Remember in the Digital to Analog section when Operational Amplifiers were discussed? In all the examples there, the output of the amplifer was connected back to the inverting input, or feedback was employed to use the high gain of the amplifier to accurately carry out some mathematical function or operation. The transfer function of the amplifier was Vout = A (V+ - V-), with A large. Recall that the output of an operational amplifier can be no lower than the potential coming from the negative side of the power supply nor can it be higher than the positive side of the supply (there are engineering details that prevent going to exactly the "power supply rails," but don't let those details obscure the big picture). Suppose an operational amplifier is run on a +5 V power supply (negative supply provided by 0 V).
(click shows answer, mouse exit from button hides answer)