This circuit for light modulated in frequency (FM) operating at 50 kHz was found in a 90’s Electronics Now. The circuit has an active filter. Modern equivalent transistors can be used.
This circuit converts square (rectangular) signals into triangular signals with typical 16 V...
The transistors of this circuit found in an English 1980s documentation may be the modern BC548...
In the figure we have an AND gate with the LM3900 operational transconductance amplifier. The...