Solid state relay (CB8518E)

This circuit was found in a 90s Popular Electronics. Both the coupler and the MOSFET can be of any type.

Logical optical driver (CB7300E)
We found this circuit in a Motorola documentation from 1968. Phototransistors are common and...
Temperature to Frequency Converter (CB4905E)
This circuit was found in an application note of the Linear Technology LTC1145. The circuit can be used...
Heterodyne Mixer (CB4291E)
This is a typical 80's FM radio mixer circuit. The circuit was found in a documentation of the...