Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Converting input voltage to output current or vice versa
Reexamination Certificate
2002-11-19
2004-12-28
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Converting input voltage to output current or vice versa
C327S513000, C327S538000, C323S315000
Reexamination Certificate
active
06836160
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to electronic circuits and components therefore, and is particularly directed to a new and improved voltage-controlled, modified Brokaw cell-based current generator, which is operative to generate an output current that exhibits a linear temperature coefficient.
BACKGROUND OF THE INVENTION
A variety of electronic circuit applications employ one or more voltage and/or current reference stages to generate precision voltages/currents for application to one or more loads. In order to accommodate parameter (e.g., temperature) variations in the environment in which the circuit is employed, it is often desirable that the reference circuit's output conform with a prescribed behavior. In the case of a voltage reference, for example, it is common practice to employ a precision voltage reference element, such as a ‘Brokaw’ bandgap voltage reference circuit, from which an output or reference voltage having a relatively flat temperature coefficient may be derived.
A reduced complexity circuit diagram of such a Brokaw bandgap voltage reference circuit is shown in
FIG. 1
as comprising a pair of bipolar NPN transistors Q
1
and QN, having their bases connected in common and to a bandgap voltage (V
BG
) output node
11
. In a typical integrated circuit layout, transistors QN and Q
1
are located adjacent to one another and differ only in terms of the geometries by their respective emitter areas by a ratio of N:1. Alternatively, transistor QN may correspond to a plurality of N transistors coupled (or ‘lumped’) in parallel. The collectors of transistors QN and Q
1
are coupled to respective ports
21
and
22
of a current mirror
20
. The current mirror and amplifier makes an equal current flowing though the collector of QN and Q
1
. Transistor Q
1
has its base-emitter junction voltage Vbe
Q1
derived from the series connection of the base-emitter junction of transistor QN and resistor R
1
, and its emitter Q
1
e
coupled to the current summation node
12
. Current summation node
12
is coupled through a resistor R
2
to ground.
In the Brokaw cell voltage reference circuit of
FIG. 1
, the voltage on the R
1
is equal to the VBE difference of the transistor Q
1
and QN, which is proportional to absolute temperature (or PTAT) and is definable as (kT/q)lnN, where k is Boltzman's constant, q is the electron charge, T is temperature (in degrees Kelvin), N is the ratio of the emitter areas of transistors QN/Q
1
. The PTAT current
11
supplied through the resistor R
2
produces a PTAT voltage thereacross, which is (2*R
2
/R
1
)*(kT/q)*lnN, where R
1
and R
2
are the resistance of resistor R
1
and R
2
respectively. This PTAT voltage V
PTAT
is summed with the VBE voltage across transistor Q
1
(which is complementary to absolute temperature or CTAT), to derive an output voltage reference V
BG
at output terminal
11
. As shown in
FIG. 2
, the output reference voltage V
BG
produced by the Brokaw bandgap reference circuit of
FIG. 1
has a first-order compensated temperature coefficient, which typically varies in a ‘squeezed’, generally parabolic manner between 20 to 100 ppm/° C.
In addition to the need for circuits that exhibit an essentially flat voltage vs. temperature characteristic, such as the Brokaw voltage reference described above, there are a number of applications where it is desired that an output current vary in a prescribed manner with change in temperature. For example, in the case of a battery charger, it may be desirable to generate an output current that exhibits a well defined linear slope over a given temperature range for the thermal fold back.
SUMMARY OF THE INVENTION
In accordance with the invention, this objective is realized by employing the temperature dependency functionality exhibited within the circuitry used to generate Brokaw voltage reference, so as to realize a modified Brokaw cell-based circuit that produces an output current whose temperature coefficient varies linearly with temperature. In the modified Brokaw cell based circuit of the invention, Q
1
and QN is exchangeable. The collector-emitter current flow path the transistor QN of the Brokaw circuit of
FIG. 1
, rather than being connected to the current mirror port, is connected to a diode connection in series with the collector-emitter current flow path of a control transistor. The base of the input transistor is coupled to receive an input or ‘reference’ (control) voltage VREF, whose value defines a limited linear range of variation of output current with temperature. The collector of the output transistor Q
1
is coupled to an input port of a current mirror, which mirrors the collector current from output transistor at an output port thereof.
Unlike the conventional Brokaw circuit of
FIG. 1
, whose output is ‘voltage’ and whose input is a ‘current’ supplied by a current mirror connected to two the legs of the voltage reference circuit, the output of the modified Brokaw circuit of the invention is a ‘current’ that varies linearly with temperature, and its input is a control ‘voltage’ applied to the base of its control transistor. For a given reference voltage applied to its base, the control transistor will produce a prescribed (PTAT) output current, which is applied to the collector-emitter current flow path of the diode-connected transistor QN and thereby to the series connected resistors R
1
and R
2
. The collector current of the output transistor Q
1
is defined in accordance with the sum of the voltage drop V
R1
across the resistor R
1
and the base emitter voltage Vbe
QN
of transistor QN. Since the voltage variation across the resistor R
1
is PTAT (and is dominant) and that of the Vbe
QN
of transistor QN is CTAT, the resultant Vbe of the output transistor is the sum of a dominant PTAT component and a CTAT component, and has a linear temperature coefficient.
Operational conditions, such as slope and DC offset, of the current generator of the invention may be selectively defined in accordance one or more parameters or relationships among parameters of the circuit. For example, the slope of the linear variation of the output current with temperature may be varied by varying the ratio of the emitter areas of transistors Q
1
and QN and/or by the ratio of the values of resistors R
1
/R
2
. For a given temperature, the output current may be varied by changing the magnitude of the control voltage applied to the base of the control transistor.
The ability of the invention to produce an output current that exhibits a very linear variation with temperature makes its readily adaptable to a variety of applications requiring customized temperature-based current behavior characteristics. For example, multiple current generators of the present invention having different parameter settings may be combined to produce a composite piecewise linear variation with temperature. As a non-limiting example, a first output current whose variation with temperature has a zero slope may be combined with a second output current having a substantial non-zero slope over its linear temperature variation, to produce a piecewise flat then inclining or declining variation with temperature current behavior.
REFERENCES:
patent: 4789819 (1988-12-01), Nelson
patent: 5394078 (1995-02-01), Brokaw
patent: 5666046 (1997-09-01), Mietus
patent: 5926062 (1999-07-01), Kuroda
patent: 5952873 (1999-09-01), Rincon-Mora
patent: 6002293 (1999-12-01), Brokaw
patent: 6078208 (2000-06-01), Nolan et al.
patent: 6091286 (2000-07-01), Blauschild
patent: 6157245 (2000-12-01), Rincon-Mora
patent: 6232829 (2001-05-01), Dow
patent: 6271710 (2001-08-01), Ooishi
patent: 2004/0066180 (2004-04-01), Harrison
patent: 0492117 (1991-11-01), None
Callahan Timothy P.
Englund Terry L.
Intersil America's Inc.
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