05-10-2012, 05:23 PM
OPERATIONAL AMPLIFIER
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Introduction
Operational Amplifier is popularly known as “Op-Amp”. It is an integrated circuit (IC) amplifier consisting of many transistors, resistors and a few capacitors within it. It has acquired its name from the fact that it performs many mathematical operations such as differentiation, integration, summation etc. in addition to amplification. It has very high gain, high input impedance and low output impedance.
Circuit Symbol and Terminals
The circuit symbol for an operational amplifier is shown in figure 1. It shows there are two input terminals, one output, and two supply terminals. The terminal with negative sign (-) is know as inverting terminal and the terminal with plus sign (+) is known as non-inverting terminal.
Parameters of Op-Amp
Every Op-Amp has a maximum input voltage range and a maximum output voltage range. Due to bias voltage change, supply voltage variation and AC ripple on the supply voltage, unwanted output can occur. There is a limit on how fast an Op-Amp output can be made to change and also there is limit on the highest signal frequency that may be employed. What follows here is the discussion on various parameters of Op-Amp.
Input offset voltage
Even when both the inputs of an Op-Amp are equal, some voltage may appear at the output. This is known as output offset voltage. To make this voltage zero, some amount of voltage is fed at one of the inputs. This voltage is known as input offset voltage. The typical input offset voltage for 741 Op-Amp is 1 mV.
Input offset current
The difference in two input currents is known as input offset current. The typical input offset current for 741 Op-Amp is 20 nA.
Applications of Op-Amp
Since the Op-Amp has very high gain, it is not suitable for use in open loop condition in many cases. With suitable feedback it is used as voltage follower, Inverting Amplifier. Non inverting Amplifier, differentiator, Integrator, Summing Amplifier, Precision Rectifier, Active Filter, Log Amplifier, Sample and hold circuit, Precision Peak Detector, Comparator, etc. Some of these are discussed below.
Voltage Follower
The voltage follower circuit is given in figure 5. The inverting terminal has been directly connected to the output terminal. The input Vi has been fed to the non-inverting terminal. The output voltage follows the input voltage.