Re: low noise amplifier
The book "Trade-Offs in Analog Circuit Design" can tell you the detail.
The main trade-off to be made in amplifier designs is among the amplifier
noise production, distortion generation and its bandwidth. Especially when a
high-performance level is required, it is difficult (or even impossible) to satisfy
all requirements simultaneously within one amplifier stage. For this reason,
we will attempt to make these requirements “orthogonal” and optimize them
separately. A particularly suitable concept that enables such an “orthogonalization”
is overall negative feedback; it localizes the various design requirements
in different parts of the amplifier circuit.
Essentially, a negative feedback amplifier consists of two parts; an active part
that provides the amplification, and a feedback network that accurately fixes the
overall gain to a predefined value. The active part consists of a combination of
transistors that approximates a “nullor”; a circuit theoretical twoport element
with infinite gain (all elements of its chain matrix equal zero).
In theory, the nullor and the feedback network establish complete orthogonality
between the overall amplifier gain at one side, and requirements with
respect to noise, distortion and bandwidth at the other side. The feedback network
only fixes the overall gain, and has no effect on the noise, distortion or
bandwidth. The nullor implementation completely determines the noise, distortion
and bandwidth of the amplifier and has no influence on the overall gain.
Further, inside the nullor, the noise, distortion and bandwidth requirements can
be made orthogonal by localizing them to different stages
The nullor implementation has to comply simultaneously with the requirements
with respect to noise, distortion and bandwidth. To achieve this, we
“orthogonalize” the requirements by confining them to different stages inside
the nullor in the following way.
Since small signals are more vulnerable to noise than large ones, it is likely
that the amplifier input has a dominant influence on the noise performance;
the power contents of the information signal is minimum here. Therefore, it
makes sense to design the nullor input stage for minimum noise, and assure that
the noise contribution of other stages is negligible. Similarly, the distortion is
likely to be dominated by the output stage, since the signal levels are maximal
there. By proper design, it can be assured that all distortion is confined to the
nullor output stage. Similarly, the bandwidth requirements can be localized
into intermediate stages .
The input stage of the nullor in a low-noise amplifier has to comply with
two requirements:
It has to assure orthogonality between noise and the other requirements.
Its own noise production should be minimal.
To assure orthogonality, the gain of the nullor input stage should be made as
large as possible