Highly linear CMOS RF amplifier using transconductance cancellation and its volterra series analysis

Abstract

This thesis consists of three parts, which deal with linearity enhancement method for CMOS RF amplifiers. First part investigates various topologies which are suitable for highly linear amplifiers, such as common source, source follower, and push-pull structure using MESFET and balun. Complementary CMOS parallel push-pull (CCPP) amplifier topology is also proposed. Prototype CCPP amplifier is implemented using 0.5㎛ CMOS process, and the linearity performance is measured. Although its gain characteristic is not much inferior to that of common source amplifier, linearity enhancement is not sufficient. This is because that $g_m````$ values of NMOS and PMOS are both negative, and are not cancelled each other. Moreover, low gain of PMOS at RF band has negative effect on $OIP_3$ of RF amplifiers. In the second part, a simple linearization technique using multiple gated common source transistors is proposed where gate width and gate drive ($V_{gs}$ - $V_{th}$) of each transistor are chosen to compensate for the nonlinear characteristics of the main transistor. To demonstrate the feasibility of this approach, a prototype double gated RF amplifier using two MOSFET is implemented and its RF characteristics are compared with those of single one. The results show that, compared with conventional single gate transistor amplifier, the 3rd order intermodulation $(IMD_3)$ is improved by 6 dB with similar gain, fundamental output power, and DC power consumption. Because the auxiliary transistor is smaller than the main one and biased at subthreshold, adding this does not affect amplifier characteristics appreciably other than the nonlinearity. Using this linearization method, MMIC amplifier is also fabricated. The linearity figure of merit $(IP_3/DC power consumption)$ is increased more than 3 dB, and this means that less than half of DC power consumption is needed to obtain the same linearity performance. The last part is devoted to the nonlinear Volterra series analysis o...