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I am working on AM modulator. I am using NMOS as a voltage controlled resistance, and opamp in inverting aplifier configuration to get carrier signal multiplied by input signal.

I created a simulation here and built it on a test breadboard. The problem with real circuit is that although positive peak amplitude is properly modulated by input signal, the negative peak is saturated (always stays at same very negative level).

I can replicate this in simulation circuit although I do not understand how to apply fix that solves problem with simulation circuit to my real circuit.

Case 1 Working circuit

If NMOS in simulation circuit is configured with:

  • Simulate Body Diode : YES
  • Body terminal : YES

then all is good.

enter image description here

Case 2 Negative peak saturation issue

If NMOS in simulation circuit is configured with:

  • Simulate Body Diode : YES
  • Body terminal : NO

then negative peak amplitude is not modulated, it saturates to -15 V (opamp's VCC-).

enter image description here

I know that by changing "Body terminal" I am changing body bias, but I am not sure what to do with my NMOS to mirror on my breadboard ticking of that setting in simulation circuit. I am using 5LN01SP NMOS and UA741CN opamp. This is how it manifests with my circuit:

enter image description here

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  • \$\begingroup\$ Think about asking a really clear question. At the moment you post seems to be a series of observations and grumbles. \$\endgroup\$
    – Andy aka
    Commented Jan 17, 2021 at 11:38

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Saturation to negative means very low resistance of the MOSFET at positive input, obviously short caused by body diode. You can see current of the feedback resistor is not equal to drain-source current in your simulation. The same thing may be happening in your board.

I'm not sure of how to make bidirecrional variable resistor with MOSFET, but hope this helpful to you; https://www.edn.com/a-guide-to-using-fets-for-voltage-controlled-circuits-part-1/

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