Found in Motorola documentation from the 90's, this circuit has a high input impedance. The power supply must be symmetrical. The circuit can be implemented with modern operational amplifiers.
The figure shows how to implement an alarm buzzer in an LM3914 to warn about crossing the scale...
This circuit can act as a shield being controlled by a microcontroller by pin 1 or 2. The...
This circuit was obtained from a 1980's QST magazine. The circuit can generate 1 to 15 MHz signals...