Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2000-10-31
2001-11-13
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, C327S513000, C323S312000, C323S315000
Reexamination Certificate
active
06316990
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a constant current supply circuit for supplying constant current to an electric load via a bipolar transistor.
FIG. 5
shows a constant current supply circuit disclosed in the unexamined Japanese patent publication No. 5-60623. An electric load L has one end connected to a ground terminal and the other end connected to an emitter of a primary transistor Q
100
. The primary transistor Q
100
is an NPN-type transistor which controls the current supplied to the electric load L. A first resistor
101
, a second resistor
102
, a third resistor
103
, and a fourth resistor
104
are serially connected between a high-potential terminal VCC and a ground-potential terminal of a power source. A secondary transistor Q
200
, being an NPN-type transistor, has a collector connected to a connecting point of the second resistor
102
and the third resistor
103
, a base connected to a connecting point of the third resistor
103
and the fourth resistor
104
, and an emitter connected to the ground-potential terminal of the power source. A base of the primary transistor Q
100
is connected to a connecting point of the first resistor
101
and the second resistor
102
. A positive voltage terminal
105
, having an electric potential VD higher than the ground potential, is connected to a collector of the primary transistor Q
100
.
According to this conventional constant current supply circuit, the electric load L receives a constant current (i.e., load current) I from the terminal
105
via the primary transistor Q
100
. The electric load L has temperature characteristics in its resistance value R. To compensate such temperature characteristics, the relationship among resistance values of the first to fourth resistors
101
to
104
(especially, a resistance ratio of the third resistor
103
to the fourth resistor
104
) is determined in such a manner that a voltage E applied between both ends of the electric load L adequately varies in accordance with the temperature. With this setting, the load current I is maintained at a constant value irrespective of temperature change.
However, the above-described conventional constant current supply circuit has the following problems.
It is now assumed that TCRL represents a resistance temperature coefficient of the electric load L, Rtyp represents a typical resistance value of the load resistance R at a reference temperature Ttyp, and &Dgr;T represents a temperature deviation from the reference temperature Ttyp.
Using the above, the load resistance R can be expressed by the formula Rtyp(1+TCRL×&Dgr;T). The temperature characteristics of the load resistance R is Rtyp×TCRL×&Dgr;T. In other words, the load resistance R causes a variation equivalent to Rtyp×TCRL×&Dgr;T in response to the temperature deviation &Dgr;T from the reference temperature Ttyp. Thus, the temperature characteristics of the electric load L varies in accordance with a change of the typical resistance value Rtyp of the load resistance R.
However, according to the above-described conventional constant current supply circuit, the resistance values of the resistors
101
to
104
are determined in such a manner that the voltage E applied between the both ends of the electric load L varies in accordance with the temperature so as to compensate the temperature characteristics of the load resistance R. Accordingly, the optimum resistance values of the resistors
101
to
104
vary in response to the deviation of the typical value Rtyp of the load resistance R.
Hence, the resistance values of the resistors
101
to
104
need to be adjusted for each electric load L. This forces the workers to perform very complicated adjustment which is not practically feasible.
The electric load L may be a pressure sensing element of a Wheatstone bridge circuit consisting of four strain gauges made of diffused resistors. The resistance value of each diffused resistor has a dispersion range of approximately ±10~20% due to manufacturing error of the diffusion density of impurities or the width of resistor wire. It is therefore difficult to adopt the above-described conventional constant current supply circuit to this kind of pressure sensor.
SUMMARY OF THE INVENTION
In view of the foregoing problems, the present invention has an object to provide a constant current supply circuit capable of supplying constant current to an electric load irrespective of temperature, even when the resistance value of the electric load is not constant due to the manufacturing error.
To accomplish the above and other related objects, the present invention provides a constant current supply circuit for supplying constant current to an electric load. The constant current supply circuit comprises a primary transistor having a collector connected to one end of the electric load for controlling current supplied to the electric load. A current path resistor is connected between an emitter of the primary transistor and a reference voltage terminal for forming an electric path supplying the current to the electric load via the primary transistor. First, second, third and fourth resistors are serially connected in this order between one potential terminal of an electric power source and the other potential terminal of the electric power source. A secondary transistor, being identical in type with the primary transistor, has a collector connected to a connecting point of the second resistor and the third resistor, a base connected to a connecting point of the third resistor and the fourth resistor, and an emitter connected to the other potential terminal of the electric power source. The primary transistor has a base connected to a connecting point of the first resistor and the second resistor.
According to this arrangement, load current I is supplied via the primary transistor to the electric load. The load current I is substantially identical with current I′ flowing across the current path resistor. The type and the resistance value of respective first to fourth resistors can be optimized so that a voltage applied between both ends of the current path resistor is maintained at a constant value irrespective of temperature. Thus, it becomes possible to supply constant load current I to the electric load even when the resistance value of a manufactured electric load (i.e., actual load resistance) is different from a designated value.
Preferably, the first and second resistors are identical in type with the third and fourth resistors, and resistance values of the first, second, third and fourth resistors satisfy the following relationship:
0.5
<
γ
=
R1
R1
+
R2
·
R3
+
R4
R4
<
1.5
where R
1
represents a resistance value of the first resistor, R
2
represents a resistance value of the second resistor, R
3
represents a resistance value of the third resistor, and R
4
represents a resistance value of the fourth resistor.
For example, to obtain preferable characteristics, the resistance values R
1
to R
4
of the first to fourth resistors are set to satisfy &ggr;=1.
Preferably, the current path resistor is a thin-film resistor.
Preferably, a direct-current amplification factor of the primary transistor to the secondary transistor is equal to or larger than 50.
Preferably, each of the primary transistor and the secondary transistor is constituted by a pair of transistor elements connected in a Darlington pattern.
REFERENCES:
patent: 3956661 (1976-05-01), Sakamoto et al.
patent: 4591778 (1986-05-01), Holland
patent: 4990846 (1991-02-01), Buck et al.
patent: 5780921 (1998-07-01), Mitsuishi
patent: 5-60623 (1993-03-01), None
Denso Corporation
Lam Tuan T.
Law Office of David. G. Posz
Luu An T.
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