Found in a 1968 EEE, this circuit accepts frequencies from 1 to 1000 Hz and control voltages from 0.1 to 15 V. The operational amplifier is from the era and can be replaced by modern equivalents.
This fully passive circuit was found in documentation from the 1970s. The circuit uses an optical...
This circuit of old documentation is based on an operational amplifier that is no longer used...
This circuit was found in a Japanese publication from the 1960s. It uses germanium transistors and is powered...