transconductance of gm the three switches 308 310 and 316 are controlled by a clock generator 322 as described in more detail below with reference to fig3 b in many applications difference amplifier 300 is constructed on an integrated circuit typically comprising part of an analogue to digital converter the skilled person will understand that standard building blocks may be employed for the various elements and the transconductance amplifier may be implemented for example using a long tailed bipolar or mos pair referring now to fig3 b the circuit has two clock phases phi1 and phi2 fig3 b shows the respective statuses 350 352 354 of switches s 1 308 s 2 310 and s 3 316 during these two clock phases as well as respective voltages 356 358 at node y 317 and node x 318 during clock phase phil switch s 3 316 is closed to connect together the output and the inverting input of transconductance amplifier 312 so that the voltage at node x 318 settles to the same voltage as that on the inverting input 320 of transconductance amplifier 312 that is vgnd this is because node x 318 is a virtual earth although the skilled person will understand that in this context the voltage at node x is substantially the same as the voltage on non inverting input 320 but in some embodiments this.