Amplifiers – With semiconductor amplifying device – Including current mirror amplifier
Reexamination Certificate
2002-04-26
2004-01-20
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including current mirror amplifier
C330S257000, C330S289000, C327S362000, C323S315000
Reexamination Certificate
active
06680651
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor integrated circuits and, more particularly, to a current mirror for providing a large current ratio and a high output impedance and a differential amplifier having the same.
2. Description of the Related Art
A current mirror is a circuit for mirroring an amount of current flow of one circuit branch into another circuit branch. The current mirror serves as a constant current source for supplying a predetermined amount of current regardless of load and is generally used to set a DC bias of a circuit.
FIG. 1
is a circuit diagram of a conventional current mirror. Referring to
FIG. 1
, an output current I
out
, for example, a mirrored current, is obtained by multiplying the ratio of a size A
2
of an output transistor
13
to a size A
1
of a reference transistor
12
with a reference current I
ref
. However, to reduce current consumption in a radio frequency (RF) circuit, in general, current smaller than 100 &mgr;A is used as the reference current I
ref
, the output current I
out
must be greater than 1 mA to be used as a bias current. Thus, in a case where the size A
1
of the reference transistor
12
is 1, the size A
2
of the output transistor
13
must be greater than 10.
In this case, an output impedance of the current mirror in low frequency operation relates only to the amount of the output current I
out
(V
e
/I
out
) (V
e
is early voltage), and does not relate to the size A
2
of the output transistor
13
. However, parasitic components, for example, parasitic capacitance between a base and a collector of the output transistor
13
and parasitic capacitance between a substrate and a collector, occur in radio frequency operation, thereby lowering the output impedance of the current mirror. And the parasitic components are increased as the size A
2
of the output transistor
13
increases.
A current mirror is generally used as a current source of a differential amplifier. If an output impedance of the current mirror decreases, a common mode rejection ratio (CMRR) of the differential amplifier is lowered, and thus precise differential signals are not generated. Further, a differential amplifier in which one terminal is grounded, is used as an active balun in a local oscillator (LO) of a RF mixer. However, the CMRR of the differential amplifier used in the LO of the RF mixer is low and precise differential signals are not generated, the characteristics of the mixer are greatly lowered. Accordingly, a need exists for a current mirror for providing a large current ratio and a high output impedance even in radio frequency operation and a differential amplifier for generating precise differential signals without lowering a common mode rejection ratio (CMRR).
SUMMARY OF THE INVENTION
A current mirror is provided, which includes: a current source for supplying a reference current, the current source having a first terminal connected to a first reference voltage and a second terminal; a reference transistor having a control electrode, a first current carrying electrode connected to the second terminal of the current source, and a second current carrying electrode connected to a second reference voltage; an output transistor having a control electrode, a first current carrying electrode connected to an output terminal to which a mirrored current flows, and a second current carrying electrode connected to the second reference voltage; and a proportional-to-absolute temperature (PTAT) voltage generator connected between the control electrode of the reference transistor and the control electrode of the output transistor, for increasing the voltage of the control electrode of the reference transistor to a predetermined voltage and supplying the increased voltage to the control electrode of the output transistor. The first reference voltage is a supply voltage, and the second reference voltage is a ground voltage.
According to an embodiment of the present invention, the current mirror further includes: a first resistor connected between the second current carrying electrode of the reference transistor and the second reference voltage; and a second resistor connected between the second current carrying electrode of the output transistor and the second reference voltage. The first reference voltage is a supply voltage, and the second reference voltage is a ground voltage. The predetermined voltage is obtained by
V
t
×
ln
(
Z
×
A
1
A
2
)
,
wherein V
t
is a thermal voltage, Z is the resistance ratio of the first resistor to the second resistor, and A
1
and A
2
are the sizes of emitters of the reference transistor and the output transistor, respectively.
According to an embodiment of the present invention, the PTAT voltage generator includes: a first transistor having a first current carrying electrode connected to the second terminal of the current source, a control electrode connected to the first current carrying electrode, and a second current carrying electrode; a second transistor having a control electrode connected to the control electrode of the first transistor, a first current carrying electrode connected to the first reference voltage, and a second current carrying electrode; a third transistor having a control electrode connected to the second current carrying electrode of the second transistor, a first current carrying electrode connected to the second current carrying electrode of the first transistor, and a second current carrying electrode connected to the first current carrying electrode of the reference transistor; and a fourth transistor having a control electrode connected to the second current carrying electrode of the first transistor, a first current carrying electrode connected to the second current carrying electrode of the second transistor, and a second current carrying electrode connected to the control electrode of the output transistor. The first through fourth transistors of the PTAT voltage generator are bipolar transistors.
According to an embodiment of the present invention, the PTAT voltage generator includes: a first transistor having a first current carrying electrode connected to the first reference voltage, a control electrode, and a second current carrying electrode; a second transistor having a control electrode connected to the control electrode of the first transistor, a first current carrying electrode connected to the first reference voltage, and a second current carrying electrode connected to its control electrode; a third transistor having a control electrode connected to the second current carrying electrode of the first transistor, a first current carrying electrode connected to the second current carrying electrode of the first transistor, and a second current carrying electrode connected to the first current carrying electrode of the reference transistor; and a fourth transistor having a control electrode connected to the control electrode of the third transistor, a first current carrying electrode connected to the second current carrying electrode of the second transistor, and a second current carrying electrode connected to the control electrode of the output transistor. The first and second transistors are MOS transistors, and the third and fourth transistors are bipolar transistors.
According to an embodiment of the present invention, the reference transistor and the output transistor are bipolar transistors.
A differential amplifier is also provided, which includes: a differential amplifying unit for differentially amplifying two input signals; and a current mirror for generating a mirrored current proportional to a predetermined reference current and supplying the mirrored current as a current source to the differential amplifying unit, wherein the current mirror includes: a current source for supplying a reference current, the current source having a first terminal connected to a first reference voltage and a second terminal; a reference transistor having a control electrode, a first current carryi
Choe Henry
Samsung Electronics Co,. Ltd.
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