Source Degenerated Derivative Superposition Method for Linearizing RF FET Differential Amplifiers

The second-order interaction effect in a field-effect transistor (FET) differential amplifier is analyzed using the Volterra series analysis method. The analysis results reveal that the second-order interaction is inherent in the fully differential amplifier structure, and thus can never be cancelled out. In contrast, it is found that the second-order interaction is possibly cancelled out by adding source degeneration impedance in the pseudodifferential amplifier (PDA) structure. In addition, the second-order interaction cancellation condition in the PDA can be made more robust and wider over the input signal swing by adopting the derivative superposition (DS) method. By combining the second-order interaction cancellation technique and the DS technique, a differential source degenerated DS method is proposed for linearizing FET differential amplifiers. A 2-GHz differential amplifier based on the proposed structure is designed for a power amplifier driver in an RF transmitter. Fabricated in 0.13-μm CMOS, it operates from a 1.2-V supply with the power dissipation of 30.2 mW. Measurement results show that it achieves +30.4 dBm of peak output third-order intercept point, 43 dBc of C/I at 0-dBm output power, +9.7 dBm of output-referred P1dB, and +10.6 dB of power gain.