Miscellaneous active electrical nonlinear devices – circuits – and – External effect – Temperature
Reexamination Certificate
2001-10-24
2003-12-16
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
External effect
Temperature
C327S539000, C327S538000, C323S312000, C323S315000
Reexamination Certificate
active
06664843
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electronic circuits and systems. More particularly, this invention relates to circuits that generate biasing currents for circuits. This invention, especially, relates to circuits that generate biasing currents that provide variations in these biasing currents to compensate for functional circuit variations due to changes in operating temperature of the functional circuits.
2. Description of Related Art
Presently designed analog circuits generally employ current biasing rather than voltage biasing. Current biasing, firstly, allows the operating points of the transistors to be relatively independent of the fabrication process parameters. Secondly, current biasing is less prone to noise pickup. Thirdly, the temperature coefficient of the biasing current can be easily altered to provide temperature compensation to some of the small signal parameters, particularly, transconductance (g
m
) of transistors. For the purpose of current biasing, a current master-bias circuit is usually employed. However, the slope of the temperature characteristic of the bias current from the master-bias circuit might have to be different for different circuits and even for the same circuit using different fabrication processes, if reasonably precise temperature compensation is required. Therefore, a master-bias current circuit must be able be easily adaptable to provide different characteristics for the bias current.
A Proportional To Absolute Temperature (PTAT) current generator as shown in
FIG. 1
is very widely used as a temperature compensated current master-bias circuit. The NPN bipolar transistors Q
1
and Q
2
, resistor R
1
, and an active current mirror circuit CM
1
form the PTAT current generator. The current mirror circuit CM
1
forces the collector currents of transistors Q
1
and Q
2
to be equal which is shown as I
C1
and I
C2
. If the small base current of Q
1
is ignored, it can be shown that the collector currents I
C1
and I
C2
of transistors Q
1
and Q
2
is determined by the equation:
I
C1
=
I
C2
=
V
be
Q2
-
V
be
Q1
R
1
Eq
.
1
where:
V
beQ1
and V
beQ2
are the voltages developed between the base and emitter respectively of the transistors Q
1
and Q
2
.
R
1
is the resistance of the resistor R
1
.
It is known that the base emitter voltages V
beQ2
& V
beQ1
of the transistors Q
1
and Q
2
are determined by the equation:
V
be
=
V
T
ln
(
I
C
J
S
A
)
Eq
.
2
where:
V
T
is a thermal voltage given by the equation:
V
T
=
kT
q
Eq
.
3
where:
k is Boltzmann's constant,
T is the operating temperature of the transistor generally in degrees Kelvin, and
q is the electrical charge of an electron.
I
C
is the collector current of an NPN transistor.
J
S
is the saturation current density per unit area.
A is the emitter area.
By substituting Eq. 2 and Eq. 3 into Eq. 1, it can be shown that the collector currents I
C1
and I
C2
of transistors Q
1
and Q
2
are equal to:
I
C1
=
I
C2
=
V
T
R
1
ln
(
I
C2
J
S
A
2
)
-
ln
(
I
C1
J
S
A
1
)
=
V
T
R
1
ln
(
A
2
A
1
)
=
AV
T
Eq
.
4
If the current mirror CM
1
is designed such that the MOS transistors M
1
, M
2
, and M
3
are of equal sizes, then the PTAT current I
PTAT
is equal to collector currents I
C1
and I
C2
and is given by the equation:
I
PTAT
=AV
T
Eq. 5
where:
A
=
1
R
1
ln
(
A
2
A
1
)
and is the constant simplified from the terms of Eq. 4.
V
T
is the thermal voltage of Eq. 3.
FIG. 2
shows the temperature behavior of the PTAT current I
PTAT
versus temperature. The constant
A
k
q
is the slope of the line. This kind of linear characteristic is usually very effective for providing temperature compensation for Bipolar transistors.
