Linear Transmitter Design Using Nonlinear Analog Circuits
Sammanfattning: This dissertation deals with analog techniques at both the architecture and circuit design levels for designing power efficient linear radio-frequency (RF) transmitters. The use of nonlinear analog integrated circuits to implement certain critical signal processing functions for the multiplicative feedback and LINC techniques is investigated with the aim of improving the system performance in terms of complexity, linearity and power efficiency. For the multiplicative feedback technique it is identified that matching and spectral expansion are problematic issues for the previous designs based on envelope detectors. To overcome these problems, the use of power detectors is proposed, and the scheme is then referred to as power feedback. The fundamental properties of the multiplicative feedback technique associated with both types of detectors are studied by means of analysis. A simulation method is described to obtain the theoretical limit of distortion reduction for a given nonlinear amplifier. A prototype power feedback chip fully integrated in CMOS is presented. Simulated and measured results demonstrate the effectiveness of this technique. Much of the work regarding the LINC technique is devoted to the implementation of the crucial signal component separator (SCS). Two new SCS architectures based on analog integrated circuit (IC) techniques are developed to avoid the constraints faced by a DSP-based realization. In one scheme, a nonlinear feedback technique is used to indirectly realize the nonlinear function involved in the SCS. Thus, the ability to generate the desired phasor is limited by the loop bandwidth and gain. To obtain wideband operation capability, translinear circuits are used in the other scheme to directly synthesize this most demanding signal processing element. It is shown that by exploiting the high degree of matching offered by IC techniques a high performance analog SCS can be realized. The realization of the high power efficiency potential of the LINC technique is critically dependent on the recombination process. By the use of bilateral PA devices, a `current summing switching amplifier' topology theoretically achieves linear amplification with 100% efficiency at all output levels. In practice the achievable linearity and efficiency of such a system will be very much limited by the performance of practical devices, and this is investigated by means of circuit simulation.
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