Based on an OTA LM3600 operational amplifier, this circuit was found in the July 1988 issue of Radio Electronics magazine. The range is 6 Hz to 6 kHz and can be changed.
Found in a 1966 documentation, this “coin finder” uses components from the era operating at 2.1...
This circuit is from a documentation on the MC1539 from Motorola and may, however, be implemented...
The 747 is an old operational amplifier, but the configuration applies to modern operatives. In...