This is an ultra high input impedance AC unity gain amplifier circuit. This circuit is used to get unity gain from ultra high input impedance AC. Here is the circuit:
Ultra High Input Impedance AC Unity Gain Amplifier circuit schematic
Because we can’t reduce input capacitance of this circuit, so we use the 2N4416 which has low capacitance. It is operated as a source follower with bootstrapped gate bias resistor and resistor . [Source: National Semiconductor Application Note]