[sdiy] Non-inverting summing amplifier

[cheater cheater]

> In a non-inverting summer, the inputs must be virtually perfect voltage
> sources, with very low source resistances [...] so as not to degrade to some lower
> voltage.

Has anyone made a circuit that uses this effect? Sounds like it could
turn out to be a nice ring-modulator-style effect. Definitely xmod.

Cheers
D.

On Wed, Jan 28, 2009 at 7:23 AM, David G. Dixon
<dixon at interchange.ubc.ca> wrote:
> 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
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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 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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