Digital Signal Processing Reference
In-Depth Information
Chapter 7
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 pulse-
based receiver of Section 7.6, serving as variable gain amplifier (vga)inthe
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 us-
ing 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 ap-
plications. 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 lin-
earized 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 lim-
ited; the nonlinear transfer characteristics of the amplification stage are caused
by the physical properties of the composing transistor devices. As the transis-
tor parameters are fairly incalculable during design time - especially towards
higher frequency bands -, measures that try to counteract these non-idealities
