[sdiy] Non-inverting summing amplifier

[David G. Dixon]

Karl,

Yes, a very good reason.  

In a non-inverting summer, the inputs must be virtually perfect voltage
sources, with very low source resistances.  This is because they terminate
at the positive input terminal which has no current flow.  Each input
therefore sees an input impedance consisting of all the other input
resistors in parallel.  In a practical circuit, each input must have its own
follower feeding the input resistor so as not to degrade to some lower
voltage.

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.  The input
voltages therefore cannot affect each other.

...at least, that's how I understand it.

Cheers,
Dave

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada
 
Tel 1-604-822-3679
Fax 1-604-822-3619
 
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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 Karl Ekdahl
> Sent: Tuesday, January 27, 2009 8:42 PM
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Non-inverting summing amplifier
> 
> Hi list
> 
> Is there a reason that i never see non-inverting summing amplifier, i.e.
> simply a non-inverting amp with several resistor inputs?
> 
> Karl
> 
> 
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