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    Need to create a circuit with a linear relationship between Vin and Vout

    OK, given this circuit:



    Which has this actual relationship between the voltage at the base of the MOSFET and the voltage across the resistor at the drain:



    Notice that the saturation happens rather quickly at low gate voltages. What I'd like is to be able to vary the gate voltage from 0 to 5 volts and have the voltage drop across the resistor be more proportional across that range so that the graph would look more like this:



    How would I go about figuring out how to make a circuit like that?

    #2
    Re: Need to create a circuit with a linear relationship between Vin and Vout

    You're using a p-channel schematic drawing again for the n-channel irf520...

    Based on what you're doing here, you have absolutely no advantage to using a MOSFET. If you insist on using a MOSFET in the configuration presented, you need to make a (FAST) negative feedback loop.

    That being said, it's easier with BJTs as they have self negative feedback in emitter follower configuration. And the other hint is that we'll be describing to you something that sounds sort of similar to something else...

    hint is :
    SPEAKER

    ...
    Last edited by eccerr0r; 11-26-2020, 04:27 PM.

    Comment


      #3
      Re: Need to create a circuit with a linear relationship between Vin and Vout

      Originally posted by eccerr0r View Post
      You're using a p-channel schematic drawing again for the n-channel irf520...
      I'm not sure why then the cad program lets me chose by part number so it must have the schematic symbol wrong for that part but anyway...

      Originally posted by eccerr0r View Post
      Based on what you're doing here, you have absolutely no advantage to using a MOSFET. If you insist on using a MOSFET in the configuration presented, you need to make a (FAST) negative feedback loop.

      That being said, it's easier with BJTs as they have self negative feedback in emitter follower configuration. And the other hint is that we'll be describing to you something that sounds sort of similar to something else...

      hint is :
      SPEAKER

      ...
      I'm not married to ANY specific components ... I just need the results that I need whatever it takes to make that happen, I'm all in.

      Comment


        #4
        Re: Need to create a circuit with a linear relationship between Vin and Vout

        Well you seem to be married to a method at the moment. I can only guide you through that path now.

        The hint of what you're asking for, well, aren't audio equipment supposed to be linear to faithfully reproduce sound.

        So what you need is to build an audio amplifier - which would be linear. Bipolar transistors behavior in the forward active region is linear with respect to base current which is what you want. MOSFETs are second order so you'll need second order feedback to return to linear.

        A power audio amplifier IC or even a LM317T would easily do the job you want minus the limitations in power handling, which is another side effect to be included why this approach to solving your initial problem is nonideal, but enough said.

        Comment


          #5
          Re: Need to create a circuit with a linear relationship between Vin and Vout

          I am curious about your schematic capture and simulation program you are using, I cannot see how you can get those graphs from the circuit simulation when the wrong MOSFET is used to simulate the circuit, this is the second time your schematic shows the wrong MOSFET.
          Last edited by budm; 11-27-2020, 01:04 AM.
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

          Voltage Regulator (LDO) testing:
          http://www.badcaps.net/forum/showthr...999#post300999

          Inverter testing using old CFL:
          http://www.badcaps.net/forum/showthr...er+testing+cfl

          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
          http://s807.photobucket.com/user/budm/library/

          TV Factory reset codes listing:
          http://www.badcaps.net/forum/showthread.php?t=24809

          Comment


            #6
            Re: Need to create a circuit with a linear relationship between Vin and Vout

            Actually I think his waveform is correct ... sort of ... for how he simulated it, providing the model used in the schematic indeed is a p-channel device.

            The waveform is really bad and shows nothing like what you'd want for a real circuit indeed because the transistor being used incorrectly...

            Comment


              #7
              Re: Need to create a circuit with a linear relationship between Vin and Vout

              You mean N-CH.
              Never stop learning
              Basic LCD TV and Monitor troubleshooting guides.
              http://www.badcaps.net/forum/showthr...956#post305956

              Voltage Regulator (LDO) testing:
              http://www.badcaps.net/forum/showthr...999#post300999

              Inverter testing using old CFL:
              http://www.badcaps.net/forum/showthr...er+testing+cfl

              Tear down pictures : Hit the ">" Show Albums and stories" on the left side
              http://s807.photobucket.com/user/budm/library/

              TV Factory reset codes listing:
              http://www.badcaps.net/forum/showthread.php?t=24809

              Comment


                #8
                Re: Need to create a circuit with a linear relationship between Vin and Vout

                After looking at the scale of the initial waveform something is indeed messed up even for N-channel as the threshold voltage hasn't been reached though subthreshold leakage may be occurring.

                I'd still say throw this all away, go back to the drawing board, and forget about sinusoidal output prior the the output stage.

                Comment


                  #9
                  Re: Need to create a circuit with a linear relationship between Vin and Vout

                  The simulator is out to lunch, the 10MEG is too large and the mosfet drawn as P-ch, and the body diode would just stay on, never letting Vout go above 0.7V but the graph does not agree. It looks like a poor simulator.

                  The Falstad simulator is better for understanding circuits:
                  https://falstad.com/circuit with many examples https://falstad.com/circuit/e-index.html

                  IRF520 has leakage current of over 10uA so a 10MEG would not work.

                  For sanity, the IRF9520 is not a IRF520.
                  Attached Files

                  Comment


                    #10
                    Re: Need to create a circuit with a linear relationship between Vin and Vout

                    It looks like crappy schematic capture/simulator program with bad component libraries.
                    I mean you look at the schematic you can just see what is not right right away, I.E. the effect of body diode of that P-CH MOSFET. This where basic understanding of of how components works come in instead of on relying on the CAD program alone.
                    Like I said, this is not he first time OP shows us bad schematic using this CAD program.
                    https://www.badcaps.net/forum/showpo...08&postcount=1
                    Last edited by budm; 11-27-2020, 06:32 PM.
                    Never stop learning
                    Basic LCD TV and Monitor troubleshooting guides.
                    http://www.badcaps.net/forum/showthr...956#post305956

                    Voltage Regulator (LDO) testing:
                    http://www.badcaps.net/forum/showthr...999#post300999

                    Inverter testing using old CFL:
                    http://www.badcaps.net/forum/showthr...er+testing+cfl

                    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                    http://s807.photobucket.com/user/budm/library/

                    TV Factory reset codes listing:
                    http://www.badcaps.net/forum/showthread.php?t=24809

                    Comment

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