The most fundamental circuitry for operational amplifier is the differential amplifier, or difference amplifier. Some people confused the differential amplifier with differentiator which deals with the calculus’s differentiation operation.
Differential amplifier amplifies the differential voltage vd which is the voltage difference between V+ and V–, called non-inverting and inverting inputs. Difference amplifier just perfect the operation to calculate the difference of the two input voltages VA and VB to have Vo = VA-VB or weighted difference Vo = A1V1-A2V2. Differential amplfier uses this equation: Vo = A Vd where Vd = V+ – V–.
To understand the basics of operational amplifier or simply differential amplifier, the SUPERPOSITION principle in electronics should be the first thing to know:
Because the differential amplfier needs two inputs for their difference, so superposition is the way to see it. Now let’s examine the basic differential amplifier with two inputs, one at the base and one at the emitter. Input at the base accounts for the amplifier called common emitter amplifier and the output at the collector is inverted, ie, negative of applied input. Input at the emitter accounts for the amplifier called common base amplifer and the output at the collector is not inverted, ie, in phase with the applied input, so called non-inverting.
In order to elliminate or reduce the difference between two input characteristics such as their inputs’ resistances (capability of drawing current into the amplifier), a pair of mirror transistors or match transistors are used. Here is the diagram:
Most operational amplifier like LM741 is designed with this input circuitry and then connect the output to various stages of amplification to achieve a gain of over 200,000.
Here is a pin-out diagram for the 741 operational amplifier each of the pins has its purpose: