Voltage Follower vs. two stage unity gain amp.

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meridic
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Voltage Follower vs. two stage unity gain amp.

Post by meridic » Wed Feb 24, 2010 4:16 pm

I continually see people throwing on the added complexity of the cascaded inverting unity gain opamps on output circuits when it seems to me the simple voltage follower circuit would be more then adequate. Am I missing something?

The only thing I can come up with is that any noise or crosstalk picked up by the cable is fed back to the inverting input causing error, but if the cable is picking up signals from other modules error is already presant. Not to mention most modulars seem to have abit of cross talk between modules anyways.

I can see the use of the cascaded amp in some areas such as a quantized output where error becomes detuning, but that is about it.

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Kent
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Post by Kent » Wed Feb 24, 2010 4:21 pm

Maybe I'm missing something here; quite likely actually, but is this in reference to an opamp that is flipping the polarity so that a circuit is not flipping polarity from input to output?

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meridic
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Post by meridic » Wed Feb 24, 2010 4:25 pm

non inverting output stages.

Two cascaded inverting unity gain amps vs single opamp voltage follower with the inverting input just tied to the output.

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Post by frijitz » Wed Feb 24, 2010 8:05 pm

Often because you are adding more than one signal and it is convenient to use the virtual ground summing node. Sometimes because you also want the inverted signal or to use the inverted input to stage 2. Sometimes just laziness or ignorance.

You can always use a single stage differential summer if the source impedances are fixed. There are a number of examples of how to do this in my wind controller schematics:
http://home.comcast.net/~ijfritz/sos_circ.htm

:grin:

Ian

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meridic
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Post by meridic » Thu Feb 25, 2010 3:06 am

Yeah, I was refering to times where an inverted output is not used nor is it summing, as shown in my diagram. I have seen this done by people who I thought should know better and it made me wonder if I was missing something.

thanks

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Post by fonik » Thu Feb 25, 2010 4:04 am

another idea was to have an additional inverted output (as seen on andrew's 4stage seq schematic)
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meridic
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Post by meridic » Thu Feb 25, 2010 4:16 am

This is strictly referring to the end result of a single output with the same phase relationship as the input. Oddly enough though andrews circuit is what pushed me over the edge to find this out once and for all. At first I did not notice the inverted output and seeing all those opamps just irked me just right, I calmed down once I saw the inverted outputs though.

I spent a good amount of time researching this earlier today, there is a good amount of mis-information and half truths out there on the subject. Mostly, alot of correct information out of context.

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Post by frijitz » Thu Feb 25, 2010 7:05 am

meridic wrote:I have seen this done by people who I thought should know better ...
Yeah, me too. :roll:

Another general reason might be to leave the possibility of adding in other signals in further development of the circuit, or a place for users to hack into.

:grin:

Ian

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meridic
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Post by meridic » Thu Feb 25, 2010 1:09 pm

it kind of sound like you are trying to make excuses for them.

I went through that too, but no more, I am done being a crutch for ignorance.

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Post by Sound » Thu Feb 25, 2010 2:16 pm

PCB layout considerations: Short PCB traces are advised in inputs. So the resistor IN assures short trace if the source is far in the pcb.

Also maybe an unused opamp section, and as said add the possibility of changes in a future, like amplify or attenuate the signal or inverted signal available.

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