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Reason for using JFET transistors in sample and hold?

AFex54

Apr 10, 2015
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Apr 10, 2015
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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
 

Harald Kapp

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Nov 17, 2011
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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.
 

AFex54

Apr 10, 2015
144
Joined
Apr 10, 2015
Messages
144
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 trinsistor requires a base current to flow which will strongly influence the charge on the sampling capacitor and thus the sampled voltage.
perfect answer, thanks :)
 
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