Troubleshooting Amplitude Discrepancies in Active LPF: Sallen Key Design

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In summary, the conversation discussed a 4 pole active LPF and the discrepancy between the calculated and simulated amplitude of Vs1 and Vs2. It was noted that the output of the 1st stage is a triangular wave and the gain is affected by R5, R6, R7, and R8. The sensitivity of these resistors was also mentioned, as well as the positive feedback and potential oscillation of the op-amp. The conversation also touched on the impact of DC coupling in the circuit and the potential for offset due to imperfect duty cycle.
  • #1
hemant03
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Hi,
I have a 4 pole active LPF.
I am seeing that the amplitude of Vs1 & Vs2 is higher than the amplitude of input signal.
I am not sure how.
In my calculation, I have taken the amplitude of the fundamental harmonic of the square wave (input) into consideration, reduce it by 6db and multiply by the gain. The amplitude results don't quite match up with what I see in simulation (JPEG of the circuit and waveform attached).
Also, I am assuming that the output of the 1st stage (Vs1) is a triangular wave, so that I am considering the fundamental harmonic of a triangular for the calculation of amplitude of the 2nd stage (Vs2).
The other thing that I don't understand is how come Vs1 and Vs2 are shifted up by some DC value. Does capacitor charging/discharging has got something to do with it?

Thanks.
 

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  • #2
You do have some gain because of R5, R6, R7 and R8. Those are for adjusting the "d" of the filter, but it also have a little bit of gain. Also, there is some sort of positive feedback from the output of the opamp back to the input area. You expect some sort of peak in the pass band and the amplitude goes up some also.

The of R5 and R6, R7 and R8 are very sensitive. You change the kind of filter just by changing the ratio. You have to be particular careful when R5 and R6 come close to equal ( the op-amp has gain approach 2) it gets very touchy. Anything over that, it will burst into oscillation. Even when it get close to 2, it is very touchy. I just use this filter in my guitar electronics design two weeks ago, I got gain close to 2, every time I power up or power down, it gave me a "beep" sound. That is conditionally stable!

regarding DC, it is a DC coupled circuit. If your square wave is not perfect 50% duty cycle, you are going to have an offset( DC shift).

there is another thread on this that goes into detail analysis of this kind of filter, only difference is they concentrated on the op-amp being only a voltage follower. This is one of the very few circuit I am not interested in going into detail as the application is very straight forward but the analysis is quite long.
 
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Related to Troubleshooting Amplitude Discrepancies in Active LPF: Sallen Key Design

1. How does a Sallen Key LPF work?

A Sallen Key LPF (low-pass filter) is a type of electronic circuit that is designed to allow low frequency signals to pass through while blocking high frequency signals. It is composed of two stages - an amplifier stage and a passive RC filter stage - which work together to attenuate high frequency signals and pass low frequency signals.

2. What are the advantages of using a Sallen Key LPF?

One advantage of using a Sallen Key LPF is that it has a relatively simple design, requiring only a few components. It also has a low output impedance, making it easy to connect to other circuits. Additionally, it has a flat response in the passband and a sharp roll-off after the cutoff frequency, making it suitable for many applications.

3. What is the significance of the cutoff frequency in a Sallen Key LPF?

The cutoff frequency in a Sallen Key LPF is the frequency at which the signal is attenuated by 3dB (half its original value). This frequency is determined by the values of the resistors and capacitors in the circuit and can be adjusted to suit the desired cutoff frequency. The cutoff frequency is important because it determines the range of frequencies that will be allowed to pass through the filter.

4. What is the difference between a Sallen Key LPF and other types of low-pass filters?

One key difference between a Sallen Key LPF and other types of low-pass filters is the use of active components (such as an operational amplifier) in addition to passive components (resistors and capacitors). This allows for better control and flexibility in designing the filter's characteristics. Additionally, the Sallen Key LPF has a relatively flat response in the passband, compared to other filters which may have ripple or resonant peaks.

5. How can I calculate the values of components needed for a Sallen Key LPF?

The values of the resistors and capacitors needed for a Sallen Key LPF can be calculated using standard equations based on the desired cutoff frequency and other parameters of the filter. Online calculators and software tools are also available to assist in designing a Sallen Key LPF. It is important to choose components with appropriate tolerance and to consider the frequency range and power requirements of the circuit when selecting components.

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