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rail to rail output voltage

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The ability of the output pin to reach voltages very close to V+ (the positive supply pin) while sourcing current; and to reach very close to V- (the negative supply pin, which might be 0V if not using split supplies) when sinking current.

Older opamp designs (like the 741 are the opposite of rail to rail. They have a hard time pulling within 2.5V of either supply rail. Other opamps can pull close to V-, but not to V+; the LM324/358 comes to mind.
 
Thank you very much for the reply. can you please tell me why some design have hard time pulling to some voltage. what exactly is pulling to some voltage. Atleast suggest me the links to the material.
 
Most opamps use an NPN darlington emitter-follower to pull the load voltage positive. A darlington is two transistors, an emitter-follower has the output from the emitter. The base of the first transistor in the darlington might go close to the positive supply but its emitter will be 0.7V (or a little more) less than its base voltage and the second transistor will also have a 0.7V (or more) voltage drop. Then the maximum output high voltage (with no load) is about 1.5V less than the supply voltage.
The output going low is the same but uses a PNP darlington emitter-follower.
I attach the schematic of a darlington emitter-follower.

Most rail-to-rail opamps use Mosfets at the output that can drive the output to the supply voltages when there is no load.
 

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If the output of the op amp is made of FETs, these FETs will have an ON resistance. When turned fully on, they look like a resistor up to the top supply or a resistor down to the bottom supply. If you apply a load the output of the op amp, you will pull current through these 'resistors' and this will cause a voltage drop across the FETs, thus reducing the output voltage. this is why most op amps that are specified as rail to rail are also specified when driving a certain (fairly light) load. The more you load them, the worse the voltage drop from the top rail is
 
Rail to rail output op amps also typically have rail to rail inputs.
That means the op amp can operate with the input common-mode voltage anywhere between the plus and minus rails.

The single-supply op amp is another type. They have an output and common-mode voltage that will go to the negative rail but not all the way to the positive rail.
The common LM324 is an example of that.
 
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in simple terms can I say that when opamp reaches saturation it cannot reach to supply voltage but little less.
Most opamps have darlington ordinary transistors as emitter-followers at the output that never saturate (the emitter follows the base with a 0.75V voltage drop and a darlington is two transistors with two base-emitter voltage drops). Therefore the maximum output high is about 1.5V less than the supply voltage and the minimum output low is about +1.5V when there is no load.
Rail-to-rail outputs use Mosfets that turn on hard and produce almost the entire supply voltage when there is no load.
 
CMOS outputs have different RdsOn and tend to be higher , thus lower current capability than
COMPLEMENTARY Darlington BJT's outputs in so called Bipolar Op Amps
 
in simple terms can I say that when opamp reaches saturation it cannot reach to supply voltage but little less.

In general, yes. Rail-to-rail op-amps, however, are designed to avoid this, so that the output can be (practically) as high as the positive supply voltage and as low as the negative supply voltage. That's about as simple as I can put it.
 
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