Found in American documentation from the 1980s, this circuit has a diode feedback. At the exit we have an indicator LED. The LM3900 is a transconductance operational amplifier.
The figure shows three circuits for using headphones in amplifiers with speakers that do not have...
In this charger, the charging current is basically limited by the LEDs. The transformer is a low...
This configuration is from an old integrated circuit manual. The circuit can be applied to modern...