# Thevenin Equivalents and Opamps

1. Jun 16, 2013

### Rosaline

Hey, guys.. So im taking some electrical engineer classes and i could really use your help on one of the questions im stuck on.

So my proff talked about opamps and said we would not be using positive feedback and skipped it. Question number two has both positive and negative feed back. I have no idea how to do this question?

#3, because there is an open circuit can i say there is no current flowing in the top and bottom wire and therefore the Thevenin Voltage is just 24V? I know to find the current i have to put a shunt between a and b but any more tips?

Im struggling overall with the rest of the questions so any help would be appreciated!

Thanks guys! XOXO

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2. Jun 17, 2013

### darkxponent

Hint: Nodal Analysis for the second problem. You can also do it using feedbacks if you want to.

3. Jun 17, 2013

### psparky

Node analysis is king for almost any op amp you will see in school. Current in equals current out.

Superposition is also your friend....use it whenever possible.

Those are pretty tough problems for a relative begginer, so if you are struggling....you are not alone!

4. Jun 17, 2013

### TimeToShine

You could break the feedback loop in question two but this requires a knowledge of two ports which I doubt you have. In that case, my advice is to apply nodal analysis. Give the node at the positive terminal of the op amp a name such as Vx and remember that because the op amp is deal the node at the negative terminal is at the same voltage and therefore is also Vx. Remember that no current flows into the terminals.

Starting at the input voltage, construct an equation using the currents via KCL and fiddle around with it to find the transfer function Vo/Vi.

Q3: Remember finding the thevenin equivalent involves setting all voltage sources to zero.

Have a punt at these and then we'll tackle the other 3 questions.