Electronic – OpAmp vs.emitter follower Transistor


I have heard that Op-amps are like special transistors with none of the drawbacks and all of the advantages. What properties make an operational amplifier a "better" amplifier then a biased transistor, and how are transistors constructed to make an op-amp?

Best Answer

A transistor is a single electronic element. By itself, it can do nothing. You use the term "biased transistor" which implies a transistor with other circuit components (usually resistors) to make a more useful entity, usually referred to as a circuit.

A transistor can be made to perform many functions such as amplification, rectification, filtering, etc. when it is combined with other circuit elements (resistors, capacitors, and even other transistors).

An operational amplifier is a specific combination of these circuit elements that forms a building block for the same functions plus many more. It is more capable than a single transistor because it contains many more circuit elements that allow for very flexible circuit configurations. It is so generally useful that it is made as an integrated circuit which is easily applied as a single circuit element and is much cheaper and smaller than it would be if built from discrete components.

An operational amplifier is the equivalent of many transistors and is thus able to perform much better than a single transistor (e.g. higher input impedance, lower output impedance, higher gain, differential inputs and/or differential outputs, etc.).

There are instances, however, where a discrete transistor can outperform an operational amplifier. One is noise performance. Special discrete transistors are available which have lower voltage noise than the best operational amplifiers and are used when the very lowest noise levels are needed. One such application is in sonar hydrophone amplifiers which have to work with signals at sub-microvolt levels.

In general, though, IC operational amplifiers out-perform single transistors and discrete operational amplifiers.