Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2001-03-12
2003-03-18
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S542000
Reexamination Certificate
active
06535053
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for obtaining a temperature-independent voltage reference by means of an energy gap reference circuit using at least one bipolar transistor and a voltage source as well as a circuit arrangement for obtaining a temperature-independent voltage reference.
2. Prior Art
When using bipolar transistors as well as electronic components such as, for instance, analog-to-digital converters (A/D converters), known temperature dependences of the transistor parameters, or of the circuit, will have to be taken into account if a temperature-independent voltage reference is to be provided. In particular, the characteristic data of a bipolar transistor are strongly temperature-dependent, the temperature-dependent context between the collector current I
C
and the base emitter voltage U
BE
being of particular relevance. The dependence of U
BE
on the temperature T results from the following equation:
I
C
=
I
S
e
qU
BE
kT
(
1
)
The reason for such a temperature dependence of I
C
is the temperature dependence of the cutoff current I
S
and of the temperature voltage
U
T
=
kT
q
,
wherein, taking into account the temperature dependence of the cutoff current
I
S
=
Ae
-
qU
G
kT
T
X
,
the following relation applies:
(
2
)
I
C
=
Ae
-
U
G
q
kT
+
U
BE
q
kT
T
X
,
(
3
)
in which k is the Boltzmann constant (1.38×10
−23
VAs/K), q is the elementary charge=1.602×10
−19
As, U
G
≈1.12 V is the (band) gap voltage of silicon, T is the temperature, x is an empirical constant and A is a proportionality factor. In known circuit arrangements, the temperature dependence of U
G
is usually neglected.
With most bipolar transistors, an increase of I
c
to double its value results from the above relations at a temperature increase by 11° K. In circuits that serve to obtain voltage references, it has already been known to basically use as a voltage reference the base emitter voltage of a bipolar transistor. In such known analog circuits, a voltage having a symmetrically equal positive temperature coefficient is added in order to compensate for the known high temperature dependence, said voltage being generated in a second transistor. Therefore, the known gap voltage reference circuits used to obtain a voltage reference, as a rule, presuppose two transistors selected as to their characteristics, the selection having to be made with slight tolerances.
SUMMARY OF THE INVENTION
The invention aims to provide a method of the initially defined kind, which uses only a single bipolar transistor and, therefore, renders the selection of a second transistor tuned to the characteristics of the first transistor superfluous. Moreover, the invention aims to further reduce the temperature dependence of the measured values and to achieve a temperature compensation at a substantially higher accuracy. To solve this object, the method according to the invention essentially consists in that only a single bipolar transistor is connected in series with a resistor, that different voltages are facultatively applied, that the voltages are detected upstream and downstream of the series resistor and fed to an A/D converter and that the gain constant of the A/D converter is calculated from the digitalized measurements and used to correct the measurements. The fact that, within the context of the method according to the invention, an A/D converter is used in addition and the signals are subsequently processed in the digital form, additionally involves the temperature dependence of such ADC circuits, which must be compensated for. Within the context of the method according to the invention, the gain constant of the A/D converter, therefore, is determined from a plurality of measurenments for the respectively prevailing temperature and may each be updated accordingly such that actually corrected values will be available, which are characterized by a higher precision than is feasible with analog circuits.
According to a preferred realization of the method according to the invention, it is proceeded in a manner that, in order to correct the ADC gain constant, a value for the base emitter voltage of the bipolar transistor and a value for the cutoff current of the bipolar transistor are measured from the voltage drop on the resistor and that, by applying a computational technique, the temperature-dependent portions of the two measured values are eliminated and a gain constant applying for the respective temperature prevailing at the time of measurement is determined.
In order to determine the gain constant, it is proceeded within the context of the method according to the invention in a manner that the gain constant is calculated by
S
=
-
1
-
ln
I
x
+
x
+
ln
A
+
x
ln
q
d
ln
I
x
k
+
ln
R
-
1
+
d
ln
I
x
U
G
+
x
(
13
)
wherein lnI
x
is the natural logarithm of the measurement for the collector current, x and A are constants, R is the resistance and U
G
is the (band) gap voltage (for Si≈1.12 V). Since the gain constant always is each newly calculated from a plurality of measurements by the algorithm explained in more detail below, it is feasible within the context of the method according to the invention and in correspondence with a preferred further development that the value for S is updated continuously or at regular time intervals and applied to calculate the actual reference voltage and, if desired, to precisely determine test voltages.
The circuit arrangement according to the invention used to obtain a temperature-independent reference voltage may be designed in a particularly simple manner, requiring but a small number of components. The circuit arrangement is essentially characterized in that it comprises, placed in series, a bipolar transistor and a resistor R connected with the transistor, that an A/D converter (ADC) configured to yield digitalized voltage measurements is connected via switches to ports provided on either side of the resistor R, and that the digital ADC signals are fed to a computer to determine the gain constant, from which the corrected voltage signal can be read out digitally.
The switch in a particularly simple manner may be designed as a multiplexer component whose inputs are switched by a control signal of the computer and comprise connectors or ports at which the voltages to be measured are applied by actuation of the associated switch. The multiplexer, thus, transmits the analog signals to the analog input of the ADC as a function of the switch position. In principle, the circuit arrangement may be established using PNP or NPN transistors. In the case of PNP transistors, the emitter is connected with the resistor and the collector that is coupled with the base is connected to ground, the adjustable voltage source being connected to the other port of the resistor.
A preferred use of the circuit arrangement according to the invention is the use in a digital voltmeter, the principal mode of operation as well as the circuit arrangement being in no way limited to such digital voltmeters.
In the following, the invention will be explained in more detail by way of the computational algorithm chosen for the calculation of the gain constant and by way of an exemplary circuit used with a digital voltmeter.
Departing from the basic relationship reflecting the dependence of U
BE
on the temperature T in a bipolar transistor
I
C
=
I
S
e
qU
BE
kT
,
(
1
)
it is then further considered that not only the collector current but also the cutoff current I
S
is temperature-dependent. The temperature dependence of the cutoff current follows the relation
I
S
=
Ae
-
qU
G
kT
T
x
,
(
2
)
the meanings indicated above also applying in the instant relations.
By inserting the meaning I
S
according to equation (2) in the equation (1), the relation
I
C
=
Ae
-
U
G
q
kT
+
U
BE
q
kT
&
Austria Mikro Systeme International Aktiengesellschaft
Joyce Kevin E.
Lam Tuan T.
Nguyen Hiep
LandOfFree
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