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500 megohm input impedance amplifier
This circuit uses a 2N3819 JFET and a 2N3904 bipolar to form a "hybrid" high input impedance source follower amplifier. The input impedance of this circuit is about 500 megohms shunted by 10 picofarads. Offset biasing is applied through a voltage divider formed at the resistor R1-R2 junction. Network R4, Q2, D1, and D2 makes "source load" Q2 act as a constant-current generator with a high output (collector) impedance, which causes a quiescent current of about 1 milliampere to flow in Q1. As a result, Q1 functions as a source follower, and the collector of Q2, acting as a source load, appears as a high impedance. The voltage gain of this circuit is about 0.99.