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Opamp follower and Sallen-Key filter behaving strange

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You should consider a better OpAmp anyway. Asking 324 to work at 20kHz is way too much for it. Get at least 3MHz GBW to be safe


--- Quote from: Benta on June 13, 2023, 11:26:10 pm ---Soo... you feed around 2.5 VPP into an LM324 operating from a single 5 V supply biased at 1.67 V. This is already at (or over) the common-mode limit.
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Only the first amplifier (HPF). The signal will be very small after the HPF.

--- Quote from: Benta on June 13, 2023, 11:37:55 pm ---Sallen-Key is a filter topology, Butterworth is a filter transfer function. Sure, you can have an equal value Sallen-Key filter.

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OK, now clear :) Thanks.

--- Quote from: Circlotron on June 14, 2023, 10:35:09 am ---Well anyway, this is the response of the final filter in the OP's schematic (using a TL072) and with the feedback capacitor the both same value (dotted line) and double the value (solid line) of the cap that goes to ground. With double value the response is possibly more useful.

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Worrying, because I want the signal to drop only after around 18 kHz… I'll increase the cutoff frequency then. Thanks for that. BTW, in which software did you do this simulation?

--- Quote from: ArdWar on June 15, 2023, 04:39:35 am ---You should consider a better OpAmp anyway. Asking 324 to work at 20kHz is way too much for it. Get at least 3MHz GBW to be safe

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Yes, I already redesigned the circuit to use TL074.


--- Quote from: CircuitChaos on June 15, 2023, 10:43:17 pm ---. BTW, in which software did you do this simulation?

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Free version.


--- Quote from: mawyatt on June 14, 2023, 12:38:30 am ---There's a 3rd Order Sallen Key Type Butterworth filter with a gain of 1 that uses equal valued components. Here's a thread that discusses such, we published this in EDN long ago!!


Found this in our notebook, you can swap the R and Cs for a LP to a HP.

Edit: You can have just about any practical filter gain by scaling the first unity gain buffer to have the overall desired filter gain. We've utilized this unique filter structure many times over the years, and it's so easy with just a quad op amp and a few equal valued components.


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Here's an implementation of a 3rd Order High Pass and 3rd Order Low Pass intermingled to optimize the output offset voltage and input DC coupling. This uses three 1nF, 100nF, 8K and 80K components, is highly insensitive to component values and Op Amp effects (shown as Unity Gain VCVS for simplicity). The corners are simply for High Pass 1/(2*pi*Rl*Cl) and Low Pass 1/(2*pi*R*C). The output offset voltage is only the offset voltage of the last Unity Gain Op Amp, and the Gain can be easily configured with either the first or second Op Amp, second is better as the Op Amp input "sees" the low pass effects of R1 and C1 before amplification. You can interchange the filter "Sections" without affecting the response, and as shown they have been positioned to remove the DC Input by having the passive high pass first (C4 R4), followed by the passive low pass (C1 R1) which helps remove out of band signals, and last stage high pass (C5 C6 R5 R6) which removes previous Op Amp offsets, with only the last Op Amp offset as the output.

A truncated version is also shown below, this merges the individual high pass and low pass sections to eliminate 3 of the Op-Amps, however the response is not exactly classic Butterworth in the High and Low Pass frequency areas as can be seen in the slight deviation between the amplitude and phase responses, Green is the top schematic version, Blue the lower truncated version schematic.

Basically this filter concept trades off simplicity of passive component selection with equal values, with the benefit of reduced passive component sensitivity for additional Op Amps, which for our work over the many decades we've utilized this topology has been a good trade since good performance dual and quad Op Amps are readily available and cheap, actually cheaper than a single 1% capacitor :-+



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