Low noise bandgap voltage reference circuit

Details Low noise bandgap voltage reference circuit Low noise bandgap voltage reference circuit

2001-08-01

2002-10-08

Berhane, Adolf Deneke (Department: 2838)

Electricity: power supply or regulation systems

Self-regulating

Using a three or more terminal semiconductive device as the...

C327S539000

Reexamination Certificate

active

06462526

ABSTRACT:

TECHNICAL FIELD
This invention relates to generally to analog and mixed signal (analog and digital) integrated circuits, and in particular to bandgap voltage references used in analog and mixed signal integrated circuits.
BACKGROUND
Reference voltages are required for a variety of purposes. For example, reference voltages are used to bias circuits or to supply a reference to which other voltages are compared. Bandgap voltage references are known in the art, and provide a reference voltage that is quite stable over a range of temperatures. The basic operation of a bandgap voltage reference follows the concept of developing a first voltage with a positive temperature coefficient, combining that voltage with a second voltage having a negative temperature coefficient, and relating the two voltages in a complementary sense such that the resultant composite voltage has a very low temperature coefficient, approximately zero. The voltage produced by bandgap voltage references is related to the bandgap, which for silicon is approximately 1.2 V. Hence, the name for these references.
One known type of bandgap reference is the Brokaw bandgap reference. An example of a Brokaw bandgap reference
10
, shown in
FIG. 1
, includes a pair of bipolar transistors Q
2
and Q
1
having their base terminals connected together (although in some Brokaw references there may be a resistor connected between the base terminals). Transistors Q
2
and Q
1
are operated at different current densities, referring to the current flowing through the emitters. In this example, transistor Q
1
is operated at a smaller current density. The operation of Q
2
and Q
1
at different current densities can be achieved in several ways, for example, by transistors Q
2
and Q
1
having unequal emitter areas but operated at equal currents, by transistors Q
2
and Q
1
having equal emitter areas and operated at unequal currents, or by some combination of these arrangements. Resistor R
1
is connected between the emitters of Q
2
and Q
1
, whose base terminals are connected together (although there could also be a resistor connected between the two bases), and thus a voltage is produced across resistor R
1
which is equal to the difference in the base-to-emitter voltages of Q
2
and Q
1
(&Dgr;V
BE
). The current through resistor R
1
is therefore proportional to &Dgr;V
BE
. Because the current through resistor R
1
is proportional to, and perhaps equal to, the emitter current of Q
2
, the current through resistor R
2
is also proportional to &Dgr;V
BE
, as will be the voltage appearing across resistor R
2
.
The base-to-emitter voltage V
BE
for a transistor has a negative temperature coefficient, governed by the following equation:
V
BE
=V
G0
[1−(
TT
0
)]+
V
BE0
(
T/T
0
)+(
nkT/q
)*
ln
(
T
0
/T
)+(
kT/q
)*
ln
(
I
C
/I
C0
)
Where V
G0
is the extrapolated energy bandgap voltage of the semiconductor material at absolute zero (1.205 V for silicon), q is the charge of an electron, n is a constant dependent on the type of transistor (1.5 being a typical example), k is Boltzmann's constant, T is absolute temperature, I
C
is collector current, and V
BE0
is the V
BE
at T
0
and I
C0
. The difference in base-to-emitter voltages, on the other hand, has a positive temperature coefficient governed by the following equation:
&Dgr;
V
BE
=(kT/q)*ln(
J
1
/J
2
)
where J is current density. Reference voltage V
REF
generated at the base of transistors Q
2
and Q
1
thus has a positive-temperature-coefficient component and a negative-temperature-coefficient component. For example, the voltage across resistor R
2
(V
R2
) has a positive temperature coefficient, and the V
BE
of Q
2
has a negative temperature coefficient. Similarly, the voltage across both resistors R
2
and R
1
(V
R2+R1
) has a positive temperature coefficient, and the V
BE
of Q
1
has a negative temperature coefficient. An optional voltage divider including resistors R
F1
and R
F2
is used to achieve an output voltage V
OUT
which is a reference voltage that is temperature stable but greater than voltage V
REF
.
Operational amplifier (OA) senses voltages at the collector terminals of Q
2
and Q
1
and maintains a relatively constant ratio between the currents I
C2
and I
C1
, and thus maintains a relatively constant ratio between the current densities J
1
and J
2
of transistors Q
2
and Q
1
. Load resistors R
L2
and R
L1
are connected between a supply voltage V
B
and the collector of transistor Q
2
and the collector of transistor Q
1
, respectively. For a design having currents I
C2
and I
C1
, equal to one another, load resistors R
L2
and R
L1
will typically be equal to one another. When the output voltage V
OUT
drops below a pre-established optimal level, the ratio of collector currents I
C2
/I
C1
is larger than the ratio of resistors R
L2
/R
L1
, and thus the input to operational amplifier OA is positive. This causes the amplifier OA output V
OUT
to increase so that V
OUT
returns to its optimal level. Conversely, if the output voltage V
OUT
rises above the optimal level, the feedback action of amplifier OA will have the opposite effect.
In any circuit design, including the prior art Brokaw bandgap reference shown in
FIG. 1
, electronic noise will be generated during the circuit's operation. There are various sources of this electronic noise. Two important types of noise generated in bandgap voltage references, and which dictate a minimum quiescent current, are 1/f noise (also known as flicker noise) and wideband noise. In the
FIG. 1
circuit, flicker noise is developed at R
1
and R
2
because of the noise in the base currents of Q
2
and Q
1
which flow through R
1
and R
2
. The flicker noise level is directly related to the magnitude of these base currents. Wideband noise for V
OUT
in the
FIG. 1
circuit is due to the collector currents of Q
2
and Q
1
. Generally, the higher the collector current, the lower the wideband noise. This illustrates that different circuit designs trade reduction in one type of noise for an increase in another type of noise. Consideration of noise in circuit design is becoming increasingly important, because of the need for lower quiescent currents and also because of ever smaller device feature sizes. Different circuit designs are needed that enable circuit designers to meet more stringent noise requirements.
SUMMARY
Generally, the invention is an improved bandgap voltage reference having advantageous noise characteristics. In one aspect, the invention adds two bipolar transistors to a conventional bandgap voltage reference. One of these added transistors is Darlington configured with one of the two bipolar transistors used in a conventional bandgap reference, and the other added transistor is configured similarly with the other bipolar transistor used in a conventional bandgap voltage reference. The configuration is such that a portion of the currents that flow into the collector terminal of the two bipolar transistors of the conventional bandgap reference circuit are diverted away to the respective collector terminals of the added transistors.
In different embodiments, the inventive bandgap reference includes two diode-connected bipolar transistors, or alternatively resistors, coupled between respective emitters of the bipolar transistors used in the conventional bandgap reference and the respective additional bipolar transistors added in accordance with the invention. Different areas of emitters for the bipolar transistor are contemplated, to divert more or less current from the conventionally used bipolar transistors, and to achieve different noise profiles. In addition, the bandgap reference of the present invention may have various design difference known in the art, such as a feedback mechanism, a voltage divider, and a resistor between the base terminals of the bipolar transistors used in conventional bandgap references.
The different embodiments of the invention have one or more of the following advantages. Compared to prior art circuits, the bandgap

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