Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1998-09-04
2001-02-13
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S513000, C327S539000, C323S315000
Reexamination Certificate
active
06188270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to voltage sources for electronic devices, particularly low-power voltage sources for analog circuits, and more specifically to a circuit for providing a low-voltage reference which is highly temperature stable (i.e., has low effective temperature coefficients) and supply voltage stable.
2. Description of Related Art
Modern electronic devices employ a wide variety of integrated circuits to carry out various logic functions, including simple designs for, e.g., wristwatches, and more complicated designs for, e.g., data processing systems. An integrated circuit (IC) is essentially a multitude of interconnected circuit elements, such as logic gates, amplifiers, inverters, etc., which are formed of electrical components such as transistors, diodes, resistors, etc. These components are miniaturized, and fabricated on a common substrate.
The substrate of an integrated circuit is formed of a semiconductor material, such as silicon or germanium, which has been doped (mixed with certain impurities) in a pattern to create solid state elements, such as complementary metal-oxide semiconducting (CMOS) devices. An IC can be single layer, with interconnecting leads formed on the substrate between circuit elements, or multi-layer, with interconnecting vias formed between adjacent and non-adjacent layers.
A primary challenge in designing precision integrated circuits is to control circuit parameters, such as bias currents, in view of temperature variations and supply variations. During the design of integrated circuits, anticipating and controlling operating fluctuations due to changes in environmental conditions such as temperature, requires a complicated analysis. Temperature sensitivity, fabrication variations and supply voltage variations exhibit complex relationships among each other. In integrated circuits, precision oscillators (more particularly, oscillators with feedback) are designed to comply with exacting specifications that have little margin for deviation. Surmounting these tight tolerances over a wide range of temperatures is very challenging for a designer. For such oscillators, these difficulties are exacerbated by today's higher frequency requirements. Precision circuits which are very sensitive to temperature variations include voltage-controlled oscillators, which are used in phase-lock loop circuits that provide clock signals to, e.g., high-speed data processing systems.
A variety of techniques are known for stabilization of a circuit over a temperature range which the circuit may endure. For example, temperature variations in a voltage-controlled oscillator can be significantly reduced by temperature-independent biasing techniques. One problem with this approach, however, relates to the use of lower power voltage supplies for the reference circuits.
In general, as CMOS technology continues to evolve to lower supply voltages, most existing analog circuits designed for higher supply voltages will fail to operate due to the large degree of device stacking. Reference circuits in particular, such as thermal voltage (V
therm
) or bandgap generators, are often composed of four levels of stacking, and have poor characteristics below a power supply (V
dd
) of about 1.5 volts.
FIG. 1
shows a common V
therm
-referenced current source. Transistors Q
1
and Q
2
have areas that differ by a factor of n, and the feedback loop formed by CMOS devices M
1
, M
2
, M
3
, and M
4
forces these two transistors to operate at the same bias current. As a result, the circuit's output current I
out
is equal to [V
therm
*ln(n)]/R, where R is the resistance of resistor R, and V
therm
is kT/q (k is Boltzmann's constant, T is absolute temperature, and q is electron charge). It turns out that both R and V
therm
have positive temperature coefficients (TCs), which yield a relatively temperature-independent output current for supply voltages above 1.5 volts; however, the four levels of stacking (M
4
, M
2
, R, Q
2
) make it unsuitable for lower voltages.
Another temperature-independent biasing technique uses base-to-emitter voltage (V
be
) referencing, as shown in FIG.
2
. In this design, the same feedback loop is present, but the absence of a second transistor makes the output current I
out
equal to V
be
/R, where V
be
is the base-to-emitter voltage of transistor Q
1
. This approach may be implemented with 3-high stacking, but it still has a large overall negative TC due to the negative TC of V
be
, and to the large positive TC of diffused and polysilicon resistors.
Bandgap generators use a weighted combination of V
therm
and V
be
generators to produce very low temperature dependence, as seen in FIG.
3
. The circuit's output current I
out
is equal to V
o
/R
2
, where R
2
is the resistance of resistor R
2
and V
o
(the output voltage) is equal to V
be
+xV
therm
ln(n). The parameter “x” (the multiplier for the resistor connected to the input of the operational amplifier) represents the weighting of the V
therm
-dependent portion of the output voltage. While this design can thus be tuned to substantially reduce overall temperature dependence, it still suffers at low supply voltages (V
dd
), from the large degree of device stacking in the V
therm
generator (four levels of stacking).
In light of the foregoing, it would be desirable to devise a voltage reference circuit for low V
dd
applications which has an improved effective temperature coefficient. It would be further advantageous if the circuit could be used in high-speed oscillator applications.
SUMMARY OF THE INVENTION
It is therefore one object of the present invention to provide an improved voltage reference circuit which can be used with digital or analog circuits.
It is another object of the present invention to provide such a voltage reference circuit which has a relatively low effective temperature coefficient.
It is yet another object of the present invention to provide such a voltage reference circuit which retains its temperature-independence for supply voltages lower than about 1.5 volts, and which is insensitive to supply voltage variations.
The foregoing objects are achieved in a circuit providing a thermal voltage referenced current source, generally comprising a power supply, a plurality of current mirrors coupled in an unstacked manner, each powered by the power supply such that the current mirrors operate with approximately equal bias currents, and at least one reference voltage output connected to a node of one of the current mirrors, wherein a reference current at the reference voltage output has a temperature coefficient with an absolute value of no more than about 1000 ppm/°C. This very low temperature dependence can be achieved using this unstacked construction with power supplies having a voltage of 1.5 volts or less. In an illustrative embodiment, the reference current changes by less than about one percent within a wide range of operating temperatures (5 to 105° C.). A gain stage may be formed using a current source, and a pair of source-coupled transistors connected to the current source and controlled by the current mirrors. For an undesirable case where the currents in the circuit are near zero, means can be provided for injecting a current into the current mirrors to correct the state. A bandgap generator can be constructed using the foregoing thermal voltage reference circuit, which has a positive temperature coefficient, in combination with a negative temperature coefficient base-to-emitter voltage (V
be
) reference circuit, to yield a reference circuit having an even lower overall temperature dependence.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.
REFERENCES:
patent: 5739682 (1998-04-01), Kay
patent: 5818292 (1998-10-01), Slemmer
patent: 5838188 (1998-11-01), Taguchi
patent: 5900773 (1999-05-01), Susak
Callahan Timothy P.
Felsman, Bradley, Vaden, Gunter & Dillion, LLP
International Business Machines - Corporation
Nguyen Minh
Salys Casimer K.
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