JFET as a switch in circuit with capacitor.

Thread Starter

hulage

Joined Nov 12, 2021
6
I'm trying to design sample and hold circuit, but I can't apply transistor switch to it. Circuit works properly with analog switch, transistor switch works good in circuits without capacitor, but they don't work together. Please explain me about dealing with transistor's SG current loss.
This is a circuit with analog switch:
1638105767919.png

This is how it works:
1638105675202.png

This is circuit with transistor switch:
1638105857463.png

This is how it works
1638106250243.png

How can I get accepteble signal using transistor switch?
 

dcbingaman

Joined Jun 30, 2021
559
The problem here is the JFET must maintain a positive voltage with respect to the source to turn it off. But this does not happen here because the capacitor holds onto the charge the voltage difference between the two terminals goes to zero.
 

dcbingaman

Joined Jun 30, 2021
559
One possible solution: Use an N Channel MOSFET, for sampling set the gate at 12V to turn on the transistor and 0V to turn it off. Being that the signal to be sampled goes to 10V you need to make sure you choose a MOSFET that can be fully on with down to 2V gate to source voltage.
 

crutschow

Joined Mar 14, 2008
29,520
Below is my LTspice simulation:
I reduced the values of some of the resistors and increased the switching voltage to insure the JFET stays off..
Proper operation also depends upon the JFET model you use.
A MOSFET switch would need to be a very small device so that its gate capacitance does not significantly affect the output.

1638121670741.png
 

Thread Starter

hulage

Joined Nov 12, 2021
6
Below is my LTspice simulation:
I reduced the values of some of the resistors and increased the switching voltage to insure the JFET stays off..
Proper operation also depends upon the JFET model you use.
A MOSFET switch would need to be a very small device so that its gate capacitance does not significantly affect the output.
You was right about using MOSFET, thank you. The circuit is finally done.
Hope it will help someone building S&H sircuit in Altium.
 

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