This circuit was obtained in a 1975 documentation. The maximum frequency depends on the diodes and the FET. Type 1N34 diodes and a FET BF245 can be used for frequencies up to 100 MHz.
This is the circuit of a commercial amplifier sold in Japan in the 1960s. The circuit for hearing aid...
This circuit with variable capacitance diode (varicap) is from a publication from the 90s. The...
In the figure we have a simple pulse generator using a unijunction programmable transistor. The...