This is a circuit for basic unity gain buffer circuit. The circuit gives the highest input impedance of any operational amplifier circuit. Input impedance is equal to the differential input impedance multiplied by the open-loop gain, in parallel with common mode input impedance. Here’s the figure of the circuit;
The gain error of this circuit is equal to the reciprocal of the amplifier open-loop gain or to the common mode rejection, whichever is less. Input impedance is a misleading concept in a DC coupled unity-gain buffer. Bias current for the amplifier will be supplied by the source resistance and will cause an error at the amplifier input due to its voltage drop across the source resistance. Since this is the case, a low bias current amplifier such as the LH1026 should be chosen as a unity-gain buffer when working from high source resistances.
VOUT = VIN
R1 = RSOURCE
The cautions to be observed in applying this circuit are three, the amplifier must be compensated for unity gain operation, the output swing of the amplifier may be limited by the amplifier common mode range, and some amplifiers exhibit a latchup mode when the amplifier common mode range is exceeded. The LM107 may be used in this circuit with none of these problems or, for faster operation, the LM102 may be chosen. [Circuit diagram source: National Semiconductor Application Notes]
