US Patent 7535305 Quiescent current detecting circuit for class AB amplifier

An operational transconductance amplifier is configured with an asymmetric output current capability. When current sunk from the output exceeds a rated load, the normally equal input voltages diverge. A comparator coupled to the OTA inputs senses when the output current drain exceeds the rated load and changes state when the OTA input voltages diverge. When used in conjunction with a current mirror transistor of known proportion, current through an output transistor, such as quiescent current through the output stage of a class A/B amplifier, can be accurately detected. A current source in parallel with the OTA output can be used to offset the current sensing trigger point.