Patent ID: 7295820

Claim:
An amplifier circuit for amplifying an input signal, comprising: (a) an amplifier having an input and an amplifier output with a DC offset; (b) a coupling capacitor having first and second capacitor plates, the first plate being connected to the output of the amplifier; (c) switching means arranged to switch the circuit between a first mode, in which a DC reference voltage is isolated from the amplifier and capacitor so that the amplifier amplifies the input signal applied to the amplifier input, and the capacitor blocks the DC offset, and a second mode, in which the DC reference voltage is connected to the amplifier input and also to the second capacitor plate so that the voltage across the capacitor plates is forced to the DC offset; and (d) a detector to detect a change in a parameter of the circuit, or a change in an input thereto, which causes a change in the DC offset so that there is a new DC offset, and to cause the switching means to switch momentarily from the first mode thereof to the second mode thereof, to adjust the voltage across the capacitor to the new DC offset, before causing the switching means to switch back again to the first mode.