Design of a low noise preamplifier for ultrasonic transducer
L. Svilainis*, V. Dumbrava
Signal processing department, Kaunas University of technology,
Studentu str. 50, LT-51368 Kaunas, Lithuania, tel. +370 37 300532, E-mail.:firstname.lastname@example.org
*Corresponding Author Contact:
Email: email@example.com, Internet: www.ktu.lt/ultra/
Published in ISSN 1392-2114 ULTRAGARSAS, Nr.2(55) 2005
The low noise preamplifier, used for the ultrasonic transducer, design approach is presented. Optimization is based on noise model for both electrical and the acoustical part of receiver. Every noise component has been investigated separately, using various transducers and operational amplifiers models. Noise behavior has been presented in diagrams for analysis. Every noise component has been minimized separately. For signal propagation evaluation, narrowband signal case has been considered. In has been shown that different ultrasonic transducers amplifier circuitry and amplifier type has to be chosen individually in order to improve signal-to-noise ratio.
Development of air-coupled ultrasonics is raising
demands for electronics used. Because of large difference
of acoustic impedances of solid body under investigation
and air, testing signal is attenuated. Attenuation can reach
up to 100dB, so powerful transmitters and low noise
receivers should be used. Since a conventional ultrasound
equipment  is not suitable in this case, development of
dedicated equipment is on its way. Among high power
excitation techniques, users are trying to squeeze out better
parameters from the transducer preamplifier [2, 3].
Fig. 1. Air-coupled ultrasonic NDE system setup
The ultrasonic transducer is excited by an electrical
pulse and transmits the ultrasonic pulse into air and only
then a signal is entering the medium under investigation.
The ultrasonic pulse travels in the material and is emitting
some energy into air. This energy is picked up by a
receiving air-coupled transducer.
In this paper we concentrate on the receiving
transducer and the preamplifier in order to obtain the best
signal to noise performance.
A few possible solutions for performance optimization
- Simplest approach would be to use the amplifier
with lowest noise components specified by a
manufacturer. If an operational amplifier is used,
the corresponding datasheet will indicate the
voltage and current noise spectral densities en and
in. Minimizing those two seems to be sufficient to
reach the best noise performance. As it can be seen
from further investigation, this approach is not
giving a unique solution what is good for one type
of ultrasonic transducer might be of little or no use
- Maximize a signal level, by increasing the amplifier
input impedance, so the signal generated by the
transducer is not dampened. Another way is to
apply matching circuits [4, 5], so the signal cable or
the transducer impedance is best matched for
minimum reflections of band flatness. But this
requirement usually calls for a high input
impedance, which is expected to generate more
noise. Also the noise level is left unaccounted here.
- Minimize the preamplifier output noise, taking into
account all possible noise sources influences .
However, there is a need to separate the sources of
noise which can not be modified or which would
degrade the signal performance.
- Minimize the noise figure. The noise figure is
expressing the signal-to-noise ratio degradation
while signal is passing the amplifier electronics.
Again, as can be seen from a further investigation,
this method also has some shorts comings like
signal source noise increase is reflected as the noise
- The complex approach. Since we are speaking
about the measurement, major importance is the
signal-to-noise ratio (SNR), because SNR is
influencing the measurement errors. Simple to say,
it still requires some techniques to be developed.
Namely, as with methods above, we need some
expression of the signal level and how it is
influenced by a circuit parameters. Also, a noise
behavioural model needs to be developed. Both
should be combined in order to give a single result.
The noise model can be developed either into
analytical form  or be modelled using some circuit
oriented software like PSPICE . Despite that the
PSPICE modelling seems to offer the easiest and closer to
reality approach, we decided to use the analytical model,
since in such a case noise contributors can be analyzed
separately, cutting out the sources level of which can not
be modified or controlled. For this purpose we have
developed the analytical noise model, incorporating both
transducer and electronics units .
Fig. 2. Electrical equivalent of the circuit noise to be modeled
The noise spectral density of the is input resistance Rt
eT2 = 4kTRt; (1)
where k=1.380658.10-23 [J/C] - Bolzmann constant, T is the
absolute ambient temperature.
Fig. 3. Noise model circuit
The transducer noise spectral density is determined by
the real part of the transducer impedance 
es2 = kT Zs (2)
The feedback circuit resistors contribute the noise densities
e12 = 4kTR1 ;
e22 = 4kTR2 . (3)
With the amplifier noise gain
we get the equation for the output noise density
Signal propagation model
To simplify the development of the signal model, we
subdivide the task into two possible applications:
The transducer signal depends on various factors, like
transmitting and receiving transducer matching to air, test
object coupling to air, transducers orientation, object
defects interaction with signals etc. In order to take at least
close approach to the factors mentioned, complexity of the
model is increasing [5, 6]. There have been attempts to
simplify such a solution, with quite useful results .
