a current sensing circuit for sensing the current through a main switch , such as the pmos or nmos switches of a switching regulator , is disclosed . the circuit comprises a mirror switch , said mirror switch being substantially similar to said main switch but with a smaller aspect ratio , a difference amplifier for ensuring that the voltage across said first leg and across said second leg are substantially equal and thereby to derive from said mirror switch a sensing current nominally equal to a current flowing in said main switch divided by a sensing ratio , a current source for producing a quiescent current in said difference amplifier and a compensatory device for compensating for said quiescent current such that said current sensing circuit can sense currents in the main switch which are smaller than the quiescent current multiplied by the sensing ratio . the compensatory device may be one or two switches essentially similar to the mirror switch .