Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2003-01-23
2004-06-29
Zweizig, Jeffrey (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
06756840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to current mirror circuits, and particularly to current mirrors having reduced nonlinear distortion.
2. Description of the Related Art
Current mirrors are widely used in analog integrated circuits. In general terms, current mirrors are circuits having a reference branch through which a reference current flows and at least one mirror branch through which a current flows that is proportional to the reference current flowing through the reference branch.
Efforts to improve the performance of current mirrors resulted in the creation of a wide variety of different implementations. A relatively popular current mirror implementation is the cascoded current mirror shown in FIG.
1
. Reference branch
10
includes transistors
11
and
12
coupled together in cascode relation. Mirror branch
13
includes cascode connected transistors
14
and
15
. The control terminals of transistors
11
and
14
are connected to a reference or bias voltage Vbias. The control terminal of transistors
12
and
15
are connected to each other and to the input of the current mirror circuit. The output current of the current mirror, Iout, passes through mirror branch
13
and is proportional to the current Iin passing through reference branch
10
. The relationship between the output current Iout and input current Iin is based upon the ratio of the sizes of transistors
14
and
15
to the sizes of transistors
11
and
12
. At relatively low frequencies, the current mirror of
FIG. 1
exhibits relatively accurate proportionality and relatively low nonlinear distortion.
At high frequencies, however, parasitic capacitances
16
in the current mirror of
FIG. 1
adversely affect the amount of nonlinear distortion. Because the transconductance of a MOS transistor is inherently nonlinear, the voltage appearing at the input of the current mirror of
FIG. 1
is not linearly proportional to the input current Iin and is subject to nonlinear distortions. This may be seen with reference to
FIG. 2
, in which the input current and input voltage of current mirror
10
is shown. Excursions of the input voltage are greater when the input current is low. The typically high output impedance of a current source (providing input current Iin to current mirror
10
) and the reference current branch do not allow the input current to be affected by the nonlinearity of the input voltage at low frequencies wherein the current flowing through parasitic capacitor
16
is negligible. At high frequencies, the current passing through parasitic capacitance
16
becomes comparable with the input current Iin. Parasitic capacitance
16
is formed from the output capacitance of the current source providing input current Iin to the current mirror, the drain capacitance of transistor
11
and the gate capacitances of transistors
12
and
15
. The current passing through parasitic capacitance
16
at high frequencies adversely affects the transfer function of the current mirror. In addition, because the charge accumulated at parasitic capacitance
16
is substantially proportional to the input voltage and because the input voltage is nonlinear relative to the input current Iin, additional nonlinear distortion to the input current Iin is exhibited. These distortions are transferred to the output current Iout in mirror branch
13
.
An attempt to improve the nonlinear distortion in the current mirror of
FIG. 1
is shown in
FIG. 3. A
resistor
17
is connected to the source terminal of each transistor
12
and
15
. Resistors
17
tend to make more linear the effective transconductance of the reference branch
10
and mirror branch
13
at lower frequencies. However, the effect provided by resistors
17
dwindles at higher frequencies. A transistor
18
is connected between the input of the current mirror and the gate terminals of transistors
12
and
15
so as to decouple the input capacitances of transistors
12
and
15
from the input. Though transistor
18
reduces nonlinear distortions at higher frequencies, transistor
18
reduced the headroom of reference branch
10
by a threshold voltage. For integrated circuits having lower power supply levels, such as 1.8 v, this reduction in available headroom becomes a nontrivial effect.
Based upon the foregoing, there is a need for a current mirror having reduced nonlinear distortion at high frequency operation.
SUMMARY OF THE INVENTION
Embodiments of the present invention overcome the above-identified shortcomings and satisfy a significant need for a current mirror having reduced nonlinear distortion at relatively high frequencies. Nonlinear distortions are reduced in part by employment of a current amplifier with the input coupled through a capacitor to the input of the current mirror. An output of the current amplifier is either coupled to a node in the reference branch or a node in the mirror branch of the current mirror. The current amplifier may be a noninverting amplifier (when the output thereof is coupled to the reference branch) or an inverting amplifier (when the output thereof is coupled to the mirror branch). The current amplifier serves to restore the shape of the current mirror output signal, thereby reducing the nonlinear distortion of the current mirror.
Another embodiment of the present invention is adapted for use in applications that utilize multiple current mirrors, such as in a design in which two current mirrors are employed to provide a differential current signal. In this embodiment, the output of the current amplifier of a first current mirror of a pair of current mirrors is coupled to the second current mirror of the current mirror pair, and the output of the current amplifier of the second current mirror is coupled to the first current mirror.
REFERENCES:
patent: 4251743 (1981-02-01), Hareyama
patent: 5650746 (1997-07-01), Soltau
patent: 6346804 (2002-02-01), Ueno et al.
patent: 6657481 (2003-12-01), Rasmussen et al.
Jorgenson Lisa K.
STMicroelectronics Inc.
Szuwalski Andre
Zweizig Jeffrey
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