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NE5532: Low noise circuit with more NE5532

Wesley Huang
Expert 1030 points
Part Number: NE5532

Hi team，

It's mentioned in a book that“One of the most useful techniques in low-noise electronics is to use two identical amplifiers so that the gains add arithmetically, but the noise from the two separate amplifiers, being uncorrelated, partially cancels. Thus, we get a 3 dB noise advantage each time the number of amplifiers used is doubled. ”

In order to prove this conclusion, the book uses 5532 to increase the number of differential amplifiers in pairs and the number of input buffers for noise test. The circuit diagram is shown in (a) (b) (c) (d). The results are shown in the table below. The noise will indeed decrease with the increase of 5532.

Could you please help analysis the reason about why noise will indeed decrease with the increase of 5532? Is there any other disadvantage in increasing the number of 5532 in pairs?

 

Best Regards

Wesley Huang

  • 0
    TI__Guru**** 152056 points

    Hello Wesley,

    If one noise source is 1 then two noise sources in series are sqrt(2), 41% higher. three noise sources in series is sqrt(3) , 73% higher. So this is not helpful

    If one noise source is 1 then two noise sources in parallel are sqrt(2) / 2, 30% lower. Three noise sources in parallel are sqrt(3)/3 , 42% lower. This is helpful.

    The input offset voltage of each amplifier is different and this will setup a DC current in all the 10 ohm resistors.

    More amplifiers plus the offset current, will increase the power required. 

    In general, it is more effective to start with a lower noise amplifier.

    A unity gain amp has a self noise amplification of 1. Rf+Ri+1

    A unity gain diff amp has a self noise amplification of 2.  Rf/Ri+1