I have stuck in solving the problem 3 in my class homework. Is there anyone can explain the solution to help me finish my homework. Thanks a lot

 

It 's a matter of setting up your equations and knowing what an ideal OpAmp is.

I 'll set up the equations for you for the first circuit and let you solve it. Then you can have a go at the second one.

Since no current flows through the + and - pins of the OpAmp, we have no voltage drops in the resistors connected to those pins. Therefore:

1. V+=Vin

Also, we know that:

2. V-=V+

And

3. Va=V-

Where Va is the voltage at the node where the three resistors meet.

Now that you know the voltage Va, you can calculate the current flowing through the resistor connected to the Ground. Name it "I".

4. I=Va/R

This current can't come from the "-" pin of the OpAmp, and thus comes intact from the resistor connected to the output pin. Ohm's law says that:

5. Vout-Va=I*R

Work it out for the Vout voltage and write the transfer function F=Vout/Vin. Don't forget to mention the saturation limits.

 

Here's another hint: The second problem will not rely on V+ = V-, because the feedback is positive.