A circuit comprises a first set of serially-connected inverters comprising an input port, the first set of serially-connected inverters comprising a first subset of serially-connected inverters, the first subset of serially-connected inverters odd in number and comprising an input port and an output port; a first low-pass filter comprising an input port coupled to the output port of the first subset of serially-connected inverters, and an output port; a second low-pass filter comprising an input port coupled to the input port of the first subset of serially-connected inverters, and an output port; and a first differential amplifier comprising a first input port coupled to output port of the first low-pass filter, a second input port coupled to the output port of the second low-pass filter, and an output port coupled to the input port of the first set of serially-connected inverters.