sine wave applies to transmission gate

Discussion in 'Homework Help' started by pinoismo, Apr 21, 2011.

  1. pinoismo

    Thread Starter New Member

    Apr 21, 2011
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    a sine wave was applied to a transmission gate as an input voltage when the gate was ON and when it was OFF. then the output was generated on the scope. please! how do you explain the scope output when the gate was ON IMG_0288.jpg and when it was OFF IMG_0287.jpg .
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It would help to have the test setup and know what the "transmission gate" (analog switch?) was. Perusing the data sheet for the gate might shed light.
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As beenthere told you, a schematic would help us to give a decent explanation.

    Bertus
     
  4. pinoismo

    Thread Starter New Member

    Apr 21, 2011
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  5. BillB3857

    Senior Member

    Feb 28, 2009
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    What is the signal source for A and A-Not?
     
  6. pinoismo

    Thread Starter New Member

    Apr 21, 2011
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    the signal source for is +5VDC.
     
  7. pinoismo

    Thread Starter New Member

    Apr 21, 2011
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    the signal source is +5VDC.
    when the gate is on: A = +5vdc, A*= 0
    when the gate is off: A = 0, A* = +5VDC
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Take a look at the first illo in the data sheet. Notice the body diode inherent in the FET's. The off conduction is simply half wave rectification through one of those body diodes.
     
  9. pinoismo

    Thread Starter New Member

    Apr 21, 2011
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    the problem is that when I applied +5vdc to V(in), V(out) = 0 when the gate is off, and V(out)=+5vdc when the gate was on.
    if the
     
  10. pinoismo

    Thread Starter New Member

    Apr 21, 2011
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    but when I applied a sine wave at V(in), no matter what the gate state, I still get voltage out.
     
  11. BillB3857

    Senior Member

    Feb 28, 2009
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    I'm confused, which is not too unusual. It seems that to stop any signal from going through, both A and A* should be 0 V. To get full wave throughput, both should be +5 V. What am I missing?

    OOPS: Just now saw Beenthere's response about the diodes! Totally missed that since I had only looked at the OP's original sketch instead of going to the datasheet.
     
    Last edited: Apr 21, 2011
  12. pinoismo

    Thread Starter New Member

    Apr 21, 2011
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    the gate A is for an nomos that it's channel get inverted with logic high( +5vdc in this case). and A* gate is for a pmos that get activated with 0 logic.
    the gate works as intended when we apply +5vdc as V(in), which means when the T-gate is on V(out) = V(in), and when the gate is off V(out)= 0V.
    I get that, the irony is that I cannot explain what happen when the sine wave is applied as V(in).
    please refer to previous posts to see how the gate works
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    Thanks for straightening me out. I need to look more closely at the arrows!
     
  14. Georacer

    Moderator

    Nov 25, 2009
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    This might be farfetched, but try this: Connect the substrate (middle arrow) of the PMOS to the Vcc and the substrate of the NMOS to the Ground. Tell us if you see anything different.

    I would also like to ask if this is an actual lab experiment, because the transmission gate is a digital circuit, meant to work with logic levels, not analog signals.
     
  15. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The devices used don't seem to have the substrate / body diodes accessible other than by the source terminals. If this is so, then it will not be so easy to back-bias the body diodes. Note with the complementary FETs connected in opposite senses, the body diodes are in the same direction, so as to turn on for the negative half-cycle.

    In addition, the gates are driven between 0V and +5V only, not to any negative voltage. Because of this, negative signal excursions may tend to turn the NMOS channel on.
     
  16. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Not all transmission gate applications are digital, although for analogue applications it may be preferable to refer to such devices as analogue switches. Some manufacturers offer transmission-gate based products intended for analogue switching. Here is a rather old one, analogue or digital: http://www.national.com/ds/CD/CD4066BC.pdf
     
  17. pinoismo

    Thread Starter New Member

    Apr 21, 2011
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    thank you all for your comments
     
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