# Name of this circuit

Discussion in 'General Electronics Chat' started by sevenfold4, Apr 16, 2015.

1. ### sevenfold4 Thread Starter Member

Jan 12, 2015
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7
Hey, could you guys help me by telling me how to call three resistors connected like in the picture.
I have an idea how it works, but i can not find anything on the net to read more about it.
Thanks

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2. ### #12 Expert

Nov 30, 2010
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It is merely a voltage divider where Vin1 and Vin2 can be different voltages. It is not a, "classic" circuit that is so special that it has a name. If Vin2 was labeled as an AC voltage, it might be the input biasing configuration for a common emitter amplifier, but the labeling refutes that theory.

3. ### sevenfold4 Thread Starter Member

Jan 12, 2015
80
7
Yea, it is not AC, both are DC, just as labeled.

4. ### crutschow Expert

Mar 14, 2008
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Sometimes that is called a summing circuit as the output is a fractional sum of the two inputs.

5. ### #12 Expert

Nov 30, 2010
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Here you go. One of the 26 images on a Google Search does NOT have an active amplifier in it.
(I learn something almost every day on this site.)

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6. ### sevenfold4 Thread Starter Member

Jan 12, 2015
80
7
This is perfect guys, thanks, it is just a simple voltage summer

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7. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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Indeed! Tho' you are admonished to maintain quite an open mind as regards the definition of 'sum' in this case!

Best regards
HP

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8. ### #12 Expert

Nov 30, 2010
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Doggone it! That idea was an inkling in the back of my mind, but I'm not Jeopardy fast any more. The difference between this circuit and an active circuit is that this configuration is not a true summing junction because the inputs will interact with each other and the resistor connected to ground (R2).

Jan 12, 2015
80
7
10. ### crutschow Expert

Mar 14, 2008
13,002
3,232
Depends upon your definition of "summing" and "interact" (much as the definition of sex) I suppose.
The passive circuit can be considered a summing circuit if all the sources are ideal (no resistance) since the output is then a linear sum of the individual voltage inputs. Of course the contribution of each input to the sum is based upon the resistance of that input as compared to the equivalent impedance of all the other resistors.
The interaction is that, if one of the resistors changes value, then all the output values for all combination of inputs changes.
So does that interaction mean the summing junction is not "true" as compared to an inverting op amp summing junction, which does not have that interaction?

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11. ### #12 Expert

Nov 30, 2010
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I see that this kind of summing was expedient in 1950, but that is the year of my birth. The proverbial light bulb didn't go on in my head until 1970...and it was caused by op-amps. I appear to be as spoiled by op-amps as today's 20 year old people are spoiled by microprocessors. I simply don't think it's all that accurate to use a passive summer when active summers have been available for (about) 50 years. Similarly, the present crop of students believes you don't need an op-amp when your MPU can ADC the voltages.

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