Opamp inverting terminal calculation- help needed

Discussion in 'The Projects Forum' started by mishra87, Aug 28, 2016.

Jan 17, 2016
164
1
Hi all ,

I am missing somewhere the fundamentals of opam.

As you can see in attached figure .

Iin= If
Now In= Vin /Rin
And If = -Vout/Rf

Since Iin =If

Vin/Rin = -Vout/Rf

On solving this we get

Vout = -(Rf/Rin)Vout

So my doubt is why If is negative current I do not understand this. I tried to read the book but I did not understand can any let me understand the basic of this.

Thanks

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2. crutschow Expert

Mar 14, 2008
12,585
3,087
It's simple.
If current is going into the summing junction through Ri from a positive voltage applied to Ri, then an equal current going out of the summing junction through Rf is required to keep the summing junction at 0V.
This requires a negative voltage at the output to Rf.

Jan 17, 2016
164
1
I
I still did not understand the fundamentals.

"
If current is going into the summing junction through Ri from a positive voltage applied to Ri "

I thinks it's not true...

Could you elaborate more in details...!!!

4. crutschow Expert

Mar 14, 2008
12,585
3,087
But it is true.
Why do you think there is not a positive current into the summing junction from a positive voltage applied to Ri?
How could there not be?

The op amp functions by doing whatever is necessary at the summing (-) junction to keep it at the same voltage as the plus junction (ground in this case) due to the negative feedback through Rf.
Thus if you apply a positive voltage to Ri, there will be a positive current flowing through Ri into the summing junction.
The starts to increase the summing junction voltage.
This causes the output to start going negative from the op amp open loop gain until the current through Rf equals the current through Ri.
At this point the voltage at the summing junction is equal to the the output voltage divided by the op amp open loop gain, with the opposite polarity. (For typical op amp open loop gain this is only a few microvolts).

This means the output voltage must be the opposite polarity from the input.

5. ci139 Member

Jul 11, 2016
341
38
i do work 48 and 72h days but then it's not reading and attempting to grasp stuff from unfamiliar/uninteresting zone

get more sleep - do stuff - when you're fresh or your mind not so tired - it starts automatically fade attention