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simulating CD4007 (CD4009 ??) for linear application
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Hi!
I am a LTSpice novice who is looking to simulate a CD4009 chip for a linear application. Specifically, in the electronotes newsletter #111, it is hinted that these can be used to make a simple VCA. My very naive approach has been to just copy down the arrangement of NMOS and PMOS shown on the CD4009 datasheet as shown, without tweaking any parameters or selecting "pick new transistor". The circuit I am trying to simulate seems to qualitatively do what it is supposed to after some tweaking, but with the RC values and and supply voltage as shown in the original schematic, it does not achieve nearly the performance claimed. I am wondering if there is any way to select NMOS and PMOS parameters to improve my simulation's accuracy to the actual CD4009. Any help is much appreciated! see files: Eli_R_CD4009_VCA_Question.asc? and photo album Eli_R_CD4009_VCA_Question datasheet: [Moderator's note:? The original Subject said "CD4007", but the body of the message refers only to "CD4009".? I have modified the Subject line to suggest the CD4009.] |
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On Sun, May 11, 2025 at 10:36 PM, <Eli.rosenkim@...> wrote:
Electronotes?- wow, I have not heard of them in a long time.
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Somewhere, there might be a pair of .MODEL statements for the MOSFETs in those early CD4xxx parts.? The default NMOS and PMOS models are right for early 1970s, so the time period is about right for CD4xxx and maybe that works in your favor.? The other unknown is what are the W and L of each FET.? By default, you get W = L = 20 microns.? Fortunately the T.I. datasheet has lots of DC curves, which an enterprising student could use to compare with simulations and adjust W and L (and maybe the .MODEL parameters) to get a good fit.? Does that sound like a plan, and is the effort worth it?
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I notice the graph from the Electronotes #111 refers to a square wave audio input signal, which maybe suggests that the circuit might not be as clean as one would want for good audio.
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Andy
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Good catch with the square wave, I noticed that right after posting. In any case the simulation results I was getting had to do more with horrible control leakage (louder than the input audio signal!) than with distortion. Thank you for the tip with the W and L parameters, I may give that a try although it looks like eetech00 has uploaded a simulation so I might be lazy? Re: Electronotes, still the only place to learn most of this stuff in any sort of depth unfortunately, although Aaron Lanterman's courses cover a lot of the introductory material really well. A shame how strange Bernie is about distribution, I am lucky enough to attend a college that has a bound copy. |
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On Wed, May 14, 2025 at 08:28 PM, <Eli.rosenkim@...> wrote:
I don't remember if I said this or if I edited it out of my reply.? I think the circuit may have been intended as a volume control, where the control voltage changes very slowly or not at all.? In that case, the RC filter they used was probably sufficient.? Because it has considerable feed-through from the control input to the output, it would not go well if the control voltage can change in less than a second, as you noticed.? I suspect it is fundamental to the circuit, not dependent on the transistor models.? It varies the supply voltage to the output transistors and I think it can't help but go straight into the output.
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Or, if this was for an electronic music synthesizer, the leakage might be treated as part of the overall sound effect, warts and all.
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I somewhat doubt there is a good way to actually reduce it, except with a balanced topology that provides cancellation, which is probably more than you can do with a CD4009.
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A very long time ago I lived within miles of Bernie Hutchins of Electronotes, but did not know it.? Maybe our paths crossed.
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Andy
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