Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2002-11-15
2004-08-10
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C323S314000
Reexamination Certificate
active
06774711
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to circuits that provide a temperature independent reference voltage, and more specifically, to bandgap voltage reference circuits.
BACKGROUND ART
In an integrated circuit, a bandgap reference circuit provides a substantially constant reference voltage output that is immune to variation in fabrication process, operating temperature, and supply voltage. The bandgap reference circuit makes use of the predictable behavior of bandgap energy of semiconductor material. A typical bandgap reference circuit employs a semiconductor bipolar pn junction (diode) device that has a negative temperature coefficient (i.e. its output voltage falls with rising temperature), and complements it with a pair of bipolar junction devices, each having a different emitter cross-sectional area, that generates a voltage difference that has a positive temperature coefficient, thereby producing a voltage output that is invariant to temperature change.
FIG. 2
shows a typical voltage reference of the prior art. As shown, the reference circuit
10
could be viewed as having been constructed with three functional components: a proportional-to-absolute-temperature (PTAT) block
14
that provides a positive temperature coefficient, a diode connected bipolar junction transistor
16
that provides a negative temperature coefficient, and a current mirror
12
that joins PTAT block
14
and the bipolar junction transistor
16
together. The PTAT block
14
is made up of a first and second bipolar junction transistors
18
and
20
connected together at their bases, the first bipolar junction transistor
18
having an emitter cross-sectional area that is only a fraction of the second one. The current mirror block
12
, which consists of NMOS transistors
26
,
28
, and
30
, mirrors the current flowing though the PTAT block
14
to the diode connected bipolar junction transistor
32
. Due to the differential in emitter cross-sectional area between the first bipolar junction transistor
18
and the second bipolar junction transistor
20
, the current density going through each transistor differs, which gives rise to the effect that each transistor would have a different base-to-emitter voltage (Vbe). The difference between the respective base-to-emitter voltages, denoted as &Dgr;Vbe, can be derived by one skilled in the art to be:
Δ
V
be
=
kT
q
ln
X
where k is the Boltzmann's constant, T is the absolute temperature, q is the electrical charge, and X is the scaling factor of the emitter cross-sectional area. As shown in the equation above, the term &Dgr;Vbe is directly proportional to the absolute temperature T. The reference current I
36
can then be expressed as
I
=
V
R1
R1
=
Δ
V
be
R1
=
kT
ln
X
qR1
Since the current I
36
is also mirrored to the branch with the diode connected bipolar junction transistor
32
, the output reference voltage
34
can be expressed as
V
ref
=
V
be
+
IR
2
=
V
be
+
R
2
R
1
kT
ln
X
q
As it is shown in the equation above, the reference voltage Vref
34
is a function of the Vbe of the diode connected bipolar junction transistor
32
and the &Dgr;V
be
of the first and second bipolar junction transistors
18
and
20
, scaled by the ratio of R
2
and R
1
.
A more robust prior art bandgap voltage reference circuit, which is shown in
FIG. 3
, employs an operational amplifier
40
to take the place of the current mirror
12
. The op amp
40
provides a feedback control loop pathway that keeps the two input nodes of the amplifier
40
at approximately the same voltage in the steady state. In so doing, the voltage difference &Dgr;V
be
between the two diodes, D
1
and D
2
, is amplified, which contributes to its higher accuracy. However, the higher accuracy comes with penalties in the form of added circuit complexity and increased power consumption as a typical op amp requires a biasing circuit that draws additional power and takes up additional space. It is the object of the present invention to have a bandgap voltage reference circuit that gets the benefit of having an op amp while at the same time does not substantially increase the circuit complexity and power consumption.
SUMMARY OF INVENTION
The above object of the present invention has been achieved by a bandgap voltage reference circuit that incorporates a unique 2-stage transconductance amplifier into a feedback control loop to improve the reference voltage accuracy and stability without the need for a biasing circuit. Having a high gain circuit gives the present invention a good power supply rejection ratio, and, contributes to its higher accuracy and stability. The elimination of the bias circuit provides the present invention with low power consumption and less circuit complexity.
REFERENCES:
patent: 4797577 (1989-01-01), Hing
patent: 5451860 (1995-09-01), Khayat
patent: 5670907 (1997-09-01), Gorecki et al.
patent: 5818292 (1998-10-01), Slemmer
patent: 5900773 (1999-05-01), Susak
patent: 6157245 (2000-12-01), Rincon-Mora
patent: 6249031 (2001-06-01), Verma et al.
patent: 6281743 (2001-08-01), Doyle
patent: 6529066 (2003-03-01), Guenot et al.
patent: 2003/0030482 (2003-02-01), Mori
Atmel Corporation
Callahan Timothy P.
Englund Terry L.
Schneck Thomas
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