| Electronics > Beginners |
| Help with comparator in LTspice? Ensuring 0V output? |
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| bluebreak:
Very nice Hugo :) Are those transient peaks I see from lack of a hysterisis loop in the circuit or could those just be smoothed out with a filter cap? Otherwise it looks excellent for my usage :-+ |
| Zero999:
--- Quote from: bluebreak on October 10, 2018, 06:45:40 pm --- --- Quote from: Hero999 on October 10, 2018, 10:03:02 am --- --- Quote from: bluebreak on October 10, 2018, 02:39:34 am ---I'm trying to simulate a gate/trigger for an analog synth, providing a control voltage out if input is below a threshold. So analog switch (mosfet) driven from the comparator is a better way to go? --- End quote --- Yes that would work. Pull the output up to 15V and add a 2N7000 to the output. The functions of the + and - inputs would be reversed as the MOSFET would perform an inverting function. Another option would be a comparator with a MOSFET output, although those typically won't work with a +/-15V supply. The maximum power supply voltage of a CMOS comparator is normally 16V. --- End quote --- The reason for pulling up to 15 is because of the current required by the 2N7000 right? (the gate voltage threshold for 2N7000 is much lower) Would 12 also work in that case? --- End quote --- The 2N7000 only draws a current to charge up its gate capacitance. Once it's on, the gate takes no current at all. I only suggested 15V, because it's the power supply voltage. There's no reason why a lower voltage can't be used. Normally it's desirable to use a much higher voltage than the threshold voltage to ensure a low on resistance, but in this case the drain current is so tiny, it won't matter. --- Quote ---BTW, if I wanted to build a few of these on one board, what would be cheaper/better? A bunch of 2N7000s or a quad analog switch like a CD4066? Are there newer alternatives? --- End quote --- The 2N7000 will be better than the CD4066, because it has a lower on resistance, therefore will be able to pull the voltage down more. I don't know about cost. Look at the prices wherever you buy components from. Another possibility is a CMOS gate with an open drain output, such as the 74HC07 or 74LVC07, but they have a maximum supply voltage of around 5V. https://biakom.com/pdf/TC74HC07AP_Toshiba.pdf https://assets.nexperia.com/documents/data-sheet/74LVC07A.pdf I was going to suggest the 74C604, which will work up to 18V but it seems to be obsolete. http://www.mouser.com/ds/2/149/MM74C906-356201.pdf |
| Ian.M:
Get rid of the pullup to the control voltage (CV) level and use a SPDT analog switch to select between the desired CV level and Gnd. e.g. CD4053 triple SPDT analog switch. It will work with your existing +15V supply, if powered between +15V and Gnd. You will still need the pullup to +15V on the comparator output. |
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