Opamp buffers and gain

Discussion in 'General Electronics Chat' started by IvarsOzolsDB, Jan 13, 2014.

  1. IvarsOzolsDB

    Thread Starter Active Member

    Aug 24, 2012
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    Newbie question. :) Is there any difference between opamp in unity gain configuration (buffer) and opamp in non-inverting configuration with gain of 2? Is there output impedance difference etc?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Many opamps are not stable in a unity gain/voltage follower configuration; you need to read the datasheet (s) for the opamp(s) in question - so no; they are not the same.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    The gain of 2 circuit bandwidth will be 1/2 the unity gain configuration (equal to 1/2 the GBWP of the op amp). There may also be some peaking in the frequency response from stray capacitance at the inverting input node.

    The closed-loop output impedance will be higher than in the unity gain configuration, but will still be very low.

    Any op amp input offset will be multiplied by a factor of 2.

    I believe those are the primary differences.

    Edit: The above assumes the op amp is stable in those configurations, as SW noted.
     
  4. Veracohr

    Distinguished Member

    Jan 3, 2011
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    Yes, the input impedance is increased by a factor of the closed loop gain

    Rin = Rin*Acl

    And output impedance is decreased by the same factor:

    Rout = Rout/Acl
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Might be worth double checking that.....
     
  6. Veracohr

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    Jan 3, 2011
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  7. crutschow

    Expert

    Mar 14, 2008
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    The input impedance of a typical op amp is already so large that any increase due to the closed loop gain in a non-inverting amp configuration is generally of little practical significance.
     
  8. t_n_k

    AAC Fanatic!

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    Hi Veracohr,

    What does Acl mean in your earlier post?
     
    Last edited: Jan 13, 2014
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I assume Close Loop Gain.
     
  10. t_n_k

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    Most likely. The issue is with the input / output resistance relationships with respect to that closed loop gain - as quoted by Veracohr in post #4.
     
  11. Veracohr

    Distinguished Member

    Jan 3, 2011
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    Yes that's what I meant.

    I see now. Not the closed loop gain, but the loop gain (1+Aβ), where A is the op amp's open loop gain and β is the feedback network factor R1/(R1+R2). Correct? Sorry for giving out misinformation.
     
  12. t_n_k

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    No need to apologize. Simple mistake. BTW the loop gain is usually taken to be Aβ. Some people include a negative sign which probably pre-supposes negative feedback is being applied.
     
  13. LvW

    Well-Known Member

    Jun 13, 2013
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    Perhaps not too important in the context of this thread - nevertheless I like to mention that the loop gain is the gain of the COMPLETE loop, that means: including any sign inversion within the loop. Remember that there are circuits with another active inverting element within the loop - and in this case, the loop is closed using the non-invering input node of the main amplifier.
    Thus, the loop gain usually should be taken to be (-Aβ).

    I think, this consideration is important because very often this is the background of some misunderstandings in finding stability margins:
    Is it necessary to investigate the loop gain at a the frequency with -180deg or with -360deg phase shift ?
     
  14. IvarsOzolsDB

    Thread Starter Active Member

    Aug 24, 2012
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    Thanks guys for answers. The pratical appliacion for the question was about buffer before Linkwitz-Riley 24db/oct filter stages. Do I need add an extra unity gain buffer before subsonic filter stage or do balanced input stage will manage right output impedance etc? Schematic attached.

    [​IMG]
     
    Last edited by a moderator: Jan 15, 2014
  15. Veracohr

    Distinguished Member

    Jan 3, 2011
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    That stage is fed by an op amp, I don't see why a buffer would be necessary. The op amp has sufficiently low output impedance.
     
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