72-operational-amplifier

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Operational amplifier or op amp is linear integrated device with property of almost ideal amplifier.

Ideal operational amplifier is a three-terminal device with differential input that consists of inverting and non-inverting terminal and the output. Scheme of ideal operational amplifier is given in Figure bellow.

operational-amplifier

Transfer characteristic of ideal op-amp is given.

op_amp01

If voltage on non inverting terminal is slightly higher then the voltage on inverting terminal output of ideal operational amplifier goes high to saturation. This is due to property of ideal operational amplifier that voltage gain is infinite. On the other hand, if differential amplifier is slightly less then zero, output of ideal operational amplifier goes low to negative saturation. Ideal op-amp has also infinite CMRR (Common Mode Reduction Ratio), slew rate and input impedance. Output impedance of ideal op-amp, as well as voltage and current offset is very low. This fact makes circuit analysis with op-amp very simple.

Real operational amplifier has voltage gain very high (>106), but less then infinite. CMRR is also high (>107), high input impendence and low output impendence. What is most troublesome with real operational amplifier is that voltage and current offset is different from zero. Actually, offset depends of transistor type in input differential stage – if input transistors are JFET, voltage offset is high, and slew rate is high too. If input transistors are bipolar, voltage offset can be low, and slow rate is also smaller. This fact should be taken into account while designing amplifiers with op-amp.

Functionally, operational amplifier consists of:

All this stages are integrated in single amplifier component called operational amplifier or op-amp.

Operational amplifier is widely used for analog signal processing schemes such as:

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