.-------------.
| VCC |
| | |
| |\ | |
| | \ |
+--|- \ |
| >---+----VO
VI ------------|+ /
| /
|/ |
|
-VCC
0 V ---------------------------------------o
VO = output voltage
Vi = input voltage
This is a very straightforward application of the non-inverting amplifier. VO will be equal to VI up to the point of saturation of the amplifier (usually about 80% of VCC). See Non-inverting amplifier for details on why it works.
The main reason to use one of these widgets is to sit between a source which can only provide a very small amount of current and a device which requires a moderate amount of current to operate. A good example is a circuit involving a thermocouple (which, although it can provide a voltage of several volts, can only output a miniscule current). Because the op-amp's input resistance is very high, the op-amp only draws a very small current from the thermocouple. However, since the op-amp itself can draw current from its own power supply, it will be able to supply the next device in the chain (for instance, a MOSFET or a logic chip) with the required amount of current for operation.
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