Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2002-09-09
2004-09-28
Sherry, Michael (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
Reexamination Certificate
active
06798182
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of electronic circuit design, and in particular to the design of a current mirror that provides a high output impedance and an accurate mirror of input current across a wide range of output voltages.
2. Description of Related Art
Current mirrors are often used to provide a controlled current to a component without loading the source of the controlled current. An independent source generates a current at a given value; the current mirror provides an output current to a load, such that the output current corresponds to the value of the independently generated current. In this manner, the source of the desired current is isolated from the load that receives an equivalent current.
FIG. 1
illustrates an example circuit diagram of a basic current mirror
100
. A transistor T
1
is configured as a diode, by connecting its drain and gate, for communicating the independent source current, Iin, to ground. A second transistor T
2
has its gate connected to the gate of T
1
, and has its source connected to the same potential as the source of T
1
. Thus, the gate-to-source voltages of each of the transistors T
1
and T
2
are equal, and, if the transistors T
1
and T
2
are operationally identical, the drain-to-source current through each will be the same. The current through T
1
corresponds to the input current Iin; therefore, assuming that the source of the current lout is sufficient to provide at least this value of current, the output current, Iout, will be equal to Iin. Note, however, that the characteristics of the load that is intended to draw the current lout can affect the operation of transistor T
2
, by affecting transistor T
2
's drain-to-source voltage, Vout. If the drain-to-source voltage Vout of transistor T
2
does not equal the drain-to-source voltage Va of transistor T
1
, the current lout through transistor T
2
will differ from the current Iin through transistor T
1
. If Vout is less than Va, then lout will be less than Iin. Similarly, if Vout is greater than Va, then lout will be greater than Iin. This is due to the limited output impedance of transistor T
2
.
Output voltage compliance is defined herein as the range of output voltages through which a current mirror will provide an output current lout that corresponds to the input current Iin. The current mirror
100
exhibits relatively poor output voltage compliance, because only when Vout is equal to Va will the output current lout equal the input current Iin, due in part to the limited output impedance of the transistor T
2
.
FIG. 2
illustrates an example circuit diagram of a current mirror
200
that provides greater output impedance, and thus a wider range of output voltage compliance than the current mirror
100
of FIG.
1
. In the current mirror
200
, a differential amplifier A
1
and transistor T
3
are configured to assure that the drain to source voltages Va, Vb of the input T
1
and output T
2
transistors are equal. The amplifier A
1
and transistor T
3
control the drain-to-source impedance of transistor T
2
, such that a controlled output current lout (=Iin) is provided independent of the output voltage Vout, when Vout is greater than Vb. Because the gate-to-source voltage and the drain-to-source voltage of each of the transistors T
1
and T
2
are assured to be equal, the output current lout is assured to be equal to the input current Iin, when the voltage Vout is greater than Vb. In the current mirror
200
, the output impedance and voltage compliance is improved, compared to the current mirror
100
, because in current mirror
200
, the output current Iout will equal the input current Tin whenever Vout is greater than Vb, which is set equal to Va. In this case, the voltage compliance is limited to the lower value of Va, which is generally determined by the source of the input current Iin.
FIG. 3
illustrates an example circuit diagram of a current mirror
300
tha is operable to lower ranges of output voltages than the current mirror
200
, as taught by U.S. Pat. No. 5,612,614, issued Mar. 18, 1997 to Barrett et al., and included by reference herein. In current mirror
300
, transistors T
1
and T
4
are configured having a common channel and two gates, thereby forming a composite transistor. This composite transistor T
1
-T
4
is diode-connected, by coupling the gates of each transistor T
1
, T
4
, to the drain of T
4
, thereby forming a two-input diode device that has an intermediate node between the gates that provides the drain voltage Va of transistor T
1
. By dividing the input source voltage Vc between the transistors T
1
and T
4
, the voltage Va at the drain of transistor T
1
is lower than the input source voltage Vc. The relative sizes/transconductances of transistors T
1
and T
4
determine the value of Va relative to Vc. Because the diode arrangement requires that the transconductance of transistor T
4
be substantially higher than the transconductance of transistor T
4
, the value of Va relative to Vc is limited.
BRIEF SUMMARY OF THE INVENTION
It is an object of this invention to provide a current mirror having a large output voltage compliance. It is a further object of this invention to provide a current mirror that dynamically adjusts for differences between an input source voltage and an output load voltage, so as to provide a large output voltage compliance.
These objects and others are achieved by providing a current mirror that divides an input source voltage dynamically, to provide a controlled voltage that corresponds to an output load voltage. The correspondence between this controlled voltage and the output load voltage determines the correspondence between the output current and the input current. By dynamically adjusting the controlled voltage, the correspondence to the output load voltage can be maintained to very low voltage levels. Preferably, the output load voltage is also dynamically divided to provide a comparison voltage for comparing to the controlled voltage when the output load voltage is high, thereby providing the appropriate output current at high voltage levels.
REFERENCES:
patent: 5612614 (1997-03-01), Barrett et al.
patent: 6633198 (2003-10-01), Spalding, Jr.
patent: 0356570 (1988-09-01), None
Koniklijke Philips Electronics N.V.
Laxton Gary L
Sherry Michael
Waxler Aaron
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