Nonlinear Loaded Open-Loop Amplifiers

This section describes a wideband amplifier, which is based on a linearized open-loop topology. The amplifier itself was originally embedded in the pulsebased receiver of Section 7.6, serving as variable gain amplifier (vga) in the baseband section, but a stand-alone chip version has also been implemented to allow for more extensive characterization. Although the linearized amplifier was specifically intended for use in the signal chain of a wideband receiver, it is equally well suited for application in, for example, the high-speed residue amplifying stages of a pipelined ad converter.

While nonlinearities of the active element are commonly suppressed by using the amplifier in feedback configuration, the lack of excess loop gain in the higher frequency range renders this approach useless for use in high-speed applications. For a detailed explanation on the subject of frequency-dependent distortion suppression, the reader is referred to Appendix 7. In contrast to the feedback-oriented approach, the open-loop amplifier that is described below, relies on a completely different distortion suppression mechanism. The linearized open-loop amplifier can be split in two sections. The first section is the active amplification stage, and provides the useful gain of the amplifier. The second part, the loading stage, is in charge of counteracting the weak nonlinear effects that are introduced by the non-idealities in the amplification stage. It is easy to understand that the mathematical solution to tackle this problem would simply be to reverse the nonlinearities from the first stage. In any real-life implementation however, the options to achieve this goal are limited; the nonlinear transfer characteristics of the amplification stage are caused by the physical properties of the composing transistor devices. As the transistor parameters are fairly incalculable during design time — especially towards higher frequency bands —, measures that try to counteract these non-idealities using a straightforward approach are fairy ineffective. However, by playing off the same active devices in the amplification stage and the compensation stage against one another, the compensation method introduced further on in this section is inherently immune to variations or modelling errors in the device parameters. Before going deeper into detail, it must be recognized that the most important characteristic of the nonlinear loaded open-loop amplifier discussed in this section is that the voltage gain of one single stage is intentionally kept low, in the range of 0 … 6 dB. The exact reason for this decision will become clear later on, but the fact that a low output impedance is combined with an open-loop topology, makes the presented baseband amplifier one of the fastest currently available in a standard cmos technology.