How can we describe the op-amp which is shown in figure below?

 

The op-amp is acting as a comparator. Because there is no feedback, the op-amp will saturate based on the varying voltage applied at the negative terminal.

The green line is the bias point, which is determined by your potentiometer. Your pot is being used as a voltage divider: the voltage at the wiper will change based on the pot's position. For example, if your pot is set halfway between the ends of travel, then the voltage at the wiper will be 0V, or virtual ground.

Whenever your AC voltage goes above the bias point, your op-amp will approach the negative rail -V. But when your AC voltage goes below this point, the output will approach the positive rail +V. What voltage the output settles at depends on the op-amp. For the above example, it would go to +V when the ac voltage was below 0V, and would go to -V when above 0V.

 

What do you mean by "your op-amp will turn off"? It will always be active and the output will be near the positive rail or near the negative rail, neither of which can reasonably be said to be "off".

 

What do you mean by "your op-amp will turn off"? It will always be active and the output will be near the positive rail or near the negative rail, neither of which can reasonably be said to be "off".

I have corrected my error. See post #2.

 

Much better, though you might also revise the previous paragraph where you say that the voltage will determine when the op-amp is on and when it is off. Thanks.

 

Thanks for the catch.

 

we generally consider it bad form to provide homework answers until the question asker had presented as much of the answer he is himself capable of.
AND welcome to AAC

 

Yeah, I thought about that after I made my last post. I got too tied up in cleaning the explanation that was already given that I lost sight of the fact that the question is quite possibly a direct homework problem even though it reads like it might just be an inquiry. All-in-all, one of the softer fouls in this regard and hopefully the OP learned the important info, though the fact that they haven't been back, except possibly to collect their free solution, doesn't bode too well on that front.

 

Point well taken. Got a little too excited about helping someone out and missed the whole point of the Homework Help section. I will keep this in mind in the future.