We are taking a different approach. In this paper we
will concentrate on a narrowband inspection analysis, so
signal is close to the operating frequency and the voltage
source internal impedance is determined by Rs. In such a
case the signal propagation through amplifier circuits is
- narrowband inspection, when continuous in time
(CW) or close to (CW burst) signals are used (e.g. for
- wideband inspection, when signals used occupy some
definable bandwidth (e.g. pulsed signals).
where etr is the transducer signal internal voltage source.
Even further, since we are interested on how amplifier
circuit elements are influencing the signal level, we do not
need an absolute value of the signal received. We just need
to have some sort of indication how the signal level will
change with circuit parameters. So, we will be
investigating the signal level change, regarding the
maximum signal level achievable. Since best propagation
is achieved when Rt is infinity, Eq.6 becomes
where SF denotes the normalized signal.
Signal propagation analysis
In order to start analysis of signal propagation, we
have investigated various air coupled transducers [2, 5, 9]
in order to evaluate the range of transducer parameters. In
order to restrict ourselves we have concentrated on (400-
800) kHz frequency range transducers, because of
popularity of this range. Results of investigation are
presented in Table 1.
Transducer || Rs, [kW] ||Cs, [pF] ||Ls, [ľH] ||C0, [pF]
|Tr 1 ||9,3 ||1758 ||88,7 ||3333
|Tr 2 ||410 ||173 ||561 ||480
|Tr 3 ||1800 ||26 ||1600 ||65
|Table 1. Air coupled transducers parameters
By analyzing the data presented in Table 1, one can
see that Rs range is within 10OW to few kW. Therefore we
have chosen such a range in the signal propagation
analysis. Results of application of Eq.7 for resistance range
mentioned are presented in Fig. 4.
Fig. 4. Normalized signal propagation vs. input resistance Rt and
It can be seen that the ratio of input resistance Rt and
the transducer impedance Rs should be above 10 times in
order to have lowest signal attenuation (the highest SF).
In order to evaluate the importance and influence of
circuit parameters, we now investigate Eq.5 components.
The operational amplifier internal noise voltage source en
is of major interest. As mentioned above, the simplest
approach would be to minimize this parameter, by
choosing an appropriate amplifier. In addition to that, this
member has the simplest expression. We have chosen three
amplifiers as representatives of achievable limits. One
(LMH6624) is presenting the lowest voltage noise, an
other (OPA657) is exhibiting the lowest current noise
source and the third one (CLC425) has both current and
voltage noise in between the first two candidates. The
parameters are presented in Table 2.
We start analysis using those parameters which do not
interfere with other components of noise generation. Since
the feedback circuit is not influencing the input
components (the transducer and the input resistor) noise,
we analyse these components first of all. In order to be
able to neglect the R1 and R2 noise, their voltage spectral
density has to be 1/3 (less than 5%) of that of contributed
by en. Let as solve Eq.5 components for this requirement.
The maximum R1 value for R1 thermal noise exclusion is
The maximum R1 value for R2 thermal noise exclusion
The maximum R1 value for exclusion of the amplifier
current noise in- generated voltage when loaded by R1 and
R2 parallel connection is
It can be proved that the reasonable gain value to use
in calculations is about 100. The results obtained for this
gain level will hold true for all gain values in the range
from 2 to 200. The calculations results, basing all the
assumptions and Eq.8, 9 and 10 are presented in Table 2.
The last two columns summarize the values recommended
for R1 and R2. In order to suit all amplifiers, in subsequent
calculations we will be using the lowest values R1=10W,
| R1max (9)|
|LMH6624 ||0.95 ||2300 ||10 ||610 ||140 ||10 ||1
|OPA657 ||4.8 ||1.3 ||160 ||15610 ||1243200 ||160 ||15
|CLC425 ||2 ||800 ||30 ||2710 ||840 ||30 ||2.7
|Table 2. Amplifier parameters and corresponding limit and
recommended values for R1 and R2 at G=100
The next candidate for analysis should be the
component which we can not be modified, e.g. transducer.
Only transducer noise spectral density component e2ntrtot at
the amplifier output with the gain 100 is presented in Fig.5.
In order to reduce the complexity, the input resistance
Rt is taken to be close to infinity.
Fig. 5. Transducers noise spectral density at the amplifier output
The output noise can be calculated by integrating
over a frequency range. One can see that
noise is large at the transducer resonant frequency and is
degrading by going apart. For the sake of simplicity, we
take the integral over the wide frequency range 150 kHz to
2.5MHz, so there is no need for taking into account an
individual transducer bandwidth. The value obtained
represents the noise RMS value at the amplifier output:
In such a way we have the ability to analyse noise
RMS at the amplifier output behaviour versus the input
resistance Rt. The diagrams obtained are presented in
Fig.6. In order to be able to judge about the noise
influence, the operational amplifier voltage noise source en
contributed RMS level at the output is added. From the
signal propagation analysis it can be seen that the input
resistance Rt is desired to be over 10 times of the
transducer Rs. Therefore Rt=10 Rs values for best signal
performance are indicated for all three transducers.
