Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Combining of plural signals
Reexamination Certificate
2000-09-06
2001-05-01
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Combining of plural signals
C327S327000, C327S539000, C323S313000
Reexamination Certificate
active
06225850
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of translinear circuits, including current multipliers, bandgap reference circuits, and proportional-to-absolute-temperature sensor circuits. Specifically, the present invention relates to compensation for extrinsic series base and emitter resistance inherent in bipolar transistors within the translinear circuits.
2. Discussion of the Related Art
Proportional-to-absolute-temperature (PTAT) temperature sensors and bandgap reference circuits determine their output values based upon the difference in the voltage drops on diode junctions having different current densities. The primary property exploited by these circuits is the exponential variation of the current density across a p-n junction barrier with the intrinsic voltage applied on that barrier. A larger class of such circuits are the translinear (TL) networks. All the translinear circuits are based on the logarithmic variation of the intrinsic base-emitter voltage of a bipolar transistor with its collector current. There are several sources of errors which limit the accuracy of these translinear circuits. The V
BE
mismatches in nominal identical pairs due to emitter area differences, process gradients, and mechanical stress, can be minimized by interdigitation and common centroid layouts. The influence of base width modulation by the base-collector voltage (the Early effect) on the collector current can also be minimized by cascode configurations or active biasing for constant V
CB
. Usually, larger errors are due to the finite current gain &bgr; and finite base current. These errors are usually minimized by using driver stages or base current cancellation techniques.
Often the limiting factor in the accuracy of translinear circuits comes from the extrinsic base resistance and the series emitter resistance. While their effect is negligible at very low current densities, such as in micropower applications, the errors introduced at even moderate current densities usually dominate the non-idealities in the system. Attempts to compensate the base resistance with other resistors have only a limited success due to the poor matching between the base resistance and the compensation resistor, which also have different temperature coefficients.
A classic example of a translinear circuit is the Brokaw bandgap circuit, shown in FIG.
1
. The two bipolar transistors Q
1
and Q
2
have different emitter areas. Specifically, transistor Q
2
is K times larger in emitter area than transistor Q
1
. The feedback loop including the operational amplifier A
1
ensures that the same current I flows through both transistors Q
1
and Q
2
.
Because transistor Q
2
is larger in area than transistor Q
1
, the current densities in the junctions of transistor Q
2
are less by a factor of K than the corresponding densities in transistor Q
1
when the same total collector current I flows through both transistors. Thus, the voltage drop across the base to emitter p-n junction of transistor Q
2
is smaller than the corresponding voltage drop for transistor Q
1
. The difference in the base-emitter voltage of the two transistors &Dgr;V
be
occurs across the resistor R
1
. The equilibrium value for the bias current I is given by
I
=
Δ
V
be
R
1
(
1
)
where
Δ
V
be
=
V
be1
-
V
be2
=
kT
q
·
ln
(
I
I
sat1
)
-
kT
q
·
ln
(
I
I
sat2
)
(
2
)
and kT/q represents the thermal voltage, which is proportional to absolute temperature T measured in Kelvin degrees. Since Q
1
and Q
2
have emitter areas ratioed by a factor of K, I
sat2
=K*I
sat1
, and eq. (2) reduces to the following
Δ
V
be
=
kT
q
·
ln
(
K
)
(
3
)
The output voltage V
BG
in
FIG. 1
, can be expressed as follows.
V
BG
=
V
be2
+
R
1
+
2
R
2
R
1
·
Δ
V
be
(
4
)
For a certain ratio of the resistors R
1
and R
2
, V
BG
becomes equal to the bandgap voltage, and it is essentially temperature independent. Importantly, the cancellation of the output voltage temperature dependence is obtained independently of the absolute value of the resistors R
1
and R
2
, which is difficult to control.
Eq. (2) assumes an ideal behavior for the bipolar transistor and neglects the voltage drops on the emitter and base extrinsic series resistors. The extrinsic base-emitter voltage of a transistor can be expressed as follows.
V
be
=
kT
q
·
ln
(
I
I
sat
)
+
I
·
R
e
+
I
β
+
1
·
R
b
(
5
)
In the above equation, R
e
−is the emitter series resistance, R
b
−is the base series resistance , &bgr;−is the dc current gain, and I/(&bgr;+1) is the base current.
For many applications, such as temperature sensors and low-noise bandgap references, the current densities are large enough so the series resistance voltage drops (the last two terms in eq. (5)) become significant. The equilibrium bias current in this case can be calculated from
I
=
kT
q
·
ln
(
K
)
R
1
-
R
e
·
(
1
-
1
K
)
-
R
b
β
+
1
·
(
1
-
1
K
)
(
6
)
In eq. (6), it is assumed that the series resistances R
b
and R
c
are inversely proportional to the emitter area, such that
R
e2
=
R
e1
K
=
R
e
K
(
7
)
R
b2
=
R
b1
K
=
R
b
K
(
8
)
In practice, the resistor R
1
is technologically different from internal resistors R
b
and R
e
, and they have different temperature coefficients. In particular, it is very hard to control the ratio of these internal resistances to the other resistors in the circuit R
1
and R
2
. For a PTAT current source, even though R
1
can be made with a zero temperature coefficient (zero TC), the errors due to the extra terms in eq. (6) introduce errors in the ideal PTAT variation. In particular, the &Dgr;V
be
voltage drop variation between the two junctions with different current densities is not PTAT any longer, but includes a term dependent on the ratio of resistors R
b
, R
e
and R
1
.
V
be
=
kT
q
·
ln
(
K
)
+
kT
q
·
ln
(
K
)
·
(
R
e
+
R
b
β
+
1
)
·
(
1
-
1
K
)
R
1
-
(
R
e
+
R
b
β
+
1
)
·
(
1
-
1
K
)
(
9
)
Conventional ways to circumvent this problem in PTAT temperature sensors have been proposed by using three different current densities and then to compensate for the IR drops using complicated computations, which are inherently undesirable due to their significant complexity.
SUMMARY OF THE INVENTION
Conventional circuits for extrinsic base and emitter series resistance compensation suffer from the flaw the compensation resistor has a different temperature coefficient than the extrinsic base and emitter resistances because of the different fabrication and structural composition of the compensation resistor and the extrinsic base and emitter resistances. Therefore, conventional circuits are unable to robustly compensate for the extrinsic resistances of the bipolar transistor over all operating conditions. An object of the present invention is to provide a general purpose extrinsic base and emitter series resistance compensation circuit effective in any translinear circuit. Another object of the present invention is to provide proportional-to-absolute-temperature sensor circuits and bandgap reference circuits which effectively compensate for the extrinsic base and emitter series resistances under all operating conditions.
According to an embodiment of the present invention, a bandgap reference circuit is constructed in which the bases of the two bipolar transistors coupled to the inputs of the operational amplifier which produces the bandgap output are driven by voltages which are elevated by amounts which perfectly compensate for the voltage drops across the extrinsic base and emitter series resistances of those transistors. A feedback loop driven by the bandgap output voltage forces a fixed current through a voltage divider. The ratio of the resistances of the resistors in the voltage divider are chosen as a function of the emitter are
Fliesler Dubb Meyer & Lovejoy LLP
Nguyen Hai L.
Wells Kenneth B.
LandOfFree
Series resistance compensation in translinear circuits does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Series resistance compensation in translinear circuits, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Series resistance compensation in translinear circuits will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2518543