Erin Walters
L01 – Smith
Sonar
Distortion Reduction in Amplifiers
Introduction
Sound amplification has numerous interesting applications, but the purpose is nearly
always the same: amplify a small signal recorded by a microphone until it can drive the
connected device. For applications like sonar detection that use sound to determine the
coordinates of an object, it is important that the recorded signal is amplified with as little
distortion as possible. The amplifier circuit must be able to detect very small voltage signals and
require very low power to operate. This paper focuses on operational amplifier (op amp)
specifications that affect distortion as well as methods of reducing distortion.
Underlying Technology
There are several parameters that play an important role in how much distortion is
produced during amplification such as gain, bandwidth, and slew rate [1]. An ideal slew rate
would be infinite, meaning that there would be no delay between an increase in the input voltage
and an increase in the output voltage [2]. As the slew rate decreases, distortion increases. Wider
bandwidth op amps tend to have higher slew rates, so they are usually preferred. However, in
some cases a narrow band is needed, which would reduce the slew rate and increase the
distortion caused by delay.
Many audio detection technologies have a millivolt input signal that has to be amplified
to an output in volts. Because the signal is being amplified by three orders of magnitude, the
amplifier often contains multiple op amps and becomes a multi-stage amplifier. Each op amp
should have as little distortion as possible since distortion created by each op amp will be
amplified by the next op amp.
Variable Gain Amplifiers
One of the most common ways to compensate for distortion is to vary the gain of the
amplifier. There are two ways to vary the gain of an amplifier: either use a potentiometer at the
output of a fixed gain operational amplifier (op amp) or vary the gain of the op amp itself by
using a variable gain op amp [3]. For multi-stage amplifiers, it is better to use a potentiometer at
the output of one or more of the fixed gain op amps rather than using a variable gain op amp.
Potentiometers can be mechanical and controlled manually or digital and controlled
remotely with a microcontroller. Digital potentiometers have several advantages over
mechanical pots: better resolution, can be controlled remotely, faster adjustment, and a smaller
footprint [4]. Digital pots have recently improved significantly; their end to end resistor
tolerances have decreased to 1%, which allows for greater system accuracy [4].
Current Technologies
A type of op amp that has been identified as a good match for sonar detection is the
“Micropower, Single Supply, Rail-to-Rail Input-Output Instrumentation Amplifier.” This type
of amplifier is ideal for battery powered applications because it requires very little power input
[5]. The Intersil EL8171 op amp has a gain of 10, a bandwidth of 450 kHz, and an average slew
rate of 0.55V/µs [5]. Another op amp of this type, the Analog Devices OP291, has similar gain
and slew rate but the bandwidth is closer to three MHz, which is much larger than the EL8171
[6]. Both of these amplifiers have a maximum supply current of 95µA and offsets of over
700µV. Another amplifier is the Analog Devices AD627; it has a lower maximum supply
current of 85µA and an offset of only around 200µA, but the bandwidth and slew rate suffer a lot
(they are reduced to 80kHz and 0.05V/µs, respectively). When comparing different amplifier
options, decisions must be made about which parameters have priority. If noise and distortion
will not affect the signal much, then choosing based on power consumption or some other
parameter may be wise. However, if minimal distortion is required, power consumption or other
parameters may have to be sacrificed.
Choosing an amplifier requires many known specifications about the operation needed
and there are many different amplifiers to choose from. The important thing to remember is that
while the industry may keep making more sophisticated amplifiers, sometimes the older
technology will work just fine.
Bibliography
[1]
Bruce Carter and Thomas R. Brown, Company Technical Reference: Handbook of
Operational Amplifier Applications http://focus.ti.com/lit/an/sboa092a/sboa092a.pdf
[2]
G. B. Rutkowski. Operational Amplifiers: Integrated and Hybrid Circuits, Google Book
Search: Wiley-IEEE, 1993.
http://books.google.com/books?id=uskYzanGfjsC&pg=PA101&lpg=PA101&dq=what+i
s+ideal+slew+rate&source=web&ots=1qPgIzMrTu&sig=dBGWLULvo8TUMxpElhQXg-4fc4&hl=en&sa=X&oi=book_result&resnum=4&ct=result
[3] “Operational Amplifier Variable Gain Circuit,” [Online]. Available: http://www.radioelectronics.com/info/circuits/opamp_variable_gain_amp/op_amp_variable_gain_amp.php
[Accessed January 19, 2009].
[4] D. Tuite. “Design FAQ’s: Digital Potentiometers,” Electronic Design 7/24/2008, Vol. 56
Issue 15, p30-31. [Online]. Avaialable:
http://www.library.gatech.edu:2048/login?url=http://search.ebscohost.com/login.aspx?dir
ect=true&db=a9h&AN=34180011&site=ehost-live
[5] Analog Devices Datasheet. “Micropower, Single-Supply, Rail-to-rail input/output Op Amps:
OP191/OP291/OP491.” Analog Devices. [Online]. Available:
http://www.analog.com/static/imported-files/data_sheets/OP191_291_491.pdf.
[Accessed January 19, 2009].
[6] Intersil Devices Datasheet. “Micropower, Single Supply, Rail-to-Rail Input-Output
Instrumentation Amplifiers: EL8171, EL8172.” Intersil Devices. January 8, 2009.
[Online]. Available: http://www.intersil.com/data/fn/fn6293.pdf. [Accessed January 19,
2009].
[7] Analog Devices Datasheet. “Micropower, Single- and Dual-Supply, Rail-to-Rail
Instrumentation Amplifier: AD627.” Analog Devices. [Online]. Available:
http://www.analog.com/static/imported-files/data_sheets/AD627.pdf. [Accessed January
19, 2009].