This simple circuit found in a 1968 EEE was intended for data transmission over the telephone line. The components are from that time, some are uncommon today.
The figure below shows an AC transconductance amplifier with the National Semiconductor LM3900. The...
This common circuit using operational amplifiers was found in the 1993 issue of Radio Electronics Special...
This circuit provides a voltage at the output only when the input voltage exceeds a value that...