This tube circuit was found in the magazine 73 for radio amateurs in an issue from the 1980s. Note the transformer used to power the circuit.
Found in a Motorola documentation from the 70s, this circuit makes use of transistors of the time....
This circuit was found in a 1968 General Electric documentation. It has an input impedance of 2 M ohms...
Found in a 1980s documentation, this circuit makes use of components from that time. Modern...