# Amplifier problem

Discussion in 'Homework Help' started by boks, Dec 17, 2008.

1. ### boks Thread Starter Active Member

Oct 10, 2008
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The op-amp is ideal, and I'm supposed to find $V_{o,B}$.

I know that there is no current going in to any of the + and - terminals of an ideal op-amp. But can there be current going out from either of the two (or negative current in, if you want)?

If not, I guess I can conclude that there is no current going through $R_1$ and $R_2$?

2. ### beenthere Retired Moderator

Apr 20, 2004
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282
Not if the op amp is ideal.

3. ### Audioguru New Member

Dec 20, 2007
9,411
895
Of course there is a current in the input R2 that equals the opposite phase current in the feedback R1.

Vob is the input voltage times -R1/R2.

The opamp would work a lot better when it is powered. Since its (+) input connects to ground then the power supply must be positive and negative.

4. ### beenthere Retired Moderator

Apr 20, 2004
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282
Oops, I only considered the R1 & R2 on the non-inverting input. The others should be R3 & R4 for clarity.

5. ### steveb Senior Member

Jul 3, 2008
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I'm not sure if I'm understanding the responses correctly. Just for clarity, are we saying that the the OP (boks) is correct that there is no current flow in R1 and R2 connected to the noninverting input, but there is current for R1 and R2 on the inverting input.

I believe this is the correct answer, but the responses don't seem to state this clearly.