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WIDE-DYNAMIC-RANGE CMOS LNA
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When performing tissue harmonic
imaging or Doppler ultrasonography, the
low-amplitude echoes to be detected appear in the presence of intensive
clutter reflections. Therefore, it is indispensable
to maintain a wide linear range of the LNA in order to minimize
high order products and intermodulation between signal components of
different amplitude.
Regarding
LNA noise performance, it
is widely
accepted that active termination of the transducer cable achieves
superior noise figure as compared with a conventional (shunt)
scheme.
At present,
active
termination
is provided by the input
impedance of an LNA configured as an inverting operational amplifier.
Accordingly, the impedance looking into the inverting
input is reduced by a factor 1+G, where G is the open-loop gain.
Typically, 5 < G < 10.
While active termination
improves noise figure of
an LNA, this technique requires lowering the G. Consequently, those
amplifier
errors associated with finite open-loop gain become increasingly
elevated.
Our patent-pending
architecture solves the most contradictive problems in designing
ultrasound LNAs: low noise and wide dynamic range. The invention
discloses an LNA comprising a class AB CMOS
transconductor and an I/V converter connected in series.
Inherently exhibiting low input
impedance, the transconductor provides active
termination of a transducer cable.
Graphs below depict
simulation
results on
the LNA's large-signal linearity and noise
performance. The simulations were conducted assuming use of
a 0.25 um CMOS technology. |
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Cable
termination by
matching transconductor impedance
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Transconductor
linearity vs. matching impedance (RS
= 50 Ohm)
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