Oct 31, 2008

Basics of op-amp

The name operational amplifier was originally given to early high-gain vacuum tube amplifiers designed to perform mathematical operations of  addition,subtraction,multiplication,division,differentiation, and integration.They could also be  interconnected to solve differential equations.
               The modern successor of those amplifiers is the linear integrated-circuit op amp.It inherits the name,works at lower voltages,and is available in a variety of specialized forms.Today's op amp is so low in cost ,versatility,and dependability have expanded their use far beyond application envisioned by early designers.Some present day users for op amp are in the field of signal conditioning,process control,communications, computers, power and signal sources, displays, and testing or measuring systems.The op amp is still basically a very good high gain dc amplifier.
      One's first experience with a linear IC op amp should concentrate on its most important and fundamental properties. Accordingly, our objectives will be to identify each terminal of the op amp and to learn its purpose, some of its electrical limitations, and how to apply it usefully.

OP AMP TERMINALS
The circuit symbol for an op amp is an arrowhead that symbolizes high gain and points from input to output direction of signal flow.Op amps have five basic terminals:two for supply power,two for input signals, and one for output.Internally they are complex.It is not necessary to know much about the internal operation of the op amp in order to use it.The people who design and build op amps have done such an outstanding job that external components connected to the op amp determine what the overall system will do.
       The ideal op amp has infinite gain and infinite frequenc
y response.The input terminals draw no signal or bias currents and exhibit infinite input resistance.Output impedence is zero ohms, and the power supply voltages are without limit. We now examine the function of each op amp terminal to learn something about the limitation of a ral op amp.

Power supply Terminals

Op amp terminals labeled +v and -v identify those op amp terminals that must be connected to the power supply. Note that the power supply has three terminals: positive, negative, and power supply common. The power supply common terminal may or may not be wired to earth ground via the third wire of line cord. All voltage measurements are made with respect to power supply common.
         The power supply is called a bipolar or split supply and has typical values of +-15v.  Some op amps are now designed to operate from a single-polarity supply such as +15or +15v and ground. Note that the common is not wired to the op amp. Currents returning to the supply from the op amp must return through external circuit elements such as a load resistor RL. The maximum supply voltage that can be applied between +v and -v is typically 36v or +- 18v.



Output Terminal
The op amp’s output terminal is connected to one side of the load resistor RL. The other side of RL is wired to ground.  Output voltage v0 is measured with respect to ground. Since there is only one output terminal in an op amp, it is called a single-ended output. There is a limit to the current that can be drawn from the output terminal of an op amp, usually of the order of 5 to 10 mA. There are also limits on the output terminal’s voltage levels; these limits are set by the supply voltages and by the op amp’s output transistors.

Input terminals

There are two input terminals, labeled - and +.They are calleddifferential input terminals because output voltage Vo depends on the difference in voltage between them, Ed, and the gain of the amplifier, AoL. The output terminal is positive with respect to ground when (+) input is positive with respect to, or above the (-) input. When Ed is reversed to make the (+) input negative with respect to, or below, the (-) input, Vo becomes negative with respect to ground.
               We conclude that the polarity of the output terminal is same as the polarity of (+) input terminal with respect to the (-) input terminal. Moreover, the polarity of the output terminal is opposite or inverted from the polarity of the (-) input terminal. For these reasons, the (-) input I designated as inverting input and the (+) input the non-inverting input
            It is important to emphasize that the polarity of v
o depends only on the difference in voltage between inverting and non inverting inputs. This difference voltage can be found by
Ed= voltage at the (+) input-voltage at the (-) input.
Both input voltages are measured with respect to ground. The sign of Ed tells us

(1) the polarity of the (+) input with respect to the (-) input and

(2) the polarity of the output terminal with respect to ground.

This equation holds if the inverting input is grounded, if the noninverting input is grounded, and even if both inputs are above or below ground potential. Thus, if the polarity of Ed matches the op amp’s symbol, the output is voltage goes to +vsat. When the polarity of Ed is opposite the op amp’s symbol, the output voltage goes -vsat.