[sdiy] Inverting Amplifier as a Buffer Output.

[Harry Bissell]

MY $.02

I favor A. Most cases I'm relying on the series output resistor to decouple the
cable capacitance from the opamp ESPECIALLY if its a TL07x series.  I fear that I
don't want to send that signal back to the inverting input. I'd rather have the
opamp stable all by itself and deal with the voltage drop. I usually use 100 ohms,
so thats a 0.1% gain error with the typical 100K input. I can deal with that...

H^) harry




----- Original Message -----
From: John Speth <John.Speth at coherent.com>
To: synth-diy at dropmix.xs4all.nl
Sent: Tue, 22 Jun 2010 18:04:57 -0400 (EDT)
Subject: Re: [sdiy] Inverting Amplifier as a Buffer Output.

For both cases, the right-most resistor limits the current that the op-amp will supply when the next stage input resistance is low.  Since case B's resistor is in the feedback loop, the voltage at the next stage input will track with the input with whatever scaling the other resistors provide.  The downside is that it could drive the op-amp output out of compliance if the next stage input resistance is too low.  There is no such regulation with case A.

That's the tradeoff.  It's up to you to assign value in terms of "advantage" or "disadvantage" based on your application.

John Speth
mailto:john.speth at coherent.com


> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Oscar Salas
> Sent: Tuesday, June 22, 2010 2:11 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Inverting Amplifier as a Buffer Output.
>
>
> Hello all,
>
> I have drawn below two different configurations of an inverting
> amplifier as buffer output:
> A) The feedback resistor is taped -before- the output resistor.
> B) The feedback resistor is taped -after- the output resistor.
>
> The case A) has voltage drop when it is conected to another module
> input.
> This is solved in the case B) where there is not voltage drop.
> So, it seems a better solution.
>
> What advantages has the case A) versus the case B) if any?
>
>
>                              ----/\/\/\/\-------
>                              |                 |
> A)                           |                 |
>                              |  =              |
>                              |  =   =          |
>       -----------/\/\/\/\-------=  --   =      |
>                                 =           =-----------/\/\/\/\
>                             ----=  +    =
>                            |    =   =
>                            |    =
>                          __|__
>
>
>
>                              ----/\/\/\/\----------------
>                              |                           |
> B)                           |                           |
>                              |  =                        |
>                              |  =   =                    |
>       -----------/\/\/\/\-------=  --   =                |
>                                 =           =---/\/\/\/\----------
>                             ----=  +    =
>                            |    =   =
>                            |    =
>                          __|__
>
>
>
>
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Harry Bissell & Nora Abdullah 4eva