Found in a 90's EDN, this circuit controls both the speed and the direction of rotation of a stepper motor. Relays can be replaced by MOSFETs, with circuit adaptations.
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...