The Feedback Technique

The precision and flexibility of the operational amplifier is a direct result of the use of negative feedback. Generally speaking, amplifiers employing feedback will have superior operating characteristics at a sacrifice of gain.

                  

                                   Eoutput  = - RO

                                                Einput       RI                  

With enough feedback, the closed loop amplifier characteristics become a function of the feedback elements. In the typical feedback circuit, figure 1, the feedback elements are two resistors. The precision of the “closed loop” gain is set by the ratio of the two resistors and is practically independent of the “open loop” amplifier. Thus, amplification to almost any degree of precision can be achieved with ease.

Notation and Terminology-        

    

Texas Instruments employs the industry standard operational amplifier symbols  Power pins are often omitted from the schematic symbol when the power supply voltages are explicit elsewhere in the schematic.  Some op amp symbols also include offset nulling pins, enable / disable pins, output voltage threshold inputs, and other specialized functions

  
Symbol (a) is a buffer op amp

Symbol (b) is a differential input, single ended output op amp. This symbol represents the most common types of op amps, including voltage feedback, and current feedback.  It is often times pictured with the non-inverting input at the top and the inverting input at the bottom.

Symbol (c) is a differential input, differential output op amp. The outputs  can  be thought of as “inverting” and “non-inverting”, and are shown across  from the opposite polarity input for  easy completion of feedback  loops on schematics.