Power supply units for PCs and other devices typically contain multiple DC power supplies. The power lines provided by the power supply unit are called power rails.
The entire range from the maximum voltage (VCC) of a power supply line to its minimum voltage (GND or minimum negative voltage, VEE) is called rail-to-rail.
An op amp with a common-mode input voltage range that nearly spans the GND-to-VCC or VEE-to-VCC range is called a rail-to-rail input op amp (or full-swing op amp).
The output stage can also be rail-to-rail. An op amp with rail-to-rail input and output voltage ranges is called a rail-to-rail input/output op amp.
The differential input stage of most conventional op amps consists of P-channel MOSFETs. When the input voltage approaches VDD, an op amp with a differential input pair consisting of P-channel MOSFETs cannot provide a normal output because the current source enters the linear region and the gain is reduced.
When the input voltage approaches GND, an op amp with a differential input pair consisting of N-channel MOSFETs cannot provide a normal output.
In contrast, an op amp with a differential input pair consisting of P-channel MOSFETs can provide a normal output even when the input voltage approaches GND.
The input stage of a rail-to-rail input op amp consists of an N-channel differential MOSFET pair and a P-channel differential MOSFET pair connected in parallel, covering almost the entire voltage range from GND to VDD.
Figure 1: Differential Input Stage Consisting of a Pair of N-channel MOSFETs
Table 1: Common-Mode Input Voltage Range of an Op Amp with a Differential Input Pair Consisting of P-channel MOSFETs
Figure 2: Common-Mode Input Voltage Range of Op Amps with Different Input Configurations
Figure 3: Simplified Equivalent Input Circuit of a Rail-to-Rail Input Op Amp
BR 36859 
US 14631 
GB 3410 
MX 3233 
AR 3090 
VN 2320 
CN 2253 
DE 2203 
