This papers presents a measurement technique for the noise figure of on-wafer differential amplifiers using 4-port network analyzers. The approach is based on the determination of the correlation of output noise waves in terms of the 4-port S-parameters and of the output noise powers. The measurement setup is simple as it does not require any hybrid coupler or calibrated noise source. Measurement results of an on-wafer differential LNA demonstrate the validity of the new technique.