Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2001-02-26
2003-03-04
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S540000, C327S543000, C327S512000, C323S313000, C323S316000
Reexamination Certificate
active
06529066
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to analog circuitry and, in particular, to band gap circuitry used to generate reference voltages having a controllable temperature coefficient.
2. Description of Related Art
In analog and mixed signal circuits, a reference voltage is sometimes needed that does not vary over temperature or that varies in a predetermined way over temperature. Typical of such circuits is a circuit commonly referred to as a band gap voltage reference circuit. A band gap circuit relies upon a difference in base-emitter voltage of two bipolar transistors, with such difference voltage having a positive temperature coefficient. That difference voltage, or a voltage derived from the difference voltage, is combined with another voltage, typically a base-emitter voltage, having a negative temperature coefficient, to produce a reference voltage. In most cases, the voltages are combined so that the reference voltage has a zero temperature coefficient, but the reference voltage can also have a controlled positive or controlled negative temperature coefficient if desired. Regardless of the temperature coefficient of the reference voltage, such circuits are referred to herein as band gap circuits or band gap reference circuits.
In a CMOS process, the only bipolar transistors available are parasitic vertical PNP transistors having their respective collectors formed in a common P type substrate. This places limits of the implementation of circuits using those transistors. Various CMOS band gap voltage reference circuits have been developed, many of which have limitations on the minimum supply voltage.
Referring to the drawings,
FIG. 1
is a diagram of one such prior art band gap reference circuit. A pair of parasitic vertical PNP transistors
8
A and
8
B are included which are diode-connected with the base and collectors of each transistor being connected to the circuit common. Transistor
8
B is implemented having an emitter area which is twenty-four times (24×) as large as the emitter area of transistor
8
A (1×). The emitters of transistors
8
A and
8
B are respectively connected to the drains of a pair of similar P type MOS transistors
6
A and
6
B. Transistor
8
B is connected to MOS transistor
6
B by way of a resistor R
1
.
A third P type MOS transistor
6
C, having a gate connected in common with the gates of transistors
6
A and
6
B, is connected to an emitter of a third parasitic vertical PNP transistor
10
by way of a resistor R
2
. All three MOS transistors
6
A,
6
B and
6
C have their sources connected in common to the supply voltage. PNP transistor
10
has the same emitter area (1×) as transistor
8
B and is also connected as a diode, with base and collector connected to the circuit common. An operational amplifier
12
, which functions as an error amplifier, has an output connected to the common gates of transistors
6
A,
6
B and
6
C, an inverting input connected to a node A intermediate transistors
6
A and
8
A and a non-inverting input connected to a node B intermediate resistor R
1
and transistor
6
B.
In operation, amplifier
12
controls the gate-source voltage of transistors
6
A,
6
B and
6
C such that the voltages at nodes A and B are equal, ignoring the small input offset voltage of the amplifier. Transistors
6
A and
6
B are the same size and have the same gate-source voltage so that both transistors will conduct approximately the same current Ith. Transistors
8
A and
8
B will also conduct the same current, Ith, with transistor
8
A operating at twenty-four times the current density given that the emitter area of the transistor only 1× compared to the 24× of transistor
8
B. As is well known, transistors
8
A and
8
B will operate at different base-emitter voltages (&Dgr;Vbe) with such difference voltage being relatively independent of the absolute magnitude of the current. The equation for &Dgr;Vbe is as follows, with Ja and Jb representing the current density of transistors
8
A and
8
B, respectively:
Δ
Vbe
=
Vt
ln
Ja
Jb
(
1
)
Vt is the thermal voltage (kT/q). Assuming that transistors
8
A and
8
B conduct the same current Ith, the ratio of current density is determined solely by the {fraction (1/24)} ratio of emitter areas, resulting in &Dgr;Vbe of 80 millivolts. Thus, assuming that the Vbe of transistor
8
A is, for example, 650 millivolts, the Vbe of transistor
8
B will be 80 millivolts less or 570 millivolts at room temperature. Since the voltages at nodes A and B are equal, the &Dgr;Vbe voltage of 80 millivolts will be dropped across resistor R
1
. In a typical application, resistor R
1
will be set to about 160 kohms thereby setting current Ith to 500 nanoamperes (80 millivolts/160 kohms). As can be seen in equation (1), voltage &Dgr;Vbe has a positive temperature coefficient since Vt has a positive temperature coefficient of +0.085 millivolts/° C. Thus, current Ith will also have a positive temperature coefficient.
The band gap output voltage Vbg is the sum of the base-emitter voltage of transistor
10
, voltage Vbe(
10
), and the voltage drop across resistor R
2
, voltage V(R
2
). Since the base-emitter voltage Vbe(
10
), typically 650 (millivolts), has a negative temperature coefficient (−2 millivolts/° C.), the value of resistor R
2
is selected so that a positive temperature coefficient voltage V(R
2
) is produced having a magnitude sufficient to offset the negative temperature coefficient of voltage Vbe(
10
). Setting resistor R
2
to 1.2 Meg ohms will produce a voltage V(R
2
) of about 600 millivolts. This will produce a band gap output voltage Vbg of 1.25 volts having the desired first order zero temperature coefficient.
One of the limitations of the
FIG. 1
prior art circuit relates to the implementation of the operational amplifier
12
.
FIG. 2A
is a simplified diagram of the input stage of an amplifier utilizing N type MOS devices and
FIG. 2B
is a diagram of an input stage utilizing P type devices. Referring first to
FIG. 2A
, input V+, the gate of transistor
16
A, is connected to node A of FIG.
1
and input V−, the gate of transistor
16
B, is connected to node B. As previously noted, both nodes A and B are at 650 millivolts, the base-emitter voltage of transistor
8
A.
In order for the
FIG. 2A
amplifier to operate properly, inspection of the input indicates that the voltage at the inputs, the common mode input voltage, must be at least as large as the sum of the gate-source voltage Vgsn of N type transistor
16
A and voltage Vdsatn of tail current source N type transistor. Voltage Vdsatn is the minimum drain-source voltage necessary for transistor
18
to operate in the saturation region where the transistor functions as a current source. The gate-source voltage Vgsn is equal to Vdsatn+Vtn where Vtn is the threshold voltage of the N type transistors. Assuming that Vtn is 700 millivolts and Vdsatn is 200 millivolts, it can be seen that the
FIG. 2A
amplifier requires a minimum common mode input voltage of 1.1 volts, well above the actual voltage of 650 millivolts at the amplifier inputs. This presents a problem.
One solution to the above noted problem is to use MOSfet having a reduced threshold voltage, usually in the range of 200 millivolts. However, such devices are typically not available on standard CMOS processes. Another approach is to use an input stage having P type devices as shown in FIG.
2
B. Inspection of the
FIG. 2B
circuit shows that the supply voltage must be at least equal to the sum of the voltage applied to the gates of input transistors
22
A and
22
B, the voltage at nodes A and B, plus the sum of the gate-source voltage Vgsp of P type transistor
22
A/
22
B and voltage Vdsatp of tail current source transistor
20
. Voltage Vdsatp is the minimum drain-source voltage of transistor
20
which will permit the transistor to operate as a current source. Again, the value of Vgsp is the sum of the threshold voltag
Guenot Stephane
Kotowski Jeffrey P.
Girard & Equitz LLP
Lam Tuan T.
Luu An T.
National Semiconductor Corporation
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