# op amp circuit help

Discussion in 'General Electronics Chat' started by kaznov, Apr 11, 2012.

1. ### kaznov Thread Starter Member

Mar 10, 2012
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0
Hi guys
for this circuit is't V0 due to V1 is just the non inverting circuit which is
V0 = ( 1+ R4/R3) * V1 !! Becasue that's wht it seems like but my teacher told me that Vo due to V1 is just zero !!!
thanks

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2. ### panic mode Senior Member

Oct 10, 2011
1,328
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opamp will do everything it can to ensure voltage difference at it's inputs is zero.

since inputs are high impedance, there i no current in or out of inputs.
using voltage divider we can write:

Vin(-) = Vo*R3/(R3+R4)

Vin(+) = Vin(-) = Vin

(Vo-Vin)/R2 = (Vin - V1)/R1

because current through R1 and R2 is same (recall - high impedance input of opamp).

then use algebraic manipulation to get Vo as function of V1 (input voltage)
should be something like this:
[Vo-Vo*R3/(R3+R4)]/R2=[Vo*R3/(R3+R4) - V1]/R1
[Vo-Vo*R3/(R3+R4)]/R2-Vo*R3/[R1(R3+R4)] = - V1/R1
Vo{1-R3/[R2(R3+R4)]- R3/[R1(R3+R4)} = - V1/R1
Vo = - (V1/R1)/{1-R3/[R2(R3+R4)]- R3/[R1(R3+R4)]}

and then (after you get correct expression that looks something like previous line) you can play with it. for example you can calculate what happens when V1=0 or when all resistors are same etc.

Mar 24, 2008
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4. ### atferrari AAC Fanatic!

Jan 6, 2004
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Hola Bill,

Who wrote chapter 8?

Whoever the author is, he did a good job. Found it easy to follow.

5. ### panic mode Senior Member

Oct 10, 2011
1,328
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is that document maintained? the last update was some three years ago and there is still lot of work to be done.

6. ### Wendy Moderator

Mar 24, 2008
20,772
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Not me, but I have written other parts of the book.

At the moment the editor is in absentia. I do expect him to eventually return though.

7. ### atferrari AAC Fanatic!

Jan 6, 2004
2,663
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And what makes impossible to know his name?