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I try to test this circuit and I get the following behaviour:

a. with MOSFET gate floating the voltage at the input propagates to the output with some delay.

b. with 12V on MOSFET gate the voltage at the input propagates to the output fast like a follower.

c. With -12V on MOSFET gate I get the Hold function for voltages below the "hold" voltage but when I raise the voltage to a higher level it still propoagates (no hold). It works like a "max" value circuit. What I am doing wrong?

I have ommited the 10k to the output and the 10k pot between 741 offset pins. for N-channel enhancement i use a 2n7000 and a BS170 (this works in reverse propagates with negative voltages)

enter image description here

edit: after following suggestions in comments i have used a tl082 instead of 741 and 2 mosfets back to back with their base shorted and now the circuit holds ok in both directions.

John Am
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    Look in the datasheet of the 2N7000: https://www.onsemi.com/pdf/datasheet/nds7002a-d.pdf Pay special attention to the **diode** that is inside the MOSFET between Drain and Source. Personally I would replace the MOSFET with an "analog switch IC" and that 741 also needs to go the way of the Dinosaurs, please use a more modern opamp. – Bimpelrekkie Nov 08 '21 at 18:43
  • Thanks, I guess I the mosfet switch does not cloase right. Ok I will read the datasheet. I have some 741 among many others. I use them for testing and they work ok. – John Am Nov 08 '21 at 18:45
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    @JohnAm if you wonder why basically everyone will tell you not to use the 741: https://electronics.stackexchange.com/questions/304521/reasons-not-to-use-a-741-op-amp we've seen this a thousand times, people using the 741 and then spending about as much time trying to figure out why under some conditions it won't work as on their actual problem. Personally, no stash of 741s would be worth my time, especially if the electronics are for hobby, not job usage. – Marcus Müller Nov 08 '21 at 18:51
  • No need pricey analog switch. Add one more, the same, FET in series, but reverse direction, D&S -> S&D, then tie the gate together. BTW, use a better OPA. One more.. that is not right way to construct a good S&H anyways. – jay Nov 08 '21 at 19:04
  • This came up recently: https://electronics.stackexchange.com/questions/592494/why-is-voltage-across-sampling-capacitor-going-below-0v/592505#592505 The short take is, mind the FET body diode, consider a JFET instead, use a better op-amp like a TL074. – hacktastical Nov 08 '21 at 19:05
  • thanks people i will follow your advices. @jay thanks i will try this. I found the circuit in a tutorial and wondered why it didn't work right away. If you can recommend a better tutorial please do it. – John Am Nov 08 '21 at 19:35
  • @hacktastical thanks. it seems the 741 is the culprit here. i will use tl084 or tl082 – John Am Nov 08 '21 at 19:40
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    John, I just checked @hacktastical 's link. That is good to start, though not yet enough. – jay Nov 08 '21 at 19:51
  • A LF398 don't do the job ? – Antonio51 Nov 08 '21 at 20:14
  • @Antonio51 I did not mean about the LF398. It is a good proven device. What I said was the circuitry construction, not the best. – jay Nov 09 '21 at 15:52
  • @jay Sorry, it was just a question to the OP ... – Antonio51 Nov 10 '21 at 10:31
  • @Antonio51 Hi, I don't have this ic. I will check this out. – John Am Nov 10 '21 at 10:34
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    Very good chip. Used it some 30 years ago. also low cost (about 10 for 3$ ?) – Antonio51 Nov 10 '21 at 11:26

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