Talk:Sallen–Key topology

message: This reads more like someone's university EE textbook than an encyclopaedia entry.  It would only explain Sallen-Key filters to someone who already knows what they are and how they work.  — Preceding unsigned comment added by Gordonjcp (talk • contribs) 06:47, 11 August 2017 (UTC)
replies: text: I agree, they dive right into formulas without even defining terms! A bunch of gobbleygook, even to many EE students. The formulas are minimally useful if the terms are defined. MacroMyco (talk) 04:54, 10 November 2017 (UTC)

text: Not very encyclopaedic is inappropriate and should be struck.  This article is very well written and succinctly covers the topic.  The term Advanced topic might be more appropriate, with a link at the beginning of the article to an introductory wiki on active filters, of which there are several.  — Preceding unsigned comment added by 64.201.178.210 (talk) 14:07, 13 November 2018 (UTC)
thread_title: Not very encyclopaedic

message: The figure shows the non-inverting input of the amp as a node labeled V-. I would think that this would be labeled V+? Busfault (talk) 16:32, 15 October 2010 (UTC)
replies: text: The image is trying to show that the operational amplifier in this negative-feedback configuration enforces that the non-inverting input and inverting input match. So the image's author used a v_- at the non-inverting input to make the point that the amplifier has negative feedback, which simplifies the analysis. I agree that the image is a little confusing at first (especially if you don't know what to expect). Labeling it with the output voltage may have made more sense. Hopefully the accompanying analysis helps. —TedPavlic (talk/contrib/@) 17:47, 17 May 2011 (UTC)
thread_title: Figure 1 Node naming

message: Any particular reason the transfer function is given in terms of r1, r2, c1, c2 for the lowpass filter and in terms of the q factor, cutoff frequency and stuff for the highpass filter? If no one objects before monday morning utc I'll add the transfer function in terms of the component values.
The current transfer function:
 G = \frac{s^2}{s^2+2\pi(\frac{F_c}{Q})s+4\pi^2(F_c^2)} 
Here's what I get for the component value transfer function:
 G = \frac{s^2}{s^2+\frac{s}{\sqrt{C_1C_2R_1R_2}}+\frac{1}{C_1C_2R_1R_2}} 
131.252.212.137 21:34, 24 May 2007 (UTC)
replies: text: The choice of Q is different for the different filter types. For example, a Butterworth has a Q of 1/\sqrt{2}. The form of the filter is always the same -- it's a generic second-order topology. --TedPavlic | talk 17:31, 19 November 2007 (UTC)
thread_title:  High-pass configuration transfer function 

message: The expressions for Q seem wrong for the filters here. In particular, it looks like "1/Q" is what's meant here, and I think that (at least for the low-pass case) the C2 needs to be swapped for C1. Thoughts? --TedPavlic | talk 19:43, 16 November 2007 (UTC)
replies: text: Everything is fixed now (and improved?). --TedPavlic | talk 17:24, 19 November 2007 (UTC)
thread_title:  Q Expressions Wrong 

message: Figures 2,3,4 renumber the Z3/Z4 components from Figure 1 in reversed order. For the low-pass calculations there is a compensation Z3=1/sC2, Z4=1/sC1 (swaps C1 and C2). This leads to compensation in the calculation for 'n' which makes up for it. For the high-pass filter it is however not trivial that 'm' needs to be calculated differently from what is described in the low-pass section. For high-pass the following holds: R1 = R, R2 = mR, C1 = C, C2 = nC. As I'm not all that familiar with filters, I haven't applied this on the page yet and want your opinion on it first.
Would it maybe be easier to modify the images, so both high/low pass can use the same 'm' and 'n' formulas?  —Preceding unsigned comment added by 78.27.11.131 (talk) 16:32, 7 March 2009 (UTC)
replies: text: I also find the index nr swapping irritating. Neels (talk) 01:40, 7 April 2011 (UTC)
replies: text:  The indexes on the first image can be changed easily. It's an SVG file, and so they can be changed with a text editor. The changes would then just have to be pushed down through everything else in the page. —TedPavlic (talk/contrib/@) 16:43, 17 May 2011 (UTC)
replies: text:  Changes have been made. Ordering should be consistent throughout. Hopefully I caught all of the text fixes. —TedPavlic (talk/contrib/@) 17:02, 17 May 2011 (UTC)
thread_title:  High pass example possibly misleading 

