MOSFET switch problem
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Posted by Saran on July 16, 2005, 11:14 am
Please log in for more thread options Hey all, I feel dumb to ask this but I am having a very very (what I think is) basic MOSFET switch question. Can anyone tell me what the output Vout of the following schematic is going to be.. Vdc = -3V | | | | ---+--- +----+ VIN | | +-------+ +------+- VOUT | | | | | | | -+-- c = 10pF -+--- vdc = 3V | | | l | | Gnd GND I would expect the gate to be turned off and the Vout should be ideally zero + some leakage...I am trying to design switches for my DRAM which pass the voltage input to the output of the pass transistor when the control signal to the gate is turned on. I am trying to store that voltage on the capactitor. However, the problem I am getting is that the MOSFET is always on irrespective of the control gate voltage and I am always getting my Vout to follow Vin (almost) whatever I do... I went back to my basics checked everything but I am really stumped. Can anyone change my line of thinking ? What am I doing wrong here??? Thanks a lot, Saran | ||||||||||||||||
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Posted by Bob Monsen on July 16, 2005, 2:44 pm
Please log in for more thread options Saran wrote: You are designing an analog switch. Take a look at "The Art of Electronics", chapter 3, for some useful tips. Alternately, you can buy analog switches in DIP packages. 4066 comes to mind. http://www3.sympatico.ca/lloyd.gordon3/images/4066.pdf Many mosfets have built-in diodes, which will cause problems in this application if you aren't careful. In your drawing (assuming it's an N-channel MOSFET) if VOUT is supposed to stay high, and VIN is low, the cap will drain through the diode no matter what the gate voltage is. You can defeat this by using back-to-back MOSFETs, or use JFETs, which don't have the diode. -- Regards, Bob Monsen If a little knowledge is dangerous, where is the man who has so much as to be out of danger? Thomas Henry Huxley, 1877 | ||||||||||||||||
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Posted by Saran on July 16, 2005, 4:21 pm
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Thanks a lot Bob, for your suggestion. I understand I am trying to design an analog switch. But, the fundamental concepts I know clashes with the simulation results I am observing when I simulated the _schematic_in_question_. If I apply a gate voltage to turn the device off....wouldn't the voltage on the capacitor be zero ideally? How will Vin be reflected at Vout? I know I must be missing something here...but I am not able to see what I am missing. I will go thru AOE one more time in regards to ur suggestion, thanks. Regards, Saran | ||||||||||||||||
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Posted by Jamie on July 16, 2005, 7:27 pm
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Saran wrote: > Hey all,
trying to do a Sample and hold circuit?
> > I feel dumb to ask this but I am having a very very (what I think is) > basic MOSFET switch question. Can anyone tell me what the output Vout > of the following schematic is going to be.. > > > > Vdc = -3V > > | > | > | > | > ---+--- > +----+ > VIN | | > +-------+ +------+- VOUT > | | > | | > | | > | -+-- c = 10pF > -+--- > vdc = 3V | > | | l > | | > Gnd > GND > > I would expect the gate to be turned off and the Vout should be ideally > zero + some leakage...I am trying to design switches for my DRAM which > pass the voltage input to the output of the pass transistor when the > control signal to the gate is turned on. I am trying to store that > voltage on the capactitor. However, the problem I am getting is that > the MOSFET is always on irrespective of the control gate voltage and I > am always getting my Vout to follow Vin (almost) whatever I do... > > I went back to my basics checked everything but I am really stumped. > Can anyone change my line of thinking ? What am I doing wrong here??? > > Thanks a lot, > Saran > have a look at the old standby 4066 chip it has 4 bilateral mos switches,. i used one of those a few years ago to convert a tone from a radio through a Voltage comparator to generate square waves then using a low valued capacitor from the square wave to generate a short pulse signal, i was able to control a charging cap state that would then set a another stage as the holding circuit.. with this, i created a Tone to Analog Voltage that would represent the freq in a scale. this then drove a already supplied Analog input that was there on the computer. the end results was a Weather Fax picture receiver... any ways, the use of the 4066 chip gives a lot of idea's it can operate from a single supply and does not require - input to turn it off. -- Real Programmers Do things like this. http://webpages.charter.net/jamie_5 | ||||||||||||||||
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Posted by Saran on July 16, 2005, 8:25 pm
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Thanks a lot Bob and Jamie for your helpful suggestions. I figured out the problem. Basically, as Bob said, the problem was due to the internal diode in MOSFET. And, the 4066 data sheet was very helpful too. Thanks. Regards, Saran | ||||||||||||||||
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>
> I feel dumb to ask this but I am having a very very (what I think is)
> basic MOSFET switch question. Can anyone tell me what the output Vout
> of the following schematic is going to be..
>
>
>
> Vdc = -3V
>
> |
> |
> |
> |
> ---+---
> +----+
> VIN | |
> +-------+ +------+- VOUT
> | |
> | |
> | |
> | -+-- c = 10pF
> -+---
> vdc = 3V |
> | | l
> | |
> Gnd
> GND
>
> I would expect the gate to be turned off and the Vout should be ideally
> zero + some leakage...I am trying to design switches for my DRAM which
> pass the voltage input to the output of the pass transistor when the
> control signal to the gate is turned on. I am trying to store that
> voltage on the capactitor. However, the problem I am getting is that
> the MOSFET is always on irrespective of the control gate voltage and I
> am always getting my Vout to follow Vin (almost) whatever I do...
>
> I went back to my basics checked everything but I am really stumped.
> Can anyone change my line of thinking ? What am I doing wrong here???
>
> Thanks a lot,
> Saran
>