The utility of op-amp increases considerably if it is used in a closed loop mode. The Closed Loop Configuration of Op amp is possible using feedback. The feedback allows to feed some part of the output back to the input. In linear applications the op-amp is always used with negative feedback. The feedback helps to control gain which otherwise drives op-amp into saturation.
The negative feedback is possible by adding a resistor as shoWn–in ihe Pig. —2:15:’ Called feedback resistor. The feedback is said to be negative as the feedback resistor connects the output to the inverting input terminal.
The gain resulting with feedback is called closed loop gain of the op-amp. Due to feedback resistance there is reduction in the gain. The closed loop gain is much less than the open loop gain and is independent of it. Most of the linear circuits use op-amp in a closed loop mode with negative feedback with R f. This is because, due to reduced gain, the output is not driven into the saturation and the circuit behaves in a linear manner.
In order to understand the operation of circuits with feedback, let us make two realistic assumptions about op-amp.