Understanding Output and Input Resistance of an Op Amp in a Lab Setting

[slashrulez]

Homework Statement

The problem is within a lab, so I guess I need to give some background first. Pretty much, I'm starting with the circuit shown here.

[URL]http://imgup.com/image-EC69_4D91309A.jpg[/URL]

For the part of the lab I'm interested in, the values of R1 and Rf were put as 1 kohms and 10 kohms respectively. The lab then asks to measure the output resistance of the amplifier, by putting a variable resistor (starting at 100 kohms) across Vo. Then it says to reduce the resistance until my output voltage reduces to half of the value at 100 kohm. This happens at 5 ohms, which is reasonable I suppose, since it should be zero ideally. Then I'm asked to measure the input resistance. I get rid of the variable resistor, and instead put it in series with R1 (starting at 0 ohms), that is, before the Vin node on the diagram. Then I'm supposed to set Vin = 5V, and increase the resistance until Vin is half the original value. This happens when the variable resistor is around 1 kohms.

Now, that's the set-up. The actual question that's giving me difficulty is:

4.) Indicate how the measurements of 2.5 and 2.6 determine Ro and Rin. In particular, explain why Rin should equal 1 kohm.

The question is basically asking me how the experimental procedure actually equates to the internal resistances of the op amp, and how I can predict the input resistance.

Homework Equations

Nothing special I would imagine.

The Attempt at a Solution

I honestly don't have much. The lab I'm in doesn't fit very well with my lecture, and we haven't even covered op amps yet. I've done some reading on them, but everything about non-ideal op amp impedances says the exact same thing more or less, and I'm having a hard time applying it to my problem.

Well, I guess the first thing I did was assume that the op-amp in the question could be treated like this, since it's a pretty simple course.

[URL]http://imgup.com/image-6A91_4D9134BB.jpg[/URL]

Looking at the first question and that figure, I applied a resistance to make Vout go to half its original value, so I would need to show how that R equals the R in the model. My only stab at a solution is to treat the system as a voltage divider. For the voltage to be cut in half, the two resistances in the divider (op-amp output and variable) would need to equal each other, so the entire resistor part reduces to 1/2. I have no idea if that's correct or not, but staring at it long enough, it's the only reasonable way I see to end up with anything being halved.

I'm guessing I would need a similar approach to the second question as with the first, treating it as a voltage division. I guess I could use that to show that my measured Rin actually represents Rin, but then I'm not sure how to actually figure out what the resistance is from that.

 

[slashrulez]

This might break a record for speed or something, but I figured it out on my own. Reading over my own question made the answer pretty simple, so this can be ignored/locked/deleted.