This circuit was found in a 90s Electronic Engineering. The quadruple operational amplifier can be replaced by an equivalent. RL determines the tested resistance range.
The presented circuit is suggested by Texas Instruments (www.ti.com) in its Application Report...
This circuit has the muting action delayed by the action of C5. The circuit is from the SGS linear...
This circuit from a 1970s General Electric publication applies to motors of the series winding type....