[sdiy] Non-inverting summing amplifier

[Scott Nordlund]

Well there's really nothing nonlinear happening here (unless you're connecting things together in a way that could possibly result in damage- connecting the sources together directly), so I wouldn't expect anything more exciting than a mixer that possibly doesn't mix very well, depending on the source impedances.

----------------------------------------
> Date: Wed, 28 Jan 2009 09:45:58 +0000
> Subject: Re: [sdiy] Non-inverting summing amplifier
> From: cheater00 at gmail.com
> To: dixon at interchange.ubc.ca; synth-diy at dropmix.xs4all.nl
> CC:
>
>> 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
>  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
>> 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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