OP amp use as Non-inverting Amplifier

No current flows into the input, Rin = ∞

The output adjusts to bring Vin- to the same voltage as Vin+. Therefore Vin- = Vin and since no current flows into Vin- the same current must flow through R1 & R2. Vout is therefore VR1 + VR2 = Vin- + IR1 = Vin- + (Vin/R2)R1
Vout = Vin (1+ R1/R2).

Application hint:  When dealing with larger signals keep in mind that the output can't exceed the power supply voltage (i.e. if the op-amp is powered from +/- 15V and have a one volt input and a gain of 20 you won't get 20V he output.  The output will most likely stop a few volts
below the supply rail (around 13V).  There are special op-amps designed to handle inputs and outputs that go all the way to the power supply rail but these op-amps usually only operate at lower voltages (i.e. 0-5V).

OP amp use as inverting amplifier

  Because no current flows into the input pins there can't be a voltage drop across ground connection.  Vin+ is therefore at 0V (this is called a virtual ground).  The output will adjust such that Vin- is at zero volts.  This makes Rin = R1.The current through R1 & R2 have to be the
same since no current goes into the input pins.Therefore I = Vin/R1.  Vout = Vin+ - IR2 =0 - (Vin/R1)R2.  Therefore Vout = -Vin(R2/R1)
The negative sine is because the current flows from the input to the output where as in the earlier examples the current flows from the output to the input.

Application hint: Why not connect Vin+ directly to ground? Actually, many people do and the circuit works fine. The reason to have R1||R2 is because real op-amps aren't perfect and draw a small bias current into both inputs. By adding R1||R2 the voltage drop at the + input is offset by the same amount as the voltage drop at the - input. This is called input offset voltage. edited by RDP

OP amp use as inverting amplifier

  The operational amplifier is arguably the most useful single device in analog electronic circuitry. With only a handful of external components, it can be made to perform a wide variety of analog signal processing tasks. It is also quite affordable, most general-purpose amplifiers selling for under a dollar apiece. Modern designs have been engineered with durability in mind as well: several "op-amps" are manufactured that can sustain direct short-circuits on their outputs without damage.

  One key to the usefulness of these little circuits is in the engineering principle of feedback, particularly negative feedback, which constitutes the foundation of almost all automatic control processes. The principles presented here in operational amplifier  circuits, therefore, extend well beyond the immediate scope of electronics. It is well worth the electronics student's time to learn these principles and learn them well.

The most common Op-Amp is the 741 and it is used in many circuits.

The OP AMP is a ‘Linear Amplifier’ with an amazing variety of uses. Its main purpose is to amplify (increase) a weak signal - a little like a Darlington Pair.

The OP-AMP has two inputs, INVERTING ( - ) and NON-INVERTING (+), and one output at pin 6.

This is the diagram of circuit using transisters which do the job of OP Amp. It is very advanced and big cost for production.

Transformers

  The transformer is the most common application of the concept of mutual inductance. In the transformer, the effect of the mutual inductance is to cause the primary circuit to take more power from the electrical supply in response to an increased load on the secondary. For example, if the load resistance in the secondary is reduced, then the power required will increase, forcing the primary side of the transformer to draw more current to supply the additional need.