What is Op-Amp (operational amplifier) ? | operational amplifier definition

Image may be copyright

Op-Amp:

The operational amplifier is one of the most useful and important components of analog electronics. They are widely used in popular electronics. Their primary limitation is that they are not especially fast: The typical performance degrades rapidly for frequencies greater than about 1 MHz, although some models are designed specially to handle higher frequencies.

The primary use of op-amps in amplifier and related circuits is closely connected to the concept of negative feedback. Feedback represents a vast and interesting topic in itself. We will discuss it in rudimentary terms a bit later. However, it is possible to get a feeling for the two primary types of amplifier circuits, inverting and non-inverting, by simply postulating a few simple rules (the \golden rules"). We will start in this way and then go back to understand their origin in terms of feedback.

The Golden Rules:

The op-amp is, in essence, a differential amplifier of the type we discussed in Section 5.7 with the refinements we discussed (current source load, follower output stage), plus more, all nicely debugged, characterized, and packaged for use. Examples are the 741 and 411 models which we use in the lab. These two differ most significantly in that the 411 uses JFET transistor sat the inputs in order to achieve a very large input impedance (Z_in~10⁹Ω ), whereas the 741 is an all-bipolar design (Z_in~10⁶Ω ).

The other important fact about op-amps is that their open-loop gain is huge. This is the gain that would be measured from a configuration like Fig. 29, in which there is no feedback loop from the output back to the input. A typical open-loop voltage gain is ~ 10⁴-10⁵. By using negative feedback, we throw most of that away! We will soon discuss why, however, this might actually be a smart thing to do.

Image may be copyright

The golden rules are idealizations of op-amp behavior but are nevertheless very useful for describing the overall performance. They are applicable whenever op-amps are conjured with negative feedback, as in the two amplifier circuits discussed below. These rules consist of the following two statements:

1.     The voltage difference between the inputs, V₊- V_-, is zero. (Negative feedback will ensure that this is the case.)

2.     The inputs draw no current. (This is true in the approximation that the Z_in of the op-amp is much larger than any other current path available to the inputs.)

When we assume ideal op-amp behavior, it means that we consider the golden rules to be exact. We now use these rules to analyze the two most common op-amp configurations.