Fig. 6. Transducers noise RMS at amplifier output vs. Rt
One can see that using Rt which is optimal for signal
propagation will cause an increase in the output noise
level. For Tr1 this increase is insignificant with any
amplifier, Tr2 noise will increase by about 2 times for
the lowest en amplifier and will be insignificant with other
two. Tr3 noise will increase about 3 times if the lowest en
amplifier is used or increase 1.7 times if average en
amplifier is used, but remains almost the same in the case
of a relatively large en.
In the same way we can calculate the output noise for
the resistance Rt thermal noise component e2nRttot getting
the EnRtRMS at the amplifier output (Fig .7).
Fig. 7. Rt thermal noise RMS at the amplifier output versus it's value
This time we see that it makes sense to increase the Rt
value further above 10Rs, so the Rt thermal noise
component can become negligibly low. This behaviour can
be explained by damping of the thermal noise source
voltage by a lower impedance of the transducer circuit.
Taking the same integral over the frequency range we
calculate the current noise source in the generated voltage
contribution to the noise RMS. The Ein+RMS voltage at the
amplifier output is presented in Fig. 8.
Fig. 8. Current noise source in+ generated output noise RMS versus
It is evident, that the LMH6624 amplifier is not
suitable for the transducer Tr3 with high Rs and Roe values.
The Ein+RMS value for this combination is extending ten
times above the en contributed noise level. The same
comes to Tr2 and LMH6624 combination, only difference
is not that large. Application of the OPA657 amplifier
significantly reduces the Ein+RMS level it is lowest part of
the drawing, so is not even notable. But it should be noted
here that the OPA657 exhibits quite large en (5 times that
of LMH6624), so this fact should be taken into account
when shifting to the OPA657 amplifier.
Finally, we calculate the total amplifier output noise
EntotRMS. The results are presented in Fig. 9.
Fig. 9. Total output noise RMS versus Rt
It is clearly visible that major noise contribution is by
en. But for transducers with large Rs and Roe (Tr2 and Tr3)
current noise at the desired values of Rt starts prevailing
over en. So, basing only noise analysis it can be predicted
the that transducer Tr3 should be used with OPA657 or
with CLC425 amplifiers with a lower current noise in+.
Taking all noise sources into account and calculating
the signal level at the output we normalize the value
obtained to SNR value at Rt infinity and the lowest Rs
transducer operating with the best en level amplifier
LMH6624. Then the circuit with best noise performance
SNR is 0dB. The SNR analysis results are presented in
Fig. 10. Normalised SNR change at the amplifier output versus Rt
Analysing the results obtained one can see that lowest
en amplifier is suitable for a quite large range of transducer
Rs values (10W to 400W the transducers Tr1 and Tr2).
But such en does not leave much room for noise reduction
the in case of large transducer Roe and Rs (the transducer
Tr3). Even further bias currents of amplifiers
investigated were not taken into account, because the
resistance at the negative amplifier input is quite low
(10W) and the positive input DC resistance is high (it is
desired to have it above 10kW), the bias currents skew will
cause output DC value out of limits. In literature 
authors recommend using a low bias current amplifier in
such a case (e.g.OPA657). Our solution is different we
break amplifier feedback loop, so the amplifier DC gain
gets equal 1. Therefore, the output DC shift due to bias
currents skew is low. Taking into account that DC
coupling is not needed in ultrasonic applications, the bias
shift may be neglected. So, the high bias current amplifier
can be used even with a high level of biasing mismatch. As
a result of the analysis we present final schematics for the
preamplifier (refer to Fig.11). The preamplifier will be
used with relatively low impedance transducers [5, 9].
Fig. 11. Final amplifier schematics
Some additional components have been added to
schematics to keep the calculated noise performance. The
capacitor C6 in series with R1 is introducing the high pass
filter with the -3 dB cutoff frequency of 150 kHz. The
capacitor C5 will add a low pass filter with the 2.5 MHz
Several approaches have been used to improve noise
performance of a transducer preamplifier for relatively
narrowband signal case. By analysing every noise
component it has been shown that en has major
contribution to a noise level. The noise current source is of
next importance. The approach have been demonstrated
how in- component can be reduced. Unfortunately, in+
component influence is dependant an transducer
parameters, so it can not be completely taken out. For
some transducer types, with high Rs or Roe this component
starts prevailing over the en part. In order to have full noise
performance evaluation, the SNR analysis has been used.
The final design is best suited for relatively low source
impedance transducers. If the amplifier for other type of
transducer is needed, the circuit parameters should be
adjusted in order to get the best noise performance.
We would like to thank the Dr. Darius Kybartas for his
consultations and useful literature support.
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