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Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 3611
Maxim > Design Support > Technical Documents > Application Notes > Basestations/Wireless Infrastructure > APP 3611
Maxim > Design Support > Technical Documents > Application Notes > Wireless and RF > APP 3611
Keywords: log amps, logarithmic amplifiers, DC log amp, log amplifiers, logarithmic amps
APPLICATION NOTE 3611
Integrated DC Logarithmic Amplifiers
Sep 23, 2005
Abstract: Among the classes of logarithmic amplifiers, the DC log amp continues to be a very powerful,
cost-effective solution for compressing wide-dynamic-range sensor signals. This article derives the
overall DC log amp transfer function from the V
BE
-to-IC transfer characteristic of bipolar transistors. It
discusses the circuit blocks encountered in a modern-day integrated DC log amp, the various errors
affecting log amp performance, and a design example featuring the MAX4206. Finally, suggestions for
optimizing log amp performance through calibration and design pointers are detailed.
This article was also featured in Maxim's Engineering Journal, vol.
56 (PDF, 324kB).
For over half a century, engineers have used log amps for
compressing signals and for computation. Although digital ICs have
mostly replaced the log amp in applications that require computing,
engineers continue to use log amps to compress signals.
Therefore, the log amp remains a key component in many video,
fiber, medical, test and measurement, and wireless systems.
As implied by the name, a logarithmic amplifier expresses an
output that is related to its input by the mathematical log function (the logarithmic base is not important,
as the different log-based functions are related by constants). By utilizing the log function, you can
compress the dynamic range of signals encountered by a system. Compressing wide-dynamic-range
signals has several important benefits. The combination of a log amp and a low-bit-count ADC can often
save board space and system cost, whereas a high-bit-count ADC might otherwise be required.
Furthermore, low-bit-count ADCs are often already present in a given system or on a resident
microcontroller. The conversion to a logarithmic parameter is also useful in many applications where
measured quantities are evaluated in decibels, or where sensors exhibit exponential or near-exponential
transfer characteristics.
During the 1990s, the fiber communications industry began using log-amp circuits to measure optical
intensity in certain optical applications. Before that time, precision log-amp ICs were both costly and
bulky; such expense was warranted in only a handful of electronic systems. The only alternative to these
IC solutions were log amps constructed from discrete components. In addition to consuming even more
board area, discrete-component log amps were frequently prone to temperature changes and required
careful design and board layout. Highly matched components were also necessary to guarantee
adequate performance over a wide range of input signals. Semiconductor manufacturers have since
developed smaller and less costly integrated log-amp products with reduced temperature sensitivity and
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