Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2000-10-11
2002-06-18
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
Reexamination Certificate
active
06407621
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to integrated circuits and components, such as may be employed in telecommunication circuits and the like, and is particularly directed to a new and improved transistor circuit for generating a programmable output parameter, such as a reference current that is controllably and precisely established by a user-programmable component (resistor) coupled therewith, without internal parameters of the circuit being subject to being modified (distorted) by the programming element.
BACKGROUND OF THE INVENTION
FIG. 1
diagrammatically a conventional bipolar transistor circuit that may be incorporated into a variety of integrated circuits for generating a programmable current reference. For this purpose, a precision voltage element
10
, such as a bandgap voltage reference device, is coupled between a (VCC) voltage supply rail
12
and the collector
21
of a bipolar (NPN) reference transistor
20
. Transistor
20
has its emitter
22
coupled in circuit with a resistor
25
, which is internal to the integrated circuit and is terminated at a reference voltage terminal (ground (GND)). An additional base current offset (NPN) transistor
30
, whose collector
31
is tied to the VCC supply rail
12
, has its emitter
32
coupled to the base
23
of transistor
20
and its base
33
coupled to the collector
21
of transistor
20
. The base
23
of the reference transistor
20
is further coupled in common with the base
43
of an output transistor
40
, the emitter
42
of which is coupled to a programming terminal
45
and the collector
41
of which is coupled to an output terminal
50
. The programming terminal
45
is adapted to be coupled through a programming circuit element, such as a resistor
47
, referenced to ground, while the output terminal
50
is used to supply an output current having a magnitude defined by the value of the programming resistor
47
.
Since the intended functionality of the circuit of
FIG. 1
is to generate a reference current at output terminal
50
that is precisely established by the value of the programming resistor
47
in proportion to the bandgap voltage of the precision bandgap voltage reference device
10
. The insertion of the base current offset transistor
30
serves to reduce the effect of base current errors. In operation, with a programming resistor
47
of some value Rprog coupled between the programming terminal
45
and ground, the following voltage loop equation (1) may be defined:
(
I
10
/&agr;
n
*R
25
+Vbe
20
(@
I
10
/&agr;
n
)=(
I
50
/&agr;
n
)*
R
47
+Vbe
40
(@
I
10
/&agr;
n
) (1)
Since, however,
I
10
=Vbandgap/Rint (2)
where Vbandgap is the bandgap voltage and Rint is a resistor internal to the integrated circuit which matches the resistor
25
(also internal to the integrated circuit, as noted above), then equation (1) may be rewritten as:
I
out
=I
50
=(1
/R
47
)*(Vbandgap*
R
25
/Rint+&Dgr;Vbe
30,40
) (3)
where &Dgr;Vbe
30,40
is the difference between the base-emitter voltage drops of transistors
30
and
40
. This base-emitter voltage difference may be significant, even where transistors
30
and
40
are designed to have identical geometries, since there is no way to predict their relative current densities, which are a function of the programming resistor
47
. As a result, the internal parameters of the circuit of
FIG. 1
are subject to being influenced by the programming element, so that the output current generated at terminal
50
cannot be accurately programmed as desired.
SUMMARY OF THE INVENTION
In accordance with the present invention, this problem is effectively obviated by a relatively simple circuit architecture whose internal parameters are effectively independent of the programming element. Like the architecture of
FIG. 1
, the present invention makes use of a bandgap voltage reference device incorporated in the integrated circuit. The bandgap voltage reference device generates a reference current I
K*temp
that is proportional to temperature (in degrees Kelvin (°K)). This bandgap based reference current I
K*temp
is supplied through the collector-emitter path of a reference transistor coupled in circuit with a reference resistor terminated at a reference voltage terminal. The reference resistor has a geometry that effectively matches that of the internal bandgap device's resistance and has a value such that the sum of the base-emitter voltage drop across the reference transistor and the voltage drop across the reference resistor resulting from the reference current I
K*temp
is equal to the bandgap voltage.
The base of the reference transistor is further coupled in common with the emitter of an output transistor and to a programming terminal, that is adapted to be coupled to a programming resistor. The base of the output transistor is coupled to the collector of the reference transistor, while the collector of the output transistor is coupled to an output terminal, from which a programmed current is to be supplied in accordance with the value of the programming resistor.
The loop equations for are such that the output current is effectively definable as the ratio of the bandgap voltage Vbandgap to the programming resistor, and is not affected by the base-emitter voltage drops of the reference and output transistors, as in the conventional bandgap-based circuit (shown in FIG.
1
). This means that the output current supplied by the invention may be programmed in accordance with the precision of the integrated circuit's internal bandgap device and the tolerance of the programming resistor without significant first order errors.
REFERENCES:
patent: 4604568 (1986-08-01), Prieto
patent: 5451859 (1995-09-01), Ryat
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Intersil America's Inc.
Wells Kenneth B.
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