Found in American documentation from the 90's, this JFET stage has an input impedance of 500 M ohms. Bipolar transistors are general purpose types and the FET supports an equivalent.
This circuit is from a 1977 Signetics manual. BC107 transistors can be replaced with BC547 and...
Higher power for an operational amplifier can be obtained with a step with complementary output. The...
This circuit is from Radio Electronics from March 1977. The integrated circuit is from that time...