Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2001-01-29
2002-04-16
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S512000, C323S312000
Reexamination Certificate
active
06373330
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a voltage reference circuit and in particular to a bandgap voltage reference circuit that eliminates the need for an error amplifier.
BACKGROUND
Bandgap voltage reference circuits are well known in the art. As is well understood, bandgap voltage reference circuits produce stable voltages that are nearly temperature independent. Stable and temperature independent voltages are useful in voltage regulators, and are commonly used in, for example, integrated circuit technology.
The fundamental principle of a bandgap voltage reference circuit is the summation of one voltage, which is proportional to absolute temperature, with another voltage, which is inversely proportional to absolute temperature. The proportional to absolute temperature first voltage is commonly generated by producing the difference between the base emitter voltages V
BE
of two bipolar transistor devices and is referred to as &Dgr;V
BE
. The quantity &Dgr;V
BE
can be expressed as:
Δ
V
BE
=
kT
q
ln
(
N
)
equ
.
1
where k is Boltzmann's constant, T is the absolute temperature, q is the electron charge, and N is the current density ratio of the two devices. As is well known to those skilled in the art, the current density ratio N may be generated by the two devices having a different area but the same current, or by having a different current with the same area. In bipolar technology, a device has a larger area than another device when the device has an emitter that is relatively larger than the emitter of the other device.
The voltage &Dgr;V
BE
is scaled with a multiplication factor M an d is then summed with the inversely proportional to absolute temperature second voltage. The second voltage is the base emitter voltage of a bipolar transistor and is referred to as V
BE
. Thus, the reference voltage can be expressed as:
V
REF
=
M
*&Dgr;
V
BE
+
V
BE
equ. 2
The term M*&Dgr;V
BE
is often referred to as a proportional-to-absolute-temperature (PTAT) voltage V
PTAT
. The multiplication factor M is an analytically or empirically derived factor that adjusts the proportions of the two components of equation 2 until the temperature coefficient of the resulting sum is nominally zero.
The generation of the voltages &Dgr;V
BE
and V
BE
typically requires that the current in the two transistor devices is precisely the same. To ensure that the collector currents in the two devices are the same, the difference in voltages across equal collector load resistors, e.g.,
56
and
58
in
FIG. 2
are detected and an error term is generated. The error term is then amplified and applied to the circuit in a closed loop configuration. Thus, conventional bandgap voltage reference circuits require the use of error detection and an amplifier circuit to strictly control the operation of the circuit.
FIG. 1
shows a conventional Widlar bandgap voltage reference circuit
10
, which is well known in the art. Widlar voltage reference circuit
10
includes two NPN bipolar transistor devices
12
and
14
with their bases commonly connected to the collector of transistor
12
in a current mirror configuration. The emitter of transistor
12
is connected directly to ground while the emitter of transistor
14
is connected to ground through resistor
16
. The collectors of transistors
12
and
14
are connected to a current source
18
through respective resistors
20
and
22
. Current source
18
is connected to a voltage source Vcc. A third NPN bipolar transistor
24
, acting as an error-feedback device, has its base connected to the collector of transistor
14
, while its collector is connected directly to current source
18
and its emitter connected directly to ground.
The operation of Widlar voltage reference circuit
10
is well known in the art. Widlar voltage reference circuit
10
produces a constant bandgap voltage V
REF
that is equal to the base emitter voltage V
BE
across transistor
12
and the voltage V
PTAT
across resistor
20
. Transistor
24
shunts an amount of current from current source
18
to ground, which controls the amount of current passing through transistors
12
and
14
and thereby controls the bandgap voltage V
REF
. Transistor
14
is larger than transistor
12
so that the current flowing through the two transistors can be equalized, i.e., to offset the voltage drop across resistor
16
. If voltage V
REF
begins to rise, the current passing through transistor
24
increases, which lowers V
REF
. If on the other hand, voltage V
REF
begins to decrease, the current passing through transistor
24
will decrease, which will raise V
REF
. Thus, transistor
24
is acting as an error amplifier, controlling the output of Widlar bandgap voltage reference circuit
10
.
FIG. 2
shows another well-known bandgap voltage reference circuit.
FIG. 2
is a Brokaw bandgap voltage reference circuit
50
, which includes two NPN bipolar transistors
52
and
54
with their bases connected. The collectors of transistors
52
and
54
are connected to a voltage supply Vcc via respective resistors
56
and
58
. The emitter of transistors
52
is directly connected to node
60
, while the emitter of transistor
54
is connected to node
60
through a resistor
62
. Another resistor
64
connects node
60
to ground. One input terminal of an error amplifier
66
is connected to the collector of transistor
52
, while the other input terminal is connected to the collector of transistor
54
. The output of error amplifier
66
produces a bandgap voltage V
REF
, which is fed-back to the bases of transistors
52
and
54
. Thus, the output signal from error amplifier
66
provides the base current for transistors
52
and
54
.
The operation of Brokaw bandgap voltage reference circuit
50
is well known to those of ordinary skill in the art. Typically in Brokaw circuit
50
, resistors
56
and
58
have the same values and transistor
54
has a larger area than transistor
52
so that the currents flowing through transistors
52
and
54
are equalized and transistors
52
and
54
have an area ratio of N. During operation, error amplifier
66
attempts to equalize the current flowing through transistors
52
and
54
by forcing the voltage drop across resistors
56
and
58
to be equal. Thus, the difference in base emitter voltages &Dgr;V
BE
is equal to (kT/q)lnN as described in equation 1. The &Dgr;V
BE
term is imposed across resistor
62
that connects the two emitters of transistors
52
and
54
. The resulting current, which is proportional to absolute temperature is then developed across resistor
64
thereby producing the PTAT voltage V
PTAT
. The bandgap voltage V
REF
is equal to the base emitter voltage V
BE
of transistor
52
plus the voltage V
PTAT
across resistor
64
.
As discussed in reference to equation 2, the V
PTAT
term is equal to M*&Dgr;V
BE
. The multiplicative term M in bandgap voltage reference circuit
50
can be expressed as:
M
=
1
+
R
64
R
62
equ
.
3
where R
64
is the resistance of resistor
64
, and R
62
is the resistance of resistor
62
. Thus, the ratio of the resistances of resistors
62
and
64
can be adjusted to achieve the desired target M.
As can be seen in
FIGS. 1 and 2
, conventional bandgap voltage reference circuits
10
and
50
generate two voltages, a base emitter voltage V
BE
and the PTAT voltage V
PTAT
. The generation of these two voltages requires that the collector currents in the two transistors be controlled through the detection of any differences voltages across equal collector load resistors
56
and
58
in
FIG. 2
, producing an error term. The error term is amplified in the opposite direction known as “negative feedback” to correct the differences in the base emitter voltages.
Thus, conventional bandgap voltage reference circuits require an error amplifier, which increases the complexity of the circuit, as well as the space and cost requirements. Although the ratio of two such resistors is typically much more precise than the abso
Halbert Michael J.
Kwok Edward C.
National Semiconductor Corporation
Skjerven Morrill & MacPherson LLP
Tra Quan
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