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MOSFET switch problem

Discussion in 'Electronic Basics' started by Saran, Jul 16, 2005.

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  1. Saran

    Saran Guest

    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
    | |

    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,
  2. Bob Monsen

    Bob Monsen Guest

    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.

    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.

    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
  3. Saran

    Saran Guest

    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

    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.

  4. Jamie

    Jamie Guest

    trying to do a Sample and hold circuit?
    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.
  5. Saran

    Saran Guest

    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.

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