Op-Amp Application: Unity Gain Buffer
This circuit is an Unity Gain Buffer. This circuit has a highest input impedance of any OP-AMP circuit. This circuit has a less gain error that is equal to common mode rejection or the reciprocal of the amplifier open-loop gain. The multiplication of the open-loop gain and the differential input impedance parallel with common mode input impedance will result the input impedance of this circuit. Here is the circuit:
The source resistance gives bias current to the amplifier. The bias current will cause an error at input of the amplifier. The error can be occured because the voltage drop across the source resistance. So, to build this circuit we should choose low bias current amplifier such as LH102.
There are some note that must be concerned in building this circuit, they are when the amplifier common mode range is exceeded some amplifiers exhibit a latch-up mode, the amplifier common mode range may be limit the amplifier output swing, and for unity gain operation, the amplifier must be compensated. [Source: National Semiconductor Application Note]