The transconductance g
mbip
for a bipolar transistor is given by:
g
mbip
=
I
C
V
T
Eq
.
6
where,
I
C
is the collector current.
If a bipolar transistor is biased by a PTAT current I
PTAT
, the PTAT current I
PTAT
found in Eq. 5 is substituted for the collector current I
C
in Eq. 6, the transconductance g
mbip
of the bipolar transistor becomes:
g
mbip
=A.
Eq. 7
Thus the PTAT current generator effectively forces the transconductance of the bipolar transistor to be constant over temperature.
Conversely, for MOS transistors in strong-inversion, the PTAT current generator does not provide an effective temperature compensation. The transconductance g
mMos
of a MOS transistor is given by the equation:
g
mMOS
=
2
I
D
μ
C
OX
W
L
Eq
.
8
where:
I
D
is the drain current of the MOS transistor.
C
OX
is the gate oxide capacitance per unit area of the MOS transistor.
W/L the aspect ratio of the MOS transistor
&mgr; the carrier mobility given by the equation:
&mgr;=
BT
−m
where:
B is a constant.
m is a process dependent exponent that has a typical value of 1.5.
T is temperature in degrees Kelvin.
If a MOS transistor is biased by a PTAT current I
PTAT
, the PTAT current I
PTAT
found in Eq. 5 is substituted for the drain current I
D
in Eq. 8, the transconductance g
mMos
of the MOS transistor is found by the equation:
g
mMOS
=
2
kABWC
OX
qL
T
1
-
m
2
Eq
.
9
It is known in the art the process dependent exponent m is not easily controllable and is almost never has a magnitude of 1. Thus it becomes obvious from Eq. 9 that the transconductance g
mMos
has a level of temperature dependence even if biased with a PTAT current I
PTAT
U.S. Pat. No. 6,157,245 (Rincon-Mora) describes a curvature corrected bandgap reference voltage circuit, the output voltage that is substantially linear and independent of the operating temperature of the circuit. The circuit includes a voltage divider network comprised of a first resistor and a second resistor connected in series. A first compensating circuit provides a first, linear, operating temperature-dependent current, and a second compensating circuit provides a second, logarithmic, operating temperature-dependent current. The first current is supplied to the first resistor of the voltage divider network, while the second current is supplied to the second resistor of the voltage divider network.
U.S. Pat. No. 5,952,873 (Rincon-Mora) illustrates a low voltage, current-mode, piecewise-linear curvature corrected bandgap reference circuit. The bandgap circuit includes a first current source supplying a current proportional to a base-emitter voltage, a second current source supplying a current proportional to absolute temperature, and a third current source supplying a non-linear current. Three resistors are coupled in series between a first node and ground. The first current source is coupled to the first node. The second current source is coupled to a second node between the first and second resistors. The third current source is coupled to a third node between the second and third resistors. An output coupled to the first node supplies a reference voltage.
U.S. Pat. No. 5,883,507 (Yin) describes a low power temperature compensated, current source. The current source creates a first reference current and a temperature compensating voltage-controlling circuit generates a temperature compensated voltage control signal during temperature variations. A bias controlling circuit is connected to the current generating circuit and the temperature compensating voltage control circuit to bias the temperature compensating voltage control circuit. A current output controlling circuit is connected to the current generating circuit and the temperature compensating voltage controlling circuit for controlling a second temperature compensated reference current to generate a high output source current even during low temperature conditions.
U.S. Pat. No. 5,796,244 (Chen et al.) teaches a voltage reference circuit that will remain constant and independent of changes in the operating temperature that is correlated to the bandgap voltage of silicon is de
Dasgupta Uday
Yeoh Wooi Gan
Ackerman Stephen B.
Cunningham Terry D.
Institute of Microelectronics
Knowles Billy
Saile George O.
LandOfFree
General-purpose temperature compensating current master-bias... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with General-purpose temperature compensating current master-bias..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and General-purpose temperature compensating current master-bias... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3183605