Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2002-10-15
2004-07-20
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S313000
Reexamination Certificate
active
06765431
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of bandgap references.
2. Prior Art
Bandgap references are well known in the prior art, and are commonly used in integrated circuits to provide a reference that is independent of temperature. These references make use of two characteristics of the base-emitter voltage (VBE) of a bipolar transistor. In particular, the base-emitter voltage VBE of a junction transistor may be expressed as follows:
V
BE
=
V
g0
+
(
V
BE0
-
V
g0
)
(
T
T
0
)
+
NKT
q
ln
(
T
0
T
)
+
KT
q
ln
(
I
C
I
C0
)
where:
T=temperature
I
C
=the transistor collector current
I
C0
=collector current for which V
BEO
was determined
V
g0
=bandgap voltage of silicon at temperature T
0
V
BE0
=base to emitter voltage V at T
0
and I
CO
q=electron charge
N=structure factor
K=Boltzmann's constant
The dominant terms are the first two terms:
V
g0
+
(
V
BE0
-
V
g0
)
(
T
T
0
)
and since V
g0
is larger than V
BE0
, the net result is a negative temperature coefficient for the V
BE
of a transistor.
If one subtracts the VBEs of two identical transistors Q
1
and Q
2
operating with unequal collector currents, there results:
V
BE1
-
V
BE2
=
KT
q
ln
(
I
C1
I
C0
)
-
KT
q
ln
(
I
C2
I
C0
)
or
:
V
BE1
-
V
BE2
=
KT
q
ln
(
I
C1
I
C2
)
This frequently is expressed in terms of current densities J
1
and J
2
in the two transistors as follows:
V
BE1
-
V
BE2
=
KT
q
ln
(
J
1
J
2
)
or for transistors that are of different areas (area ratio of 1 to n) but otherwise identical and having the same collector currents, can be expressed in terms of the transistor areas A as follows:
V
BE1
-
V
BE2
=
KT
q
ln
(
A
2
A
1
)
=
KT
q
ln
(
n
)
In bandgap references, two transistors are usually operated at different current densities, typically by using two transistors of different areas, but having equal collector currents. Accordingly, for specificity in the descriptions to follow, it will be assumed that the respective two transistors have different areas and have substantially equal collector currents, though this is not a specific limitation of the invention, as transistors of the same area could be operated at different collector currents, or transistors of different areas could be operated at different collector currents in the practice of the present invention.
Now referring to
FIG. 1
, a circuit diagram for a classic bandgap reference may be seen. In such a circuit, resistors R
2
and R
3
could be equal resistors with amplifier A
1
, preferably a high input impedance amplifier, driving the output voltage VBG to the voltage required to provide a zero differential input to the amplifier. Accordingly, under these conditions, the currents through resistors R
2
and R
3
are equal currents, and accordingly, neglecting the base currents of transistors Q
1
and Q
2
, provide equal collector currents to transistors Q
1
and Q
2
. In such a circuit, transistor Q
2
could have an area n times the area of transistor Q
1
, so that the current density in transistor Q
2
is only 1
times the current density in transistor Q
1
.
Amplifier A
1
forces the collector voltages of transistors Q
1
and Q
2
to be equal. Because the collector voltages are equal, the voltage V
R1
across resistor R
1
is as follows:
V
R1
=VBE
q1
−VBE
q2
Where:
VBE
q1
is the base emitter voltage of transistor Q
1
, and
VBE
q2
is the base emitter voltage of transistor Q
2
Referring back to the prior equations, it may be seen that the difference in these two VBE'S, the voltage across resistor R
1
, is proportional to absolute temperature. Also, since the current in resistor R
2
equals the current in resistor R
1
, the voltage across resistor R
2
is also proportional to absolute temperature, and can be thought of as amplifying the voltage across resistor R
1
by a factor of (R
1
+R
2
)/R
1
.
In addition to the voltages proportional to absolute temperature (PTAT) across resistors R
1
and R
2
, that leg of the circuit also includes the base emitter voltage VBE of transistor Q
2
. Again, referring to the prior equations, the VBE of a transistor linearly decreases with increases in temperature. Accordingly, by proper selection of the value of resistor R
2
in relation to the value of resistor R
1
, the linear rate of increase in the PTAT voltage across the combination of resistors R
1
and R
2
with temperature increase may be made to equal the linear rate of decrease of the base emitter voltage V
BE
of transistor Q
2
with temperature increases, so that the bandgap voltage output of the circuit VBG is substantially temperature insensitive.
In typical prior art bandgap references, the area ratio for transistors Q
1
and Q
2
may be, by way of example, on the order of 10 to 1, which area ratio will provide a VBE difference, the voltage across resistor R
1
, on the order of 60 millivolts. The output voltage of the bandgap reference needed to balance the positive temperature coefficient of the voltage across resistors R
1
and R
2
with the negative temperature coefficient of the VBE of transistor Q
2
for a silicon transistor is typically a little over 1.2 volts. Accordingly, resistor R
2
typically is approximately an order of magnitude larger in resistance than resistor R
1
.
The resistor R
2
effectively amplifies the voltage across resistor R
1
, including the noise across resistor R
1
. In a typical bandgap reference circuit, resistor R
1
is the single largest source of wideband noise. The noise across resistor R
1
includes not only the thermal noise of resistor R
1
, but also the shot noise of transistors Q
1
and Q
2
, and for that matter, the noise associated with the base resistance of transistors Q
1
and Q
2
.
In electronic systems, the voltage reference provides the known standard that the rest of the system relies upon. Electronic circuit noise present in voltage references can limit the overall accuracy and ultimately the usefulness of the reference. Previous methods of reducing noise have depended on increased circuit power consumption or expensive semiconductor process development. The present invention improves the noise performance of bandgap references using a new circuit arrangement with existing process technology.
BRIEF SUMMARY OF THE INVENTION
Low noise bandgap references of the type providing a temperature independent output by balancing the proportional to absolute temperature dependence of the difference in base-emitter voltages of two transistors operating at different current densities with the negative temperature coefficient of the base-emitter voltage of a transistor are disclosed. The bandgap references disclosed reduce the noise characteristic of such references by balancing the difference in base-emitter voltages of a first number of pairs of transistors, each pair having two transistors operating at different current densities, with the negative temperature coefficient of the base-emitter voltage of a second number of transistors, the second number being less than the first number. Various embodiments are disclosed, including embodiments having an output corresponding to the bandgap of the transistor material (silicon in the exemplary embodiment), and multiples of the bandgap of the transistor material.
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patent: 2002/0000843 (2002-01-01), Lee
Blakely , Sokoloff, Taylor & Zafman LLP
Cunningham Terry D.
Maxim Integrated Products Inc.
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