This circuit was found in a 90s Electronic Design. It uses integrated circuits that are not common today.
The transistor can be the BD135 and this circuit, found in a 1978 publication, can be powered by...
This second-order filter was found in a 1978 publication and can use common operational amplifiers like...
This circuit has a sensitivity of 2uW. We found it in documentation from the 1980s. The integrated...