message: This article would be a bit more complete were it to point out that passband gain can be altered by simply overlaying the gain resistors from a Non-Inverting Amplifier [].  — Preceding unsigned comment added by 69.140.49.200 (talk) 13:19, 3 June 2011 (UTC)
replies: text: Please read the lead (i.e., the first paragraph) of the article. Then read the section with the title "VCVS Example: Bandpass configuration". Did you miss these sections? Perhaps your point is that these things could be highlighted more to make it more obvious? —TedPavlic (talk/contrib/@) 16:51, 3 June 2011 (UTC)
thread_title: Passband Non-Unity Gain

message: Good work, Ted.  I just worked on a Sallen–Key (SK) filter wizard at work and had to go from knowing zero to being able to automate SK design.  This is the first site I used, and it got me started.  Anyway, here's three places that this article could have been more helpful to me.  First, the low pass and high pass examples do not discuss the gain resistors, which of course I had to support.  It is in the band-pass discussion, so I don't understand why it's not in low-pass and high-pass.  Second, the benefits of SK filters are somewhat buried.  Another site says SK filters are relatively insensitive to component values and that component values are fairly close to each other.  Pole control is low, making high Q difficult to control.  Third, and this is a complaint I have with all SK sites I found, the simplification described by letting R_1 = m R_2 and C_1 = n C_2 is not nearly as good as letting R_1 = m R, R_2 = R/m, C_1 = n C and C_2 = C/n.  The formulas for Q and f_c become:
:f_c = \frac{1}{2 \pi R C}
:Q = \frac{mn}{m^2 + 1 + (mn)^2 (1 - K)}
where K is the gain when we add the negative feedback resistors.  This eliminates the dependence between f_c and Q calculations.  In this case R = \sqrt{R_1 R_2} and C = \sqrt{ C_1 C_2 }.  Further simplification comes when I assume R_1 = R_2 (i.e., m = 1), although in my case I care mostly about the area on an IC occupied by the passives. R_1 = R_2 comes very close to the best possible minimum for gains up to 20 \times.  Board-level users may care more about minimizing the cost of the capacitors, and might prefer to use C_1 = C_2. WaywardGeek (talk) 11:09, 11 October 2011 (UTC)
replies: text: Thanks for your comments, WaywardGeek. If I recall correctly, much of the basic features of the web page (e.g., the examples picked) were present before I touched the page. I just cleaned up some of the math and (hopefully) some of the presentation. The gain is non-unity only for the band-pass case because a Sallen–Key (SK) filter is unity gain by definition. Once you add non-unity gain, the filter becomes a more general VCVS filter. This is explained briefly in the lede of the article. Certainly the introductory information on SK filters could (and probably should) be expanded to make their relevance more clear; however, this encyclopedia article should not turn itself into a guide on SK design. Finally, regarding the design choice of letting R_1 = m R_2 (etc) instead of setting R_1 = m^2 R_2 and introducing a fictitious R (and C), I think many references avoid this approach so that the effect of the actual components are clear. Again, because the encyclopedia article is not meant to be a design guide, different choices are made. The clarity of the content might be compromised by introducing symbols for components that only exist in the abstract. —TedPavlic (talk/contrib/@) 16:59, 11 October 2011 (UTC)
thread_title: Some feedback on article

message: This is the most useless set of links.  Does this phrase just mean 'a gain greater than 1'?  If you search for "super unity gain" in google you just get lots of identical sentences about sallen-key topology filters!  Can someone who understands this article adjust the phrasing :) thank you!  — Preceding unsigned comment added by 94.116.245.123 (talk) 09:29, 12 December 2012 (UTC)
thread_title: Super-Unity-Gain

message: Although it is questionable whether this section really belongs to an encyclopedic article, if it is present, even when just intended as an example, it would be wise to introduce a procedure that is feasible in practice. I experienced that undergraduate students, who tried to follow the previous version as a design guideline, got stuck with a few equations from which they were unable to obtain the component values. By the relative simple modification that I made to this section, the dimensioning of f_0 becomes independent from Q.WimdeValk (talk) 20:55, 12 October 2017 (UTC)
thread_title: Design Choices