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Plz explain this buffer circuit ...

Soumitra Bhattacharjee
Intellectual 800 points
Other Parts Discussed in Thread: RC4558

 Hi

I'm stuck trying to understand a part of this and would appreciate any pointers from you. This is a working circuit and reproduced as it is. 

a) The input 1uF capacitor (connected in reverse) - this is a polar cap but am not sure how this configuration works.

b) The 120K resistor and 47uF capacitor in the feedback loop - is that for nulling the offset ?

Thank you and regards

 

 




  • 0
    TI__Guru**** 189399 points

    Hi, Soumitra,

    I am going to move your post to our precision amplifier forum.

    In the meanwhile, I might suggest you throw this in TINA and run some sims. Do a frequency sweep of the input.

    I'd assume the polarity of the 1 uF cap just says the bias of the pre-stage is at a lower voltage than the bias of the input of the op amp.

    -d2

  • 0 in reply to Don Dapkus
    Intellectual 800 points

     Hi Don

    Thank you for the reply.

    Yes the polarity seems logical considering the bias potential but could it not have been better to have a non-polar capacitor there ? 

    Let me run a TINA sim and get back.

    Thank you and regards

    Soumitra

     

     

     

     

  • 0 in reply to Don Dapkus
    Intellectual 800 points

     Hi Don

    Im afraid im a newbie to TINA, though i did setup the scene and ran an AC and DC analysis to begin with.

    The voltage at points are probably not correct - the midpoint of the resistor divider shows 11.1V whereas it should be 7.5V. I have attached the file here in case it can be opened by others to verify. The opamp used in the original circuit is a JRC4558 ... and the equivalent available with TI is the RC4558 whose supply is lower than the NJR opamp.

    Any help is much appreciated.

    Regards

     Soumitra3264.Input Opamp Test.TSC