How to determine op-amp gain with active feedback?











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3
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The Howland current pump uses an op-amp in the configuration below with a resistive feedback network which gives me the gain show below:
enter image description here



But if I decide to swap the feedback resistor for an instrumentation amplifier for less noise and better resolution, what will the new gain be? I've tried searching but can't seem to find anything on this.





schematic





simulate this circuit – Schematic created using CircuitLab










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  • 1




    Your proposed circuit isn't going to work, because the input is shorted to the in-amp output.
    – The Photon
    Nov 5 at 6:06










  • Describe your theory behind this "concept", please. What do you imagine it does and how? What does the input source see here vs your earlier circuit?
    – jonk
    Nov 5 at 6:10















up vote
3
down vote

favorite
2












The Howland current pump uses an op-amp in the configuration below with a resistive feedback network which gives me the gain show below:
enter image description here



But if I decide to swap the feedback resistor for an instrumentation amplifier for less noise and better resolution, what will the new gain be? I've tried searching but can't seem to find anything on this.





schematic





simulate this circuit – Schematic created using CircuitLab










share|improve this question




















  • 1




    Your proposed circuit isn't going to work, because the input is shorted to the in-amp output.
    – The Photon
    Nov 5 at 6:06










  • Describe your theory behind this "concept", please. What do you imagine it does and how? What does the input source see here vs your earlier circuit?
    – jonk
    Nov 5 at 6:10













up vote
3
down vote

favorite
2









up vote
3
down vote

favorite
2






2





The Howland current pump uses an op-amp in the configuration below with a resistive feedback network which gives me the gain show below:
enter image description here



But if I decide to swap the feedback resistor for an instrumentation amplifier for less noise and better resolution, what will the new gain be? I've tried searching but can't seem to find anything on this.





schematic





simulate this circuit – Schematic created using CircuitLab










share|improve this question















The Howland current pump uses an op-amp in the configuration below with a resistive feedback network which gives me the gain show below:
enter image description here



But if I decide to swap the feedback resistor for an instrumentation amplifier for less noise and better resolution, what will the new gain be? I've tried searching but can't seem to find anything on this.





schematic





simulate this circuit – Schematic created using CircuitLab







op-amp amplifier current-source instrumentation-amplifier






share|improve this question















share|improve this question













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edited Nov 5 at 5:54









Niteesh Shanbog

349210




349210










asked Nov 5 at 5:49









VanGo

393315




393315








  • 1




    Your proposed circuit isn't going to work, because the input is shorted to the in-amp output.
    – The Photon
    Nov 5 at 6:06










  • Describe your theory behind this "concept", please. What do you imagine it does and how? What does the input source see here vs your earlier circuit?
    – jonk
    Nov 5 at 6:10














  • 1




    Your proposed circuit isn't going to work, because the input is shorted to the in-amp output.
    – The Photon
    Nov 5 at 6:06










  • Describe your theory behind this "concept", please. What do you imagine it does and how? What does the input source see here vs your earlier circuit?
    – jonk
    Nov 5 at 6:10








1




1




Your proposed circuit isn't going to work, because the input is shorted to the in-amp output.
– The Photon
Nov 5 at 6:06




Your proposed circuit isn't going to work, because the input is shorted to the in-amp output.
– The Photon
Nov 5 at 6:06












Describe your theory behind this "concept", please. What do you imagine it does and how? What does the input source see here vs your earlier circuit?
– jonk
Nov 5 at 6:10




Describe your theory behind this "concept", please. What do you imagine it does and how? What does the input source see here vs your earlier circuit?
– jonk
Nov 5 at 6:10










1 Answer
1






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7
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What you need to do is add a couple of resistors





schematic





simulate this circuit – Schematic created using CircuitLab



If the instrumentation amp has a gain G, then, since the current through R1 must equal the current through R2,



Vin/R1 = G iL Rs/R2,



where iL is the load current.



Rearranging the terms gives



iL = Vin(R2 /R1 G Rs)



Note that, strictly speaking, an instrumentation amp is not required, since Rs is grounded, and a simple non-inverting op amp would do the job. In practice, an instrumentation amp would be a good idea, since tiny differences in ground resistance will have a noticeable effect due to the large gain of the amp.



Also note that this configuration will almost certainly oscillate like crazy. The phase shift caused by the instrumentation amp will need careful compensation.






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    1 Answer
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    up vote
    7
    down vote



    accepted










    What you need to do is add a couple of resistors





    schematic





    simulate this circuit – Schematic created using CircuitLab



    If the instrumentation amp has a gain G, then, since the current through R1 must equal the current through R2,



    Vin/R1 = G iL Rs/R2,



    where iL is the load current.



    Rearranging the terms gives



    iL = Vin(R2 /R1 G Rs)



    Note that, strictly speaking, an instrumentation amp is not required, since Rs is grounded, and a simple non-inverting op amp would do the job. In practice, an instrumentation amp would be a good idea, since tiny differences in ground resistance will have a noticeable effect due to the large gain of the amp.



    Also note that this configuration will almost certainly oscillate like crazy. The phase shift caused by the instrumentation amp will need careful compensation.






    share|improve this answer

























      up vote
      7
      down vote



      accepted










      What you need to do is add a couple of resistors





      schematic





      simulate this circuit – Schematic created using CircuitLab



      If the instrumentation amp has a gain G, then, since the current through R1 must equal the current through R2,



      Vin/R1 = G iL Rs/R2,



      where iL is the load current.



      Rearranging the terms gives



      iL = Vin(R2 /R1 G Rs)



      Note that, strictly speaking, an instrumentation amp is not required, since Rs is grounded, and a simple non-inverting op amp would do the job. In practice, an instrumentation amp would be a good idea, since tiny differences in ground resistance will have a noticeable effect due to the large gain of the amp.



      Also note that this configuration will almost certainly oscillate like crazy. The phase shift caused by the instrumentation amp will need careful compensation.






      share|improve this answer























        up vote
        7
        down vote



        accepted







        up vote
        7
        down vote



        accepted






        What you need to do is add a couple of resistors





        schematic





        simulate this circuit – Schematic created using CircuitLab



        If the instrumentation amp has a gain G, then, since the current through R1 must equal the current through R2,



        Vin/R1 = G iL Rs/R2,



        where iL is the load current.



        Rearranging the terms gives



        iL = Vin(R2 /R1 G Rs)



        Note that, strictly speaking, an instrumentation amp is not required, since Rs is grounded, and a simple non-inverting op amp would do the job. In practice, an instrumentation amp would be a good idea, since tiny differences in ground resistance will have a noticeable effect due to the large gain of the amp.



        Also note that this configuration will almost certainly oscillate like crazy. The phase shift caused by the instrumentation amp will need careful compensation.






        share|improve this answer












        What you need to do is add a couple of resistors





        schematic





        simulate this circuit – Schematic created using CircuitLab



        If the instrumentation amp has a gain G, then, since the current through R1 must equal the current through R2,



        Vin/R1 = G iL Rs/R2,



        where iL is the load current.



        Rearranging the terms gives



        iL = Vin(R2 /R1 G Rs)



        Note that, strictly speaking, an instrumentation amp is not required, since Rs is grounded, and a simple non-inverting op amp would do the job. In practice, an instrumentation amp would be a good idea, since tiny differences in ground resistance will have a noticeable effect due to the large gain of the amp.



        Also note that this configuration will almost certainly oscillate like crazy. The phase shift caused by the instrumentation amp will need careful compensation.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Nov 5 at 6:16









        WhatRoughBeast

        48.3k22873




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