Operational amplifiers are used in a wide variety of applications in electronics. Some of the most common applications are: as voltage follower, selective reversing circuit, current-to-voltage converter, active rectifier, integrator, a wide variety of filters and a voltage comparator. This is not a complete list; in fact, we have a quarter of the NSCC curriculum where we do nothing but study the various applications that are used with operational amplifiers and integrated circuits. The first thing we are going to see is the voltage follower. Since the full output is fed back, the gain equals one. Here we have an input to the non-inverting input and we observe that all output is fed back so A is going to match one in this case. This is similar to what we look at with a follower emitter; we had the follower of emitters a little like this. We had an exit that came out here and the entrance looked like the exit and are basically the same size.
An operational amplifier commonly known as op-amp is a two-input single output differential voltage amplifier characterized by high gain, high input impedance and low output impedance.
The operational amplifier is called this because it has its origin in analog computers, and was used mainly to perform mathematical operations. Depending on your feedback circuit and polarization, an op-amp can be made to add, subtract, multiply, divide, deny and interesting even perform calculation operations such as differentiation and integration.
Today, operational amplifiers are very popular building blocks in electronic circuits. Optical amplifiers are used for a variety of applications such as AC and DC signal amplification, filters, oscillators, voltage regulators, comparators and in most industrial and consumer devices. Operational amplifiers show little reliance on temperature changes or manufacturing variations, making them ideal building blocks in electronic circuits.
![[Image: 1.-Basic-Operational-Amplifier-Circuit.jpg]](http://www.electronicshub.org/wp-content/uploads/2015/01/1.-Basic-Operational-Amplifier-Circuit.jpg)
The basic circuit of an operational amplifier is as shown in the figure above. An operational amplifier has a differential amplifier input phase and a follower emitter output stage. Practical op-amp circuits are much more complicated than the basic op-amp circuit shown above.
The transistors Q1 and Q2 form a differential amplifier, where the difference input voltage is applied to the base terminals of Q1 and Q2. Transistor Q3 functions as an emitter follower and provides a low output impedance.
The output of the basic circuit op-amp VOUT is given as,
VOUT = VCC - VRC - VBE3
VOUT = VCC - IC2RC - VBE
Where, VRC is the voltage across the resistor RC and VBE3 is the base-emitter voltage of transistor Q3.
Suppose that the transistors Q1 and Q2 are matched transistors, that is, they have equal VBE levels and equal current gains. If both transistor base terminals are connected to ground, the emitter currents IE1 and IE2 are the same and IE1 and IE2 flow through the common resistor RE. The emitter current is given by the ratio,
IE1 + IE2 = VRE / RE
If the bases Q1 and Q2 are grounded,
0 - VBE-VRE + VEE = 0
ie VRE = VEE - VBE
Therefore, IE1 + IE2 = (VEE - VBE) / RE
When a positive voltage is applied to the non-inverting input terminal, the base of Q1 is pushed upward by the input voltage and its emitter terminal follows the input signal. Since the emitters Q1 and Q2 are connected together, the emitter of Q2 is also drawn by the positive input at the non-inverting terminal. The base of Q2 is grounded, so the positive voltage at its emitter causes a reduction in its base-emitter voltage VBE2. The reduction in VBE2 causes the emitter current IE2 to decrease and, consequently, IC2 is also reduced.
It can be seen that a positive input on pin # 3 gives a positive output, hence the non-inverting input terminal name.