[sdiy] Non-inverting summing amplifier

[David G. Dixon]

Yeah, that's what I meant!  ;->

Non-inverting summer: inputs affect each other: bad!

Inverting summer: inputs independent: good!

End of story!

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
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Canada
 
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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Ingo Debus
> Sent: Wednesday, January 28, 2009 12:26 PM
> To: Tom Wiltshire
> Cc: Synth DIY
> Subject: Re: [sdiy] Non-inverting summing amplifier
> 
> 
> Am 28.01.2009 um 20:38 schrieb Tom Wiltshire:
> 
> >
> > On 28 Jan 2009, at 18:40, Ingo Debus wrote:
> >
> >>
> >> Am 28.01.2009 um 08:23 schrieb David G. Dixon:
> >>
> >>> In an inverting summer, on the other hand, the inputs act as
> >>> current sources
> >>> into the negative input terminal, which in turn acts as a summed
> >>> current
> >>> source through the feedback resistor.  Hence, the opamp makes the
> >>> voltage
> >>> inputs look like current sources with infinite impedance.
> >>
> >> You mean, the input impedance of an inverting amplifier is
> >> infinite? Sure aout this?
> >> As I understand it, the input impedance is just the value of the
> >> input resistor, since its other end is tied to virtual ground.
> >
> > I thought he meant that the inv amp arrangement makes it look like
> > the *output* impedance of the thing you feed into it is infinite.
> > So you get all the voltage that you sent out, without any losses.
> 
> Now I'm really confused.
> Why would the output impedance of something look different from what
> it really was?
> And something you get losslessly "all the voltage" from would be a
> voltage source, not a current source, no?. A voltage source has zero
> output impedance.
> 
> As I view it, the difference between a inverting summer and a non-
> inverting summer (which is nothing else but a passive mixer followed
> by a non-inverting amplifier) is that the impedance of one input of
> the inverting summer is independent of what is connected to the other
> input(s). But both have finite input impedances.
> 
> Ingo
> 
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