Inverter to non-inverter cct

[Bissell, Harry]

This circuit is from the National Semiconductor Audio Ap-notes A4.8 (and
probably lots of other places). Note: The midpoint of the slider will give
attenuation up to the Common Mode Rejection Ratio of the Op-amp, and no
more. Usually that's good enough. And the point that you get the null
(minimum output) might not be at exactly the 50% point. I just set the knob
on the pot to "straight up" (or wherever...) when I'm at the null point.
Good enuf.  :-) Harry Bissell

> -----Original Message-----
> From:	Gene Zumchak [SMTP:zumchak at cerg.com]
> Sent:	Tuesday, February 23, 1999 9:28 AM
> To:	Jezz Brookes
> Cc:	Synth DIY
> Subject:	Re: Inverter to non-inverter cct
> 
> Jezz,
> 
>     Yes it works, and it is linear from -1 to +1.  It's so simple, you
> just
> should have tried it, but it's nice that you asked because some others may
> have
> need of it.
> 
>     When the pot is at the bottom, the non-inverting input is grounding
> and is
> simply a unity-gain inverter.  When at the top it is a voltage follower.
> The
> non-inverting input is Vin, the inverting input is Vin and the output must
> be
> Vin.  If you have need to go between an equal an opposite +V and -V you
> simply
> go from the input of a unity inverter to the non-inverting input.  From
> the
> non-inverting input to ground you put a switch (transistor, FET,
> transmission
> gate.  When the switch is on it is unity inverting.  When the switch is
> off it
> is unity non-inverting.
> 
> Gene Z.
> 
> 
> 
> Jezz Brookes wrote:
> 
> > I've scoured my collection of circuits but couldn't find a circuit I'm
> after
> > but I've seen it somewhere. So naturally I turn to you guys for help
> > (again!)
> >
> > I want a circuit that you can vary the gain from +1 to -1 (i.e.
> inverting)
> > and I came up with this circuit (attached). Is it going to work?
> >
> >
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> >                  Name: Invert.gif
> >    Invert.gif    Type: GIF Image (image/gif)
> >              Encoding: base64