This is a circuit suggested by Linear Technology in a 1980s documentation. The integrated circuit is from the era. Observe the input and output voltage.
In this interesting circuit from Popular Electronics, the output voltage is the square root of the...
We found this circuit in a documentation from the 90s. It lets frequencies around 1 kHz pass,...
This is one of many configurations with a 4-input mixer transistor that can be found on this...