A double balanced mixer has a DC detection circuit connected to receive the output of the mixer core. The detected level is applied to adjust the duty cycle of the transistors of the mixer core. In one example, this adjusting level is added to the local oscillator signals that are applied to the core. In another example, the level is applied to adjust the threshold of the transistors of the mixercore. In a further example, the duty cycle of the local oscillator signals is adjusted before it is applied to the mixer core Also provided is an input stage for a double balanced mixer core, which comprises a transistor having a low pass filter as a feedback circuit.