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Op Amps-Operational Amplifiers

Introduction to Operational Amplifiers

What is an operational amplifier (op-amp)?

An op-amp is a multi-stage , direct coupled, high gain negative feedback amplifier that has one or more differential amplifiers and its concluded with a level translator and an output stage.A voltage-shunt feedback is provided in an op-amp to obtain a stabilized voltage gain. Op-amps are available as Integrated Circuits (IC’s).

The main use of an op-amp is to amplify ac and dc input signals and was initially used for basic mathematical operations such as addition, subtraction, multiplication, differentiation and integration. Nowadays , the application of op-amp’s varies from ac and dc signal amplification to use in active filters, oscillators, comparators, voltage regulators, instrumentation and control systems, pulse generators, square wave generators and many more electronic circuits. For the design of all these circuits the op-amp’s are manufactured with integrated transistors, diodes, capacitors and resistors, thus making it an extremely compact, multi tasking, low cost, highly reliable and temperature stable integrated circuit. It is also designed in such a way that the external characteristics can be changed with the addition of external components like capacitors and resistors. Thus it can act as a complete amplifier with various characteristics.

Block Diagram Of Operational Amplifier (Op-amp)

 
stereo amplifier circuit BA5417
 

The op-amp begins with a differential amplifier stage, which operates in the differential mode. Thus the inputs noted with ‘+’ & ‘- ‘ . The positive sign is for the non-inverting input and negative is for the inverting input. The non-inverting input is the ac signal (or dc) applied to the differential amplifier which produces the same polarity of the signal at the output of op-amp. The inverting signal input is the ac signal (or dc) applied to the differential amplifier. This produces a 180 degrees out of phase signal at the output.

The inverting and non-inverting inputs are provided to the input stage which is a dual input, balanced output differential amplifier. The voltage gain required for the amplifier is provided in this stage along with the input resistance for the op-amp. The output of the initial stage is given to the intermediate stage, which is driven by the output of the input stage.In this stage direct coupling is used, which makes the dc voltage at the output of the intermediate stage above ground potential. Therefore, the dc level at its output must be shifted down to 0Volts with respect to the ground. For this, the level shifting stage is used where usually an emitter follower with the constant current source is applied. The level shifted signal is then given to the output stage where a push-pull amplifier increases the output voltage swing of the signal and also increases the current supplying capability of the op-amp.