With this circuit it is possible to connect a comparator to a power stage via two wires. The circuit can be used as a voltage monitor, for example.
This is an old test circuit for integrated TTL counters, feeding a Nixie tube, common at the time....
This old oscillator using germanium transistors has a high impedance headphone as inductor to...
This preamp with several inputs using germanium transistors from the time was found in Japanese...