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Reason for using JFET transistors in sample and hold?

Discussion in 'General Electronics Discussion' started by AFex54, Jul 3, 2015.

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  1. AFex54

    AFex54

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    Apr 10, 2015
    I notice even the most basic sample and hold circuits use a JFET transistor rather than simply using a bipolar npn.
    all the transistor has to do is act as an on off switch to allow a sample of the input voltage through,
    JFET are voltage controlled , I cant see the point of using one if all you need is switch
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    2,628
    Nov 17, 2011
    Here's why:
    • A FET (JFET or MOSFET) has no gate current, therefore the influence of the controlling voltage to the sampled voltage is minimal (note that is is not null because of effects like charge injection etc.).
    • A bipolar transistor requires a base current to flow which will strongly influence the charge on the sampling capacitor and thus the sampled voltage.
     
    hevans1944, Arouse1973 and AFex54 like this.
  3. AFex54

    AFex54

    144
    2
    Apr 10, 2015
    perfect answer, thanks :)
     
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