Datasheet

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LOW-NOISE AMPLIFIER (LNA)
VOLTAGE-CONTROLLED ATTENUATOR
R
S
Attenuator
Input
Attenuator
Output
A1-A8AttenuatorStages
Control
Input
Q
1
V
B
Q
2
Q
3
C
1
V1
Q
4
Q
S
C -C ClippingAmplifiers
1 8
Q
5
Q
6
Q
7
Q
8
C
2
V2
C
3
V3
C
4
V4
C
5
V5
C
6
V6
C
7
V7
C8
V8
A1 A2 A3 A4 A5 A6 A7 A8
V
CNTRL
VCA8500
SBOS390A – JANUARY 2008 – REVISED MARCH 2008
The attenuator is essentially a variable voltage divider
that consists of the series input resistor (R
S
) and
As with many high-gain systems, the front-end
eight identical shunt FETs placed in parallel and
amplifier is critical to achieve a certain overall
controlled by sequentially activated clipping amplifiers
performance level. Using a proprietary new
(A1 through A8). Each clipping amplifier can be
architecture, the LNA of the VCA8500 delivers
understood as a specialized voltage comparator with
exceptional low-noise performance, while operating
a soft transfer characteristic and well-controlled
on a very low quiescent current of only 8.3mA per
output limit voltage. Reference voltages V1 through
channel. This current consumption is significantly
V8 are equally spaced over the 0V to 1.2V control
lower compared to CMOS-based architectures with
voltage range. As the control voltage rises through
similar noise performances.
the input range of each clipping amplifier, the
amplifier output rises from 0V (FET completely ON) to
The LNA performs a single-ended input to differential
V
CM
– V
T
(FET nearly OFF), where V
CM
is the
output voltage conversion and is configured for a
common source voltage and V
T
is the threshold
fixed gain of 20dB (10V/V). The ultralow
voltage of the FET. As each FET approaches its off
input-referred noise of only 0.7nV/ √ Hz, along with the
state and the control voltage continues to rise, the
linear input range of 250mV
PP
, results in a wide
next clipping amplifier/FET combination takes over for
dynamic range that supports the high demands of
the next portion of the piecewise-linear attenuation
PW and CW ultrasound imaging modes. Larger input
characteristic.
signals can be accepted by the LNA, but distortion
performance degrades as input signals levels
Thus, low control voltages have most of the FETs
increase. The LNA input is internally biased to
turned on, producing maximum signal attenuation.
approximately 2.4V; the signal source should be
Similarly, high control voltages turn the FETs off,
ac-coupled to the LNA input by an adequately-sized
leading to minimal signal attenuation. Therefore, each
capacitor. Internally, the LNA directly drives the VCA,
FET acts to decrease the shunt resistance of the
avoiding the typical drawbacks of ac-coupled
voltage divider formed by R
S
and the parallel FET
architectures, such as slow overload recovery.
network.
(VCA)
The amplified differential signal swing that comes
from the LNA is reduced by the subsequent VCA
stage. The VCA is designed to have a linear-in-dB
attenuation characteristic; that is, the average gain
loss in dB is constant for each equal increment of the
control voltage (V
CNTL
). Figure 61 shows the
simplified schematic of this VCA stage.
Figure 61. Voltage-Controlled Attenuator Simplified Schematic
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