We found this circuit in a 1977 factory documentation. The operational amplifiers can be 741 and the symmetrical power supply. Capacitors can be changed for operation at other frequencies.
This circuit uses integrated 555 and an amplification step with transistors, as we can see in the...
This circuit was found in Radio Electronics from the 90s. It converts capacitance into frequency...
This circuit is from the Linear Applications Handbook of the National Semiconductor of 1994. The...