Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Logarithmic
Reexamination Certificate
2001-09-06
2003-10-21
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Logarithmic
C327S359000, C327S361000, C327S513000
Reexamination Certificate
active
06636099
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electronic signal metrology technology. In particular, the present invention relates to logarithmic detectors or amplifiers.
2. Description of the Prior Art
Logarithmic detectors or amplifiers are used to measure signals having a large dynamic range. For example, applications requiring compression of a wide range of analog input data and linearization of transducers having exponential outputs. Logarithmic amplifiers are used mainly in communication applications for measuring receive signal strength indicator (RSSI) and for controlling the radio frequency (RF) power transmitted in a power amplifier. A logarithmic amplifier (logamp) is a device that represents RF signals at its input by an equivalent decibel-scaled DC voltage at its output.
FIG. 1A
shows the output of a typical logarithmic amplifier at
10
. An ideal response of a logamp is shown as a straight line
12
when plotted against a logarithmic/decibel scaled x-axis. This ideal response is approximated by the successive compression of a cascade of amplifiers. For small input signals a cascade of amplifiers will have large combined gain that progressively diminishes as larger input signals force latter stages of the cascade of amplifiers into compression. Increasing the gain increases the sensitivity of the logamp to small input signals. The actual response of a three stage logamp is shown as a series of three curves
14
when plotted on the same graph.
FIG. 1B
shows the outputs of
FIG. 1A
plotted on a linear graph at
20
. The ideal response of the logamp is shown as a curve
12
when plotted on a linear scale. An actual response of a three stage logamp is shown as line segments
14
. As can be seen from
FIG. 1B
the deviation of the actual response
14
from the ideal response
12
can be reduced by simultaneously increasing the number of stages in the logamp and reducing the gain of each stage such that the small signal gain remains constant. Such action would result in a greater number of segments of the actual response curve
14
(
FIG. 1B
) and thereby reduce deviation between the two curves
12
and
14
.
FIG. 2
shows the most common circuit implementation of a logamp at
50
. This configuration is referred to as a current mode approach. A voltage in
52
is applied to a cascade of amplifiers
54
. The voltage at the output of each amplifier
56
is converted to current at V/I converter
58
. This current is rectified by rectifier
60
. The rectified currents from all of the amplifiers
56
is summed across resistor
62
, which after filtering results in a decibel-scaled DC voltage at the output node
64
. Small input signals
52
will produce small combined rectified currents because only latter stages of the cascade of amplifiers
54
will convert the voltage into current. These small rectified currents will result in a small DC output voltage at output node
64
. A large input signal will cause a larger number of currents to sum onto the output node
64
thereby producing a large DC output voltage.
The implementation shown in
FIG. 2
is relatively insensitive to temperature variations. Each of the gain stages in the cascade
54
is biased with a proportional to absolute temperature (PTAT) current source and the combination of V/I converter
58
with the rectifier
60
is biased with a constant current source derived from a bandgap reference. In this way, the voltage at the output of each amplifier
56
remains constant despite changes in temperature, which results in constant current at the output of each rectifier
60
and an overall output voltage that is insensitive to temperature.
One problem with the current mode amplifier described above is that it can only function in a relatively limited bandwidth due to the use of current rectifiers
60
. Another problem with the current mode amplifier is that such a device consumes a relatively large amount of current to operate.
Therefore, it is desirable to provide logarithmic amplifier that operates at a broad range of input frequencies. Furthermore, it is desirable to provide a logarithmic amplifier that consumes less current than current mode logarithmic amplifiers.
SUMMARY
The present invention teaches a logarithmic amplifier that operates at a broad range of input frequencies. The present invention also teaches a logarithmic amplifier that consumes less current than current mode logarithmic amplifiers.
A first embodiment of the present invention teaches a voltage mode logarithmic amplifier comprising: at least one first gain stage for providing at least one amplified rectified voltage signal at least partially responsive to at least one input voltage signal; at least one second gain stage for providing at least one further amplified rectified signal at least partially responsive to the at least one input voltage signal; and at least one output node for producing at least one output voltage signal that is at least partially responsive to the at least one amplified rectified voltage signal and the at least one further amplified rectified voltage signal.
The voltage mode logarithmic amplifier further including: at least one self-biased replica stage operative to provide at least one voltage offset signal responsive to temperature; and at least one differential amplifier operative to receive said at least one voltage offset signal and provide a temperature corrected output voltage signal responsive to said at least one input voltage signal, wherein said at least one differential amplifier is communicatively coupled to both said at least one first gain stage and said at least one second gain stage.
REFERENCES:
patent: 4442549 (1984-04-01), Main
patent: 5057717 (1991-10-01), Kimura
patent: 5345185 (1994-09-01), Gilbert
patent: 5481218 (1996-01-01), Nordhold et al.
patent: 5677561 (1997-10-01), Jensen
patent: 6144244 (2000-11-01), Gilbert
Cunningham Terry D.
Maxim Integtated Products, Inc.
Nguyen Long
Perkins Coie LLP
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