Trouble With Inverting Op-Amp

[Kimly]

R1=1kΩ, R2=10kΩ, R3=910Ω
Determine the effect on the output if the circuit shown has the following fault (one fault at a time)
a. Output pin is shorted to the inverting input.
b. R3 is open.
c. R3 is 10kΩ instead of 910Ω.
d. R1 and R2 are swapped.

I have no idea what the answer to "a" is. I believe the answer to "b" is the output signal would be in saturation. I believe the answer to "c" is there would be no change. Finally I believe the answer to "d" is the closed loop gain would decrease significantly. Please let me know if any of my answers are correct and what the solution to "a" might be. Any help with this would be greatly appreciated.

Thank you

[Jerome Trent]

• For output pin shorted to input pin, this would give you closed loop feedback of 100% of your output signal. This would mean that R2, the feedback resistor is equal to zero. Here is your gain formula for a closed loop inverting op amp.

If the gain equals zero, then the output voltage should be zero. Of course if this occurred in your circuit, your op amp would probably fry.

• For R3 open, we would have to look at the differential amplifier on the input of the op amp to see what happens.

R3 would be the base resistor of Q1 in this differential amp using inverting input (base of Q2). So the question is what would happen if the base resistor of Q1 were to be open. This would mean no Q1 base current. With no Q1 base current there would be no emitter or collector current. It would be just as if Q1 and its base and collector resistors were not there. Take them out and what do you have left?

This is nothing more than a common emitter amplifier. So you should have an inverted output but you would not have the gain and bandwidth that you would expect if the circuit was operating normally.

• If the value of R3 is changed from 910 ohms to 10K ohms, what would happen? Well R3 is not used in calculating the gain of the circuit and the resulting output voltage. So the gain and output voltage should be essentially unchanged but the CMRR (Common Mode Rejection Ratio) is changed by an imbalance in the input impedance of + verses - in our op amp. The inverting input has an impedance of about 910 ohms (R1 in parallel with R2). So the non-inverting input impedance needs to be about 910 ohms which is what we started with. Changing that to 10K ohms would cause the non-inverting input impedance to be higher. This in turn would cause the CMRR to be lower and our output would be susceptible to more noise in it.

• If we swap R1 and R2, what happens? R1 is the input resistor and R2 is the negative feedback resistor. They are both used in calculating the gain of the circuit, so our gain would change. Making R2 the input and R1 the negative feedback we would get: