Bandgap voltage reference circuit with an increased...

Details Bandgap voltage reference circuit with an increased... Bandgap voltage reference circuit with an increased...

1999-11-18

2001-05-15

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

C327S513000, C323S315000

Reexamination Certificate

active

06232829

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bandgap voltage reference circuit and, more particularly, to a bandgap voltage reference circuit with an increased difference voltage &Dgr;V
BE
.
2. Description of the Related Art
A bandgap voltage reference circuit is a circuit that provides a reference voltage that is ideally temperature independent. Bandgap voltage reference circuits are commonly used as stand-alone voltage sources, and as building blocks in analog-to-digital converters, digital-to-analog converters, bias line generators, and other common analog circuits.
FIG. 1
shows a schematic diagram that illustrates a conventional bandgap voltage reference circuit
100
. As shown in
FIG. 1
, circuit
100
includes a current source
110
that outputs a current I that is proportional to absolute temperature (PTAT), and transistors Q
1
, Q
2
, and Q
3
. The collectors of transistors Q
1
and Q
2
are connected to current source
110
through resistors R
1
and R
2
, respectively, while the collector of transistor Q
3
is directly connected to current source
110
.
In addition, the emitters of transistors Q
1
and Q
3
are connected together, while the emitter of transistor Q
2
, which has an emitter area that is N times larger than the emitter area of transistor Q
1
, is connected to the emitter of transistor Q
1
through resistor R
3
. Further, the bases of transistors Q
1
and Q
2
are connected to the collector of transistor Q
1
, while the base of transistor Q
3
is connected to the collector of transistor Q
2
.
In operation, circuit
100
provides a nearly temperature independent reference voltage V
REF
between the collector and emitter of transistor Q
3
by summing a voltage that has a positive temperature coefficient with voltage that has a negative temperature coefficient of equal value.
For example, when the temperature increases by one degree, the voltage with the positive temperature coefficient increases by, for example, 2 mV while the voltage with the negative temperature coefficient decreases by 2 mV. Since the voltages vary an equal amount in opposite directions, the reference voltage V
REF
remains unchanged when the temperature increases by one degree.
With respect to the voltage with the positive temperature coefficient, it is known that the difference between the base-to-emitter voltages of a pair of bipolar transistors that are forced to operate with unequal emitter current densities is a voltage with a positive temperature coefficient.
In circuit
100
, since transistor Q
2
has an emitter area that is N times larger than the emitter area of transistor Q
1
, transistors Q
1
and Q
2
operate with unequal emitter current densities. As a result, a difference voltage &Dgr;V
BE
, which is equal to V
BEQ1
−V
BEQ2
, has a positive temperature coefficient.
As shown in
FIG. 1
, the base-to-emitter voltage V
BEQ1
of transistor Q
1
is equal to the base-to-emitter voltage V
BEQ2
of transistor Q
2
and the voltage VR
3
across resistor R
3
, i.e., V
BEQ1
=V
BEQ2
+VR
3
. Rearranging yields V
BEQ1
−V
BEQ2
=VR
3
.
Since the difference voltage &Dgr;V
BE
is equal to the difference between the base-to-emitter voltages (&Dgr;V
BE
=V
BEQ1
−V
BEQ2
), the difference voltage &Dgr;V
BE
is also equal to the voltage VR
3
across resistor R
3
. Since the difference voltage &Dgr;V
BE
has a positive temperature coefficient, the voltage VR
3
across resistor R
3
must also have a positive temperature coefficient.
The voltage VR
3
across resistor R
3
(and the value of resistor R
3
) define the resistor current which, in turn, defines the emitter current I
EQ2
of transistor Q
2
. As a result, the emitter current I
EQ2
is proportional to the difference voltage &Dgr;V
BE
and, therefore, must have a positive temperature coefficient.
In addition, the collector current I
CQ2
of transistor Q
2
is approximately equal to the emitter current I
EQ2
of transistor Q
2
due to the beta of transistor Q
2
. As a result, the collector current I
CQ2
of transistor Q
2
is proportional to the difference voltage &Dgr;V
BE
and, therefore, must have a positive temperature coefficient.
Thus, since the collector current I
CQ2
is proportional to the difference voltage &Dgr;V
BE
, the voltage VR
2
across resistor R
2
is proportional to the difference voltage &Dgr;V
BE
, and therefore must also have a positive temperature coefficient.
The voltage VR
2
is also known as an amplified difference voltage &Dgr;V
BE
because the voltage VR
2
is approximately equal to R
2
/R
3
times the voltage VR
3
which, in turn, is equal to the difference voltage &Dgr;V
BE
.
With respect to the voltage with the negative temperature coefficient, it is known that the base-to-emitter voltage of a bipolar transistor has a negative temperature coefficient when the collector current of the transistor is proportional to absolute temperature.
As noted above, current source
110
outputs a current I that is proportional to absolute temperature. As a result, the base-to-emitter voltage V
BEQ3
of transistor Q
3
has a negative temperature coefficient.
Thus, circuit
100
provides a nearly temperature independent reference voltage V
REF
between the collector and emitter of transistor Q
3
by summing the voltage VR
2
, the amplified difference voltage &Dgr;AV
BE
, with the base-to-emitter voltage V
BEQ3
across the base-to-emitter junction of transistor Q
3
.
The amplified difference voltage &Dgr;AV
BE
(VR
2
) has a positive temperature coefficient of approximately +2 mV/° C., while the base-to-emitter voltage V
BEQ3
has a negative temperature coefficient of approximately −2 mV/° C. Thus, by summing voltages which have equal and opposite temperature coefficients, the total voltage, i.e., the reference voltage V
REF
, remains unchanged as the temperature changes. (See also U.S. Pat. No. 3,617,859 to Dobkin which is hereby incorporated by reference.)
FIG. 2
shows a schematic diagram that illustrates a conventional bandgap voltage reference circuit
200
. Circuit
200
is similar to circuit
100
and, as a result, utilizes the reference numerals to designate the structures which are common to both circuits.
As shown in
FIG. 2
, circuit
200
differs from circuit
100
in that circuit
200
eliminates both current source
110
and transistor Q
3
, and instead utilizes an operational amplifier (op amp)
210
and a resistor R
4
. As with circuit
100
, transistor Q
2
of circuit
200
has an emitter area that is N times larger than the emitter area of transistor Q
1
of circuit
200
.
Op amp
210
has a positive input connected to the collector of transistor Q
1
, a negative input connected to the collector of transistor Q
2
, and an output connected to the bases of transistors Q
1
and Q
2
. Resistor R
4
, in turn, has a first end connected to resistor R
3
and the emitter of transistor Q
1
, and a second end connected to ground.
In operation, the resistances of resistors R
1
and R
2
are equal, and develop voltages at the collectors of transistors Q
1
and Q
2
which are equal when the collector currents are equal. When the collector currents, which are proportional to absolute temperature, are not equal, op amp
210
responds to the unequal collector voltages by changing the base voltages of transistors Q
1
and Q
2
until the collector currents of transistors Q
1
and Q
2
are equal.
In circuit
200
, transistors Q
1
and Q
2
are again forced to operate with unequal emitter current densities due to the difference in emitter areas. As a result, the difference voltage &Dgr;V
BE
is again equal to the voltage VR
3
across resistor R
3
, and the voltage VR
3
again has a positive temperature coefficient.
The voltage VR
3
across resistor R
3
defines the emitter current I
EQ2
of transistor Q
2
. As a result, the emitter current I
EQ2
is proportional to the difference voltage &Dgr;V
BE
, and must have a positive temperature coefficient.
Since the collector currents, the base currents, and the betas of transistors Q
1
and

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