This circuit is an ultra high input impedance AC unity gain amplifier and used to get unity gain from ultra high input impedance AC. Here is the schematic :
Ultra-High input impedance AC unity gain amplifier circuit diagram
2N4416 IC is used to reduce the input capacitance. The IC has low capacitance. It’s operated as a source follower with bootstrapped gate bias resistor and the resistor.
Source : National Semiconductor Application Note