Amplifiers – With control of power supply or bias voltage – With control of input electrode or gain control electrode bias
Reexamination Certificate
2002-12-03
2004-05-11
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With control of power supply or bias voltage
With control of input electrode or gain control electrode bias
C330S295000, C330S310000
Reexamination Certificate
active
06734730
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an amplifier for switching a gain depending on the level of an input signal, and particularly to a variable gain amplifier which can be effectively applied to a receiver for wireless communication.
In a wireless mobile communication system such as a mobile telephone or the like which has already formed a wide industrial market, communications are realized using the frequency of GHz band. An amplifier of the receiver used in the GHz band for wireless communication is designed to assure high sensitivity and to obtain good linearity. For example, an amplifier, which can switch the gain thereof, namely provide a high gain when an input signal is low level and a low gain when an input signal is high level, is employed. With the switching of gain, distortion of signal due to saturation of circuit can be suppressed and more excellent linearity can be obtained.
The first example of the existing gain switching amplifier, namely variable gain amplifier is illustrated in FIG.
9
. In this example, the current sources
201
-
1
,
201
-
2
,
201
-
3
of different current values are switched with the ON/OFF operation of the switches
203
-
1
,
203
-
2
,
203
-
3
under the control of a selection circuit (SEL)
103
for three differential amplifiers consisting of loads (Z
L
)
104
p
,
104
n
, transistors
202
p
-
1
,
202
n
-
1
, transistors
202
p
-
2
,
202
n
-
2
and transistors
202
p
-
3
,
202
n
-
3
. A differential input signal to the input terminals
101
p
,
101
n
is amplified and outputted to the output terminals
102
p
,
102
n.
A gain of amplifier is determined with product of a mutual conductance gm of transistor and an impedance of a load
104
, but since the mutual conductance gm corresponds to an operation current of transistor, a gain of amplifier can be switched through the switching of a current source
201
. Since the mutual conductance gm also corresponds to an element size of transistor, a method for switching the gain by introducing transistors of different sizes into the three differential amplifiers can also be introduced. The amplifier for switching a current or a size of element is disclosed, for example, in the Japanese Patent Laid-Open (Kokai) No. Hei 11-177357. In above amplifier, since constants of a transistor change depending on the switching of a current or an element size, an input impedance and an output impedance of the amplifier change whenever the switching is conducted.
The second example of the existing gain switching amplifier is illustrated in FIG.
10
. This amplifier includes, between the input terminal
101
and output terminal
102
, a plurality of paths consisting of a voltage-input/voltage-output type amplifier (hereinafter referred to as V—V amplifier)
106
a
of the gain A
0
and a V—V amplifier
106
b
of the gain A
1
(A
0
>A
1
).
The selection circuit
103
obtains information of an input signal amplitude from an external circuit of the amplifier and controls a bias circuit (BC)
107
. When the input signal amplitude is low, the selection circuit
103
supplies a bias voltage to the V—V amplifier
106
a
of high gain from the bias circuit
107
to select the path including the V—V amplifier
106
a
of high gain by cutting the bias voltage of the V—V amplifier
106
b
of low gain. When the input signal amplitude is high, the path including the V—V amplifier
106
b
of low gain is selected. Such amplifier is disclosed in the Japanese Patent Laid-Open (Kokai) No. 2001-223549. Even in this example, since the mutual conductance gm is varied by changing an element size of transistor forming the amplifier
106
or a current flowing into the transistor, the input impedance and output impedances when the gain is switched vary to a large extent.
As the third example of the existing gain switching amplifier, the amplifier disclosed in the Japanese Patent Laid-Open (Kokai) No. Hei 11-284460 is illustrated in FIG.
11
. This example is constituted of an amplifier unit
810
and a level detecting unit
811
having a plurality of paths allocated between the input terminal
101
and output terminal
102
via a change-over switch
105
, namely a path of the V—V amplifier
801
a
, a path of cascade connection of the V—V amplifier
801
b
and V—V amplifier
802
b
and a path of cascade connection of the V—V amplifier
801
c
, V—V amplifier
802
c
and V—V amplifier
803
c
. The V—V amplifier
801
has the gain A
0
, while the V—V amplifier
802
has the gain Al and the V—V amplifier
803
has the gain A
2
, respectively.
The level detecting unit
811
is constituted of the path of cascade connection of the V—V amplifier
801
x
, V—V amplifier
802
x
and V—V amplifier
803
x
having the identical electrical characteristics as those explained above and a level detector (DET)
116
. Any path of the amplifier unit
810
is selected depending on the detection result.
The circuit of this example changes an input impedance and an output impedance through the switching as in the case of the first and second examples. An input impedance changes at a higher frequency influenced by a coupling capacitance between the input side and output side when a load side of a preamplifier
801
changes with the switching operation. Moreover, since this example is based on the installation of the level detecting unit
811
using the same V—V amplifier as the amplifier unit
810
from the structural viewpoint, physical configuration of circuit becomes large and a current dissipation is also large and moreover installation of the change-over switch
105
accompanied by deterioration of signal is essential.
SUMMARY OF THE INVENTION
In a receiver for wireless communication, it is very important that impedance matching is attained between a variable gain amplifier and a preceding stage thereof (for example, antenna) and impedance matching is also attained between the variable gain amplifier and a subsequent stage thereof (for example, mixer). If such impedance matching is not attained, signal loss and noise increase due to generation of reflection in the signal to be transmitted and thereby sensitivity is deteriorated.
In the existing variable gain amplifier explained above, it is not considered that unmatching is generated due to the change of input and output impedances when the switching of gain is conducted. Particularly, in the method for changing a circuit parameter such as an element size or a current flowing into the element, an input impedance and an output impedance change largely. Therefore a degree of mismatching becomes large in order to change the gain.
Moreover, deterioration of sensitivity due to the switching loss cannot be neglected in the system for changing the path with a switch connected in series to the signal path.
It is therefore an object of the present invention to provide a high sensitivity variable gain amplifier which can maintain the excellent matching condition by suppressing an impedance change due to the switching of gain.
The object of the present invention can be effectively achieved by introducing a structure in which at least one path comprising at least a stage of the voltage-input/voltage-output amplifier+a stage of voltage-input/current-output amplifier and a path comprising a stage of the voltage-input/current-output amplifier are connected in parallel between the input terminal and output terminal or, a structure in which a plurality of paths each comprising at least a stage of voltage-input/voltage-output amplifier+a stage of voltage-input/current-output amplifier are connected in parallel between the input terminal and output terminal, and by switching the gain by selecting and operating any one path among the paths explained above. And then, it is desirable to keep, even when any path is selected, the input impedance viewed from the input terminal almost unchanged or the output impedance viewed from the output terminal unchanged.
An input impedance and an output impedance explained above can be realized, for example, on selecting any one of paths through control
Doi Takeshi
Maio Kenji
A. Marquez, Esq. Juan Carlos
Fisher Esq. Stanley P.
Hitachi , Ltd.
Nguyen Khanh Van
Reed Smith LLP
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