Voltage follower (thing)

                     
               .-------------.
               |    VCC      |
               |     |       |
               |  |\ |       |
               |  |  \       |
               +--|-   \     |
                  |      >---+----VO
   VI ------------|+   /
                  |  /
                  |/ |
                     |
                    -VCC

0 V ---------------------------------------o

V_O = output voltage
V_i = input voltage

This is a very straightforward application of the non-inverting amplifier. V_O will be equal to V_I up to the point of saturation of the amplifier (usually about 80% of V_CC). 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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