd order lowpass filter one capacitor should be double the value of the other. Use three capacitors with the same value and connect two capacitors in parallel.

Or use the same value for both capacitors and add two resistors to make the gain of the opamp 1.6 times.

Or fiddle with the resistor values.

Yes, same as 1.414C/0.707C = 2
Must it be exactly 2? I took the nearest value of capacitors from the calculation by 3.3 nF cap.

I just read some application note about active filter from TI, they don't use equal R or equal C, I think that will be fine as long as the Q is remained 0.707 for the 2nd order Butterworth LPF.

My previous circuit shows the Q of 0.5 which is Bessel filter. Am I right?

Yes, one capacitor is calculated to be 0.707 and the other is calculated to be 1.414 for a butterworth 2nd order filter when the opamp has a gain of 1.
Yes, the Q of a 2nd order Butterworth filter is 0.707 and the Q of a Bessel filter is 0.5.

If the Q is higher than 0.707 then the frequency response will have a boosted peak near the cutoff frequency.

I made a bass-boost circuit for my stereo. Then my speakers with 8" woofers sound like big sub-woofers.
It uses Sallen and Key 2nd order highpass filters with equal values for its resistors and capacitors like a Bessel (that has a gain of 1) but its gain is about 2.5 which causes the bass to be boosted just before the filter drops off the very low frequencies. When the gain is 3.0 or higher then it oscillates.

I made similar circuits with lowpass filters with extra gain to boost 3.5kHz on telephone conferencing systems. They sound very crisp and clear.

Thanks audioguru
College syllabus is really insufficient, student needs to do more investigation.

Hi,
I've just read through microchip notes.
**broken link removed**
https://ww1.microchip.com/downloads/en/DeviceDoc/adn003.pdf
It says that the Gain BandWidth Product must be greater than 100*G(closed loop)*fc. Why 100? Two decade?
Is that because of the gain is not as high as the low frequency gain at the GBWP frequency?

Opamps have frequency compensation so that they don't oscillate when negative feedback reduces their gain to 1.
The Gain Bandwidth Product is the frequency that the compensation has reduced the open-loop gain to 1. The compensation rolls off the open-loop gain at 20dB (10 times) per decade of frequency.

Then if your opamp has a GBP that is 100 times higher than the cutoff frequency of your lowpass filter, the opamp has an open-loop gain of 100 at the cutoff frequency which results in fairly low distortion and a closed-loop gain error of only 1%.

audioguru said:

Not many ordinary opamps work with a supply voltage as low as 3V. An old LM358 dual opamp works with a supply voltage of 3V but then its wax output voltage is only about +1.5V to +1.8V.

Click to expand...

A 324 op-amp can work with 3V but that is the lowest voltage it can work with so watch out!

An LM324 has 4 lousy old opamps.
An LM358 has 2 of the same opamps.

Their bandwidth is way too low for this project and nearly any other audio project. They also have a high amount of crossover distortion and are noisy (hissss).