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Time for another spaced query


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4/23/2000 8:28 PM
Don Symes
Time for another spaced query
I'm spicing some double-MOSFET component switches, and I think the capacitance of the FET in the ON condition is messing with me. Maybe in the OFF position too.  
 
Would paralleling a, say, 20M resistor across both FETs reduce the capacitance?
 
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4/29/2000 2:42 PM
Ray Ivers

Don,  
 
Yes, I would think so. Do you need that high an off resistance? - if not, you could go to an even lower shunt resistance (duh - I'm as sharp as a tack this AM). JFET's would have much lower junction capacitance, no? - but maybe not as high an off-resistance... I'm not really much of a design-level SS guy, Don, but I'm workin' on it; I think SS does a number of things extremely well.  
 
Ray
 
4/30/2000 2:30 AM
Don Symes

The deal is, I've got (according to Spice) soe 300V signal swings to deal with. The HV JFETS are too high an RdsON, but their Cds is much better.  
 
I'm back to LDRs for those particular switches - unless that photofet relay looks better.  
 
Thanks.
 
5/3/2000 7:48 PM
moocow
Don't believe everything Spice tells you. It's good for some things, but it will get confused by switching transients. Be careful of the switch settings you are using in your simulation (on time, on/off resistance). If your switch is switching too fast, you might be getting some unrealistic spikes.  
 
You are better off building it and then see what happens. As for photofets, I've used them before, and they are also very good for switching signals on and off. LDRs tend to have a switching delay that bothers some people.
 
5/3/2000 8:24 PM
Don Symes

It ain't transients, 'cause I've got the switches in a given mode for a run. The OFF capacitance, though, in these hi-Z environments, contributes to some significant leakage (and tone suckage). I was after a way to overcome that --> IR's PVT412 is one good way.
 

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