# Questions with op-amp building block

#### uofmx12

Joined Mar 8, 2011
55
http://i812.photobucket.com/albums/zz41/uofmx12/84_86.jpg

Here I have two figures I have a few questions about using a gain block with two op-amps. Any help on any of the questions will be appreciated.

1) Figure 8.4, What portion of the output signal is being used in the feedback? And why would it be called the feedback factor?
2) Figure 8.4, what happens when R1 is shorted and what is the feedback factor? And R2 is shorted, then what is the feedback factor?
3) Figure 8.6, What are 2 estimates of amplifiers u's input resistance.

#### MrGeorge

Joined Aug 16, 2011
5
1.
R1 and R2 for a potential divider. In this instance, the formula for the voltage in between the resistors is: R1/(R1+R2) * Vin = 0.99 * Vin. This is called feedback factor because only a proportion is being fed back into the circuit.

2.
If R1 was shorted then the inverting input of the OpAmp would be earthed, thus no voltage, 0% feedback. If R2 was shorted then the output would be connected directly to the inverting input hence 100% feedback.

3.
I'm not sure.... If I had to guess I'd say 100k but that'd only be a shot in the dark!

I'd wait for someone else to double check all this but I hope it helps!