Question on Opamp homework problem: Find Iout between these two Opamps

In summary, the conversation discusses finding the output voltage for a non-inverting op amp using two different methods. One method uses the equation for finding output voltage and the other uses Ohm's law. There is a discrepancy in the results and the conversation speculates on the cause, ultimately concluding that the polarity of the top amp is incorrect and it is in full saturation. The other op amp is correctly wired and forms an amplifier that amplifies the difference of two signals.
  • #1
james weaver
28
4
Homework Statement
find Io
Relevant Equations
ohms law
Hi, so my objective is to find Io, and I've attached a picture of my work. Here is my question: At the point Vo1, if I use the equation for finding the output voltage for a non-inverting op amp I end up getting this:

$$V_{0{1}}=\left( 1+\frac {r_{f}} {r_{1}} \right)V_{s1} = \left( 1+\frac {40k} {10k} \right)2 = 10v$$

However, I can calculate the current ##I_{3}## by using ohms law:

$$\frac{V_{s1}-V_{A}} {10k}=\frac {2v} {10k}=0.2mA$$

From there, I start at Vs1 and add the voltage drops until I reach Vo1, which gives me: ##2-50k*I_{3}=-8v##

My instinct is to go with the -8, since ##I_{3}## would have to be reversed if it were 10v. Can you please help explain to me which one is right and why? Thanks.

circ.png
 
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  • #2
First, I think the polarity of the top amp is backwards.

james weaver said:
I start at Vs1 and add the voltage drops until I reach Vo1, which gives me: 2−50k∗I3=−8v
Nope, the voltage at A is zero (virtual ground assumption), So V01 = 0 - 50KΩ⋅I3.
Alternatively, if you want to start from VS1, you have to count both the 2V from VS1 minus the 2V drop across R1, which cancel each other. So, V01 = VS1 - R1⋅I3 - RF⋅I3.
 
  • #3
DaveE said:
Nope, the voltage at A is zero (virtual ground assumption),
[edit] Or if you assume the circuit is correctly drawn: [/edit]
Since point "A" is fed with a positive battery voltage AND the positive output from its non-inverting configuration, where is the negative source to put it at zero volts?
 
  • #4
Tom.G said:
the positive output from its non-inverting configuration
Which is an unlikely HW problem without a good solution since we don't have the PS voltages. So...

DaveE said:
First, I think the polarity of the top amp is backwards.
However, you can solve it the other way if you like.
 
  • #5
However, the equation he wrote is for a positive gain amp. So it's not just the polarity that's messed up here.
 
  • #6
To make it clear: The upper opamp has positive feedback and will not work as desired. It is in full saturation.
The other opamp (at the bottom) is wired as a positive gain amplifier (for Vs1) with - of course ! - negative feedback. For Vs2 it will work as an inverting amplifer.
Together, it forms an amplifier which amplifies the difference of both signals (with a different gain factor).
 

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