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OP AMP Current Noise

Frank Huang16969
Prodigy 160 points
Other Parts Discussed in Thread: OPA134, OPA277, OPA211

Hi, I have 2 questions about the current noise difference between BJT OPA & FET/CMOS OPA:

1.  When looking at OPA134/300 and other FET/CMOS OPA's current noise density, it seems has a inverse noise shape with voltage noise... i mean the higher the bandwidth cause higher current noise (I call it white noise then f noise...).  Why?

2. FET/CMOS OPA's current noise shape is different from Bipolar OPAs such as OPA277, the BJT OPA's noise and current noise density has the same shape(1/f noise then white noise). Why BJT's current noise has the same shape with voltage noise?

Thanks a lot for your reply.

                                                                                                                                                                    Frank

 

 

  • 0
    TI__Mastermind 22277 points

    Frank,

     

    The increase in noise is called f-squared noise.  This noise is the result of an interaction of the input voltage noise and the input capacitive reactance.  I have an attached a preliminary article on this subject.  This article has not been published.  I have published a series of articles on noise on en-genius (see link below).

     

    http://www.en-genius.net/site/zones/audiovideoZONE/technical_notes/avt_120108

     

    f-squared noise.pdf
  • 0 in reply to Art Kay
    TI__Prodigy 160 points

    Art,

    I am so honored to have you answered my question. Actually, your series of articles about OPA Noise have been translated into Chinese, I have studied #1-#7 already. Thanks for introducing #8 and the latest one. I will try to understand the f squared noise from your articles. I will reply you when I have further questions.

    BTW, I have added an Word file to describe my question more clear to other visitors.

    Thanks again!

    B. Regards,

    Frank Huang

    Current Noise Difference.doc
  • 0 in reply to Art Kay
    TI__Prodigy 160 points

    Art,

    So, do you think the f-squared noise existing in all kinds of OPA? But some of them didn't mention it in their datasheet, such as OPA277, OPA211.

    B. Regards,

    Frank

     

     

  • 0 in reply to Frank Huang16969
    TI__Mastermind 22277 points

    I’m glad you were able to make use of the articles.  Thanks for the feedback.

     

    Yes.  F-squared noise exists on all op-amps.  On bipolar amplifiers the magnitude of the broadband current noise is larger then the f-squared noise, so you can’t see the f-squared noise.  OPA211 and OPA277 are bipolar amplifiers.  In FET input amplifiers, the broadband current noise is smaller so you can see the f-squared noise in measurements and simulations.  

     

    However, in new datasheets we do not show f-squared noise.  This is because f-squared noise is an interaction between voltage noise and the input captive reactance of the op-amp.  So, for example, if you were developing a spice model for the op-amp, the f-squared noise would automatically be generated by the input capacitance and the voltage noise.  We do not want people to erroneously count both f-squared noise in the current noise spectral density and f-squared noise from interaction with the voltage noise and input capacitance.  The only source of f-squared noise is from interaction with the voltage noise and input capacitance. 

     

    A good tool to help understand f-squared noise is Tina.  Try running a few Tina simulations with and without input capacitance to see the f-squared effect.

     

  • 0 in reply to Art Kay
    TI__Prodigy 160 points

    Art,

    Yes, simulation and hands-on will give me more deep understanding on it. It's great to learn from you. Thanks a lot for your help!

    B. Regards,

    Frank