Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1997-10-01
2001-01-09
Kim, Jung Ho (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C323S313000
Reexamination Certificate
active
06172555
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to voltage reference circuits and more specifically to a bandgap voltage reference circuit that provides a stable voltage reference over a range of operating temperatures.
BACKGROUND OF THE INVENTION
Circuits that provide substantially stable reference voltages under varying conditions have existed for many years. One such circuit is the bandgap voltage reference circuit which is based on the base to emitter voltage (V
BE
) of bipolar junction transistors. The circuit typically utilizes two transistors operating at different current densities. A voltage proportional to the difference between the base to emitter voltages (&Dgr;V
BE
) of the two transistors is developed within the circuit. Typically, the &Dgr;V
BE
voltage developed by the circuit increases with increasing temperature and the V
BE
voltage of the transistor decreases with increasing temperature such that the sum of the two voltages can be arranged to be substantially independent of temperature.
FIG. 1
illustrates a voltage reference circuit
10
known to the prior art described in P. R. Gray and R. G. Meyer,
Analysis and Design of Analog Integrated Circuits
, John Wiley & Sons, New York, N.Y., 1993, at 344-346. In order for a stable operating point to exist, the differential input voltage defined across input terminals
18
and
22
of the operational amplifier
26
must be zero. Thus, the voltage drop across R1
30
must equal the voltage drop across R2
34
. Assuming negligible base currents for transistors Q1
38
and Q2
42
, a &Dgr;V
BE
must exist across resistor R3
46
. As the temperature increases, V
BE
of Q2
42
decreases. The two currents I
1
32
and I
2
36
must have a ratio determined by the ratio of R1
30
to R2
34
. These two currents are the collector currents of the two diode-connected transistors Q1
38
and Q2
42
, assuming base currents are negligible. Thus the difference between their base to emitter voltages is
Δ
V
BE
=
V
T
ln
I
1
I
S2
I
2
I
S1
=
V
T
ln
R2I
S2
R1I
S1
where I
S1
and I
S2
are the device dependent saturation currents of Q1
38
and Q2
42
, respectively. V
T
is given by
V
T
=
kT
q
where k is Boltzmann's constant, T is the absolute temperature in Kelvin, and q is the charge of an electron. &Dgr;V
BE
appears across resistor R3
46
and is proportional to absolute temperature. The same current that flows in R3
46
also flows in R2
34
, so that the voltage across R2
34
must be
V
R2
=
R2
R3
Δ
V
BE
=
R2
R3
V
T
ln
R2I
S2
R1I
S1
The output voltage V
OUT
14
is the sum of the voltage across R1
30
and the voltage across Q1
38
. The voltage across R1
30
is equal to that across R2
34
indicated above. The output voltage is thus
V
OUT
=
V
BE1
+
R2
R3
V
T
ln
R2I
S2
R1I
S1
where V
BE1
is the base to emitter voltage of Q1
38
.
The resulting V
OUT
can be arranged to have an effective temperature coefficient of zero. To achieve this result, the parameters of transistors Q1
38
and Q2
42
, and resistors R1
30
, R2
34
and R3
46
must be strictly controlled.
FIG. 2
illustrates another prior art voltage reference circuit
50
as disclosed in U.S. Pat. No. 3,887,863. In this circuit, the input signals
54
and
58
to the operational amplifier
62
are proportional to the voltage drops across load resistors R1
64
and R2
68
. If the voltage drops are not equal, the operational amplifier output drives the base of transistors Q1
72
and Q2
76
so as to establish equal currents through R1
64
and R2
68
. In this example, &Dgr;V
BE
is proportional to the voltage measured across resistor R3
80
. As the temperature changes the change in &Dgr;V
BE
is compensated by the change in voltage across R3
80
such that the voltage drop across the series combination of Q2
76
and R3
80
is equal to the voltage drop across Q1
72
. The resulting output voltage (V
OUT
)
84
can be arranged to provide a temperature independent voltage reference. Again, proper functioning of this bandgap voltage reference circuit requires critical matching of R1
64
, R2
68
, R3
80
, R4
88
, Q1
72
and Q2
76
.
These prior art references are representative of efforts to improve the stability of bandgap voltage reference sources at the expense of circuit complexity and an increase in the stringency of the component matching requirements. The present invention provides a bandgap voltage reference circuit capable of operation with a low supply voltage. The circuit has a low device count and reduced component matching requirements without loss of performance.
SUMMARY OF THE INVENTION
The bandgap voltage reference circuit of the invention in one embodiment includes an operational amplifier, a first and second transistor, a voltage divider and a non-linear temperature-dependent element. The operational amplifier includes a pair of input terminals and an output terminal. The operational amplifier is sensitive to the difference in the current through its input terminals. Each operational amplifier input terminal is in electrical communication with a corresponding transistor collector. Each transistor emitter is adapted to receive an input reference voltage. The areas of the transistor emitters are unequal. In one embodiment, the applied input reference voltage is ground.
In one embodiment, the voltage divider includes a first resistor having a first terminal in electrical communication with the output terminal of the operational amplifier and a second terminal in electrical communication with the base of the first transistor. The voltage divider also includes a second resistor having a first terminal in electrical communication with the second terminal of the first resistor and a second terminal in electrical communication with the base of the second transistor. In one embodiment, the non-linear temperature-dependent device has one terminal electrically coupled to the second terminal of the second resistor and a second terminal adapted to receive a second input reference voltage. In one embodiment, the ratio of the resistance of the first and second resistors is given by the equation
R1
R2
=
V
OUT
-
V
BE
Δ
V
BE
where V
OUT
is the reference voltage provided by the circuit, V
E
is the voltage drop across the first terminal of the non-inear temperature-dependent element and the second terminal of the non-linear temperature-dependent element, and &Dgr;V
BE
is the differential voltage between the base of the first transistor and the base of the second transistor, where the base currents of the transistors are negligible. In another embodiment, the ratio of the resistance of the first and second resistors is given by the equation
R1
R2
=
V
OUT
-
V
E
Δ
V
BE
In one embodiment, the non-linear temperature-dependent element is a diode. In another embodiment, the non-linear temperature-dependent element is a bipolar junction transistor having a base electrically coupled to the second terminal of the first resistor, an emitter adapted to receive the second input reference voltage, and a collector electrically coupled to the second terminal of the second resistor. In another embodiment, the bipolar junction transistor has a base electrically coupled to its collector instead of the second terminal of the first resistor.
The invention also relates to a method for providing a bandgap voltage reference. The method includes providing a voltage reference subcircuit comprising a reference voltage input terminal, an operational amplifier, and a first and second transistor. The operational amplifier includes a first and second input terminal and an output terminal. Each transistor includes a collector in electrical communication with a corresponding operational amplifier input and an emitter in electrical communication with the reference voltage input terminal. The method includes the steps of applying an input reference voltage to the reference voltage input terminal and generating an output v
Kim Jung Ho
Sipex Corporation
Testa Hurwitz & Thibeault LLP
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