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TRANSMIT-RECEIVE OPERATION
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The
presented wide-linear-range ultrasound receiver comprises
a low-noise preamplifier (LNA) and a T/R switch. As shown below, the
switch isolates
the preamplifier from the high-voltage pulses generated by the
transmitter
during the transmission mode and connects the transducer element to the
preamplifier during reception.
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According
with the invention, the T/R switch is
embedded into the LNA input structure. This provides a local feedback
increasing the LNA's dynamic range. The circuit
is unconditionally stable while allowing to program its input
impedance.
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LNA TRANSCONDUCTANCE
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Typically, the LNA
topology consists of a differential amplifier. A bipolar differential
pair is considered to be linear as long as an
input swing remains unsaturated, i.e., between the limits of ±
50 millivolts. To expand dynamic range further, one can use a
local feedback usually called “emitter degeneration”. However,
selecting a larger emitter resistor increases noise.
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techniques have been proposed to improve linearity (input
dynamic range) of a differential amplifier. The most effective
among those methods is the one called “multi-tanh”. The idea behind the
multi-tanh technique is having multiple differential pairs with
different input offset bias operating in parallel. Although
the multi-tanh circuits provide an extended dynamic range, they suffer
from several disadvantages: |
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There
is a need in multiple offset voltage generating
circuits, a circuit for weighting output currents, and a circuit for
adding the weighted output currents.
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The
supplementary
circuits would contribute additional noise. |
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The
value of said offset voltages is in order of
0.5 to 2 thermal potentials. Thus,
high-accuracy generation of the offset voltages becomes a technical
challenge itself |
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NEW LINEARIZATON TECHNIQUE
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To increase the
linear dynamic range of a differential amplifier, we conceived using of
a dedicated negative feedback that directly controls the
transconductance
characteristics. As shown,
flatness of the transconductance curve depends on the loop gain,
G.
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The graph below
illustrates two-tone IMD simulations for the linearized circuit at a
±
150 mV input swing and G = 4.
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