This filter was found in an 80's English publication. However, it can be implemented with modern operating systems. The circuit can tune frequencies in a range from 10,000 to 1.
Found in Philips' 1968 documentation, this circuit serves as a 6.3 V reference for high stability. |...
The circuit shown uses an operational transconductance amplifier such as the LM3900. Transistors...
This circuit is from a documentation on the MC1539 from Motorola and may, however, be implemented...