The proposed mixer is designed around four current-driven transconductance amplifiers contained in an SSM2024 from Precisio
n Monolithics. To obtain a low offset and high control rejection, the four inputs should have an impedance to earth of about 200 . These impedances are obtained from resistors R5 through R8, which also form part of a potential divider at each input. With the values in the diagram, the nominal input signal is 1V (0 dBV). Distortion at that level is about 1%; at lower levels, it is not more than 0.3%. The amplification of the current-driven amplifiers (CDAs) is determined by the current fed into the control inputs. These inputs form a virtual earth so that calculating the values of the bias resistors (to transform the inputs into voltage-driven inputs) is fairly simple. The output currents of the amplifier are summed by simply linking the output pins. The current-to-voltage converter, IC2, translates the combined output currents into an output voltage. The value of R13 ensures that the amplification of IC2 is unity.