Method and circuit for compensating VT induced drift in...

Miscellaneous active electrical nonlinear devices – circuits – and – External effect – Temperature

Reexamination Certificate

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C327S350000

Reexamination Certificate

active

06507233

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to monolithic logarithmic amplifier integrated circuits and to methods for logarithmic conversion of an input signal.
Logarithmic amplifiers have been used to provide various functions. The closest prior art is believed to be the assignee's hybrid integrated circuit LOG
100
logarithmic and log ratio amplifier, the article “What's All This Logarithmic Stuff, Anyhow?”, by Robert A. Pease, Electronic Design, Jun. 14, 1989, pp. 111-113. Also see the text “Function Circuits” by Wong and Ott, McGraw-Hill Publishing Company, New York, 1976, page 58. Logarithmic amplifiers have been used in signal compression wherein the compressive effects of the logarithmic transfer function are useful. For example, use of the assignee's LOG
100
logarithmic amplifier connected ahead of an eight-bit analog-to-digital converter can produce equivalent 20-bit converter dynamic range.
FIG. 1
is a schematic diagram of the assignee's above mentioned hybrid integrated circuit LOG
100
logarithmic amplifier. Referring to
FIG. 1
, the logarithmic amplifier
1
A includes a first operational amplifier
11
(also referred to as operational amplifier A
1
) having its (−) input connected to an external input terminal
14
into which an input current I
in
is provided by the user. The (+) input of operational amplifier
11
is connected to ground. The output of operational amplifier
11
is connected by conductor
13
to the emitter of an NPN transistor Q
1
, the collector of which is connected to input terminal
14
. The emitter of transistor Q
1
is also connected by conductor
13
to the emitter of a matched NPN transistor Q
2
having its base connected to ground and its collector connected to both an external reference current terminal
15
into which a reference current I
ref
is supplied by the user, and to the (−) input of a second operational amplifier
19
(also referred to as operational amplifier A
2
) having its (+) input connected to ground. The output of operational amplifier
19
is connected to an external output conductor
17
on which an output voltage V
out
representative of the log ratio of I
in
/I
ref
is produced. The base of transistor Q
1
is connected to an external terminal
16
. V
out
is connected to one terminal of a thin film resistor R
2
, the other terminal of which is connected to conductor
16
. A “composite” temperature-dependent resistor R
1
having a large positive temperature coefficient (TC) is coupled between conductor
16
and ground. Resistor R
1
includes a 270 ohm thin film resister R
1
b
connected between conductor
16
and one terminal of a 220 ohm thermistor R
1
a,
the other terminal of which is connected to ground. Composite resister R
2
may be a selectable parallel combination of thin film resisters each of which has one terminal connected to terminal
16
and another terminal connected to enable the user to set the resistance of R
2
.
Logarithmic amplifier
1
A of
FIG. 1
is implemented as a hybrid integrated circuit. The thermistor R
1
a
is formed on a discrete chip that is bonded onto the hybrid integrated circuit. Because of its large size, the logarithmic amplifier
1
of prior art
FIG. 1
must be packaged in a larger package.
FIG. 2
shows a schematic diagram of another prior art logarithmic amplifier
1
B similar to that of
FIG. 1
except that transistors Q
1
and Q
2
have been replaced by (or are represented by) diodes D
1
and D
2
, respectively.
Generally, it is more convenient and less expensive to integrate all the elements of a circuit into a single chip. Furthermore, monolithic construction also facilitates assembly of the circuit into small surface mount packages, such as the SO-14. Accordingly, the prior art logarithmic amplifier shown in
FIG. 1
has the disadvantages that the hybrid LOG
100
product is not “compatible with” ordinary monolithic integrated circuit (IC) technology. However, adding the capability of providing a conventional thermistor in a conventional IC process would have resulted in additional complexity and cost.
Thus, the LOG
100
design shown in
FIG. 1
was considered impractical to implement on a single chip, because a thermistor which could, as a practical matter, have been provided on the same chip along with the amplifier circuitry and thin film resisters, was not available. It would have been considered impractical, in view of the benefit, to add the semiconductor processing steps that would have been needed to include a thermistor in a single-chip implementation of the LOG
100
.
Until now no one has provided a logarithmic amplifier similar to the ones shown in
FIGS. 1 and 2
integrated into a single monolithic chip and capable of being packaged in a small, inexpensive plastic package, such as a TSSOP-14 or a SO-14.
In the past, integrated circuit interconnection metallization generally has only been utilized for making very low resistance resisters. For example, very low value resisters, e.g., emitter resisters and shunt resisters having very small resistances have been formed of the integrated circuit interconnection metallization that also is used throughout the integrated circuit. U.S. Pat. No. 4,990,803 (Gilbert) issued Feb. 5, 1981 discloses a multi-stage logarithmic amplifier in which a front end PTAT resistive attenuator includes an input voltage divider circuit including a high temperature coefficient resistor and a fixed resistor in its transfer branch. The output of the attenuator is connected to a logarithmic cell circuit. U.S. Pat. No. 4,990,803 also discloses that the high temperature coefficient resistor can be a 30 ohm resistor fabricated from aluminum interconnection metallization provided during chip fabrication. An input attenuator is suitable for voltage inputs, but would shunt low level current inputs.
Thus, there has been a long-standing unmet need for a monolithic temperature-compensated logarithmic amplifier.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a monolithic integrated circuit logarithmic amplifier and method which provide essentially temperature-compensated logarithmic amplification of an input signal.
It is another object of the invention to avoid the large physical size and high cost of prior hybrid integrated circuit logarithmic amplifiers.
It is another object of the invention to provide a small, low-cost temperature-compensated logarithmic amplifier, especially one that is suitable for measurement of light intensity in fiber-optic devices.
It is another object of the invention to avoid the difficulties of using discrete large positive-temperature-coefficient thermistors in logarithmic amplifiers.
Briefly described, and in accordance with one embodiment, the invention provides a temperature-compensated monolithic logarithmic amplifier including a logarithmic amplifier cell (
26
) configured to produce a logarithmic voltage signal (V
3
) representative of a difference between a first voltage (V
1
) developed across a first PN junction device (D
1
) in response to an input signal (I
in
) and a second voltage (V
2
) developed across a second PN junction device (D
2
) in response to a reference signal (I
ref
). The logarithmic amplifier includes an output circuit (
36
) including an output amplifier (A
2
), a temperature-dependent first resistive element (R
1
) having a positive first temperature coefficient, and a second resistive element (R
2
) having a second temperature coefficient that is of substantially lower magnitude than the first temperature coefficient, the first (R
1
) and second (R
2
) resistive elements being coupled as a voltage divider between an output of the output amplifier (A
2
) and a reference conductor (GND) to provide a feedback signal to an input of the output amplifier (A
2
), the output circuit (
36
) being configured to produce a temperature-compensated output signal (V
out
) in response to the logarithmic voltage signal (V
3
). A temperature-dependent third resistive element (R
1
a
) included in the first resistive

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