LOGARITMIC AMPLIFIER
The dynamic range of the echoes is wide enough
for the wide swing to surpass the input amplitude of
the ultrasound machine monitors, after the normal
signal amplification this signal needs to be ready for
digital processing, we have explained that the
amplitude of the echoes have micro-volt levels so it
is needed to amplified them, but on account of its
wide dynamic range, after the amplification of the
echoes these have a suitable amplitude for the
diagnostic, even the smallest amplified echo has a
right level but the biggest amplified echo has a level
that is longer than the monitor standard input
accepts, let´s focus not on the possible harm it may
cause to the monitor input but what happen to the
useful information the biggest echo convey, in
figure 7 the echoes are represented as lines, this
figure shows three sets of echoes and each set have
three echoes 1, 2 and 3, the first set represents the
real echoes coming from the human body, the second
set represents the echoes after been linear amplified
which is amplified twice, and third set represents the
echoes after a logarithmic amplification, after the
linear amplification it is seen that the biggest echo
surpasses the allowed level the echo may have, so
the amplitude above the allowed area does not
appears in the monitor and the information of the
biggest echo is distorted, this distortion is called
aliasing. The set of echoes that have been amplified
in a logarithmic way does not have distortion
because although all the echoes are amplified, the
small echo is amplified significantly but the biggest
echo is slightly amplified and does not surpasses the
allowed amplitude area and there is not aliasing.
Now in figure 8 let´s focus on the big echoes only
because are the ones that may diminish the quality of
image and so the confidence of the medical
diagnostic. The echoes amplified in a linear way
surpasses the not allowed area and are shopped to
the same amplitude so the information sent to the
monitor is the same for the four echoes, the different
structures in diagnostic ultrasound are recognized by
their gray nuances, and in a gray scale all have the
same gray and the monitor is showing a false image.
None of the echoes that are amplified in a
logarithmic way surpasses the not allowed area so all
are represented with gray nuances and the image
matches the echoes received in the probe from the
human body.
Fig 7
Figure 9 shows a circuit that performs the
logarithmic amplification explained before, and the
graph of figure 10 is the volt – ampere characteristic
of a silicon diode, from 0,4 V to 1,2 V, the relation
V/A is lineal, but beyond 1,2 V it becomes
logarithmic, so for small signals the diode
performance is linear, but for large signals is
logarithmic, and this principle is applied in the
logarithmic amplifiers, of course there are more
sophisticated circuits, and there also are chips
integrated with better performance.