Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
2002-01-23
2003-10-28
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S145000
Reexamination Certificate
active
06639469
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a gain control circuit, which can be used in various parts of a low-noise amplifier used in a transmitter/receiver of mobile radio communication equipment, and which can operate as a variable gain circuit for compensating for the attenuation amount of a radio wave.
In general, in the field of radio communication, the attenuation amount of a radio wave changes with a communication distance. In mobile radio communication equipment such as mobile telephone, since the communication distance greatly changes, it has been required with a variable-gain amplifier circuit in order to compensate for the attenuation amount of the radio wave dependently upon the communication distance.
Referring to
FIG. 10
, there is shown a block diagram illustrating a fundamental construction of a transmitter/receiver section in a prior art mobile radio communication equipment. The shown transmitter/receiver section includes a receiver circuit part connected between one selection terminal of a switch
102
connected to an antenna
101
and one terminal of a base band processing part
113
, the receiver circuit part including a low-noise amplifier
103
, a band-pass filter (BP)
104
, a mixer
108
, an intermediate frequency (IF) amplifier
105
, a band pass filter (BP)
106
and a mixer
108
, which are cascade-connected in the named order. The shown transmitter/receiver section includes a transmitter circuit part connected between another terminal of the base band processing part
113
and the other selection terminal of the switch
102
, the transmitter circuit part including a mixer
109
, a band pass filter (BP)
112
, a driver amplifier
111
and a power amplifier
110
, which are cascade-connected in the named order.
In the above mentioned arrangement, the low-noise amplifier
103
and the intermediate frequency amplifier
105
in the receiver circuit part and the driver amplifier
111
in the transmitter circuit part can be constituted of the above mentioned variable-gain amplifier circuit.
Referring to
FIG. 11
, there is shown a circuit diagram illustrating a basic construction of the variable-gain amplifier circuit which can be used at various parts in the transmitter/receiver section of the mobile radio communication equipment. The shown variable-gain amplifier circuit is constructed to have a function similar to that of the variable-gain amplifier circuit disclosed in Japanese Patent Application No. Heisei 10-260734 published as JP-A-2000-091861. This variable-gain amplifier circuit includes an adjustable impedance circuit part including an NPN bipolar transistor
8
having a base connected to a control terminal
5
for receiving a controlled bias voltage, a collector connected to a power supply terminal
11
for receiving a positive fixed voltage, and an emitter connected through a resistor
6
to ground. The variable-gain amplifier circuit also includes a variable-gain amplifying circuit part having an amplifying circuit
1
having an input connected through a capacitor
9
to an input terminal
3
for receiving an input signal and an output connected to an output terminal
4
, a node between the input terminal
3
and the capacitor
9
being connected to a node between the emitter of the transistor
8
and the resistor
6
.
With this arrangement, the power supply terminal
11
connected to the collector of the transistor
8
is connected through a load (not shown) to a constant positive voltage power supply, and the control terminal
5
connected to the base of the transistor
8
is connected to a base bias circuit (not shown) which is formed of for example two bias resistors. In addition, as mentioned above, the emitter of the transistor
8
is connected through the resistor
6
to the ground. Thus, the adjustable impedance circuit part is constituted of the transistor
8
itself and the resistor
6
. Since a constant positive voltage is applied to the collector of the transistor through the load from the power supply, the capacitor
9
functions as a DC blocking capacitor against the adjustable impedance circuit part when an amplified signal is outputted from the output terminal
4
of the amplifying circuit
1
in the variable-gain amplifier circuit.
In this variable-gain amplifier circuit, the shunt amount to the ground, of an input signal supplied to the input terminal
3
, is controlled by the adjustable impedance circuit part, so that the amplification gain of the amplifying circuit
1
in the variable-gain amplifying circuit part can be varied. More specifically, since the control terminal
5
connected to the base of the transistor
8
is grounded in an AC mode, if the bias voltage applied to the control terminal
5
connected to the base of the transistor
8
is controlled, an impedance between the emitter of the transistor
8
and the ground (impedance of the transistor
8
and the resistor
6
) changes, with the result that the shunt amount of the input signal supplied to the input terminal
3
is controlled, and therefore, the amplification gain of the amplifying circuit
1
changes.
As other known technique concerning the gain control circuit and the variable-gain amplifier circuit, a variable-gain amplifier circuit shown in JP-A-6-120756 and a gain control circuit shown in JP-A-2000-151311 are exemplified.
In the above mentioned gain control circuit (variable-gain amplifier circuit), if the resistance value of the resistor
6
connected to the emitter of the transistor in the adjustable impedance circuit part is small, the input signal is bypassed through the resistor directly to the ground, so that a SIN ratio (ratio of signal power to noise power) in the amplifying circuit lowers, with the result that a noise factor (called an “NF”) in a maximum gain condition becomes large. To the contrary, if the resistance value of the resistor
6
connected to the emitter of the transistor is large, the power supply voltage required for operating the transistor becomes large. Accordingly, if the resistance value of the resistor
6
is not set at an appropriate value, a stable operation cannot be obtained.
Specifically, in the gain control circuit shown in
FIG. 11
, assuming that the power supply voltage is 3.0V, a collector current of the transistor
8
in operation is 5.0 mA, a collector-emitter voltage drop of the transistor
8
is 1.0V, a maximum value Re
max
of the resistor
6
for allowing the transistor
8
to operate is expressed as the following equation (1):
Re
max
=
3.0
-
1.0
5.0
×
10
-
3
=
400
Ω
(
1
)
FIG. 12
illustrates a relation of the noise factor NF (dB) to the resistance value Re (ohm) of the resistor
6
in the maximum gain condition of this gain control circuit (resistance dependency of NF). Here, the characteristics of {ONLY RESISTOR} is referred to. For example, if it is desired to suppress the noise factor NF to not greater than 1.2 dB, the resistor is required to have the resistance of not less than 1.2 K&OHgr;, which is apparently larger than 400 &OHgr; as mentioned above. This means that the transistor
8
cannot operate under the power supply voltage of 3.0V.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a gain control circuit which has overcome the above mentioned problems of the prior art.
Another object of the present invention is to provide a gain control circuit allowing a variable-gain amplifier circuit to operate under a low power supply voltage with a lowered noise factor in the maximum gain condition.
A further object of the present invention is to provide a variable-gain amplifier circuit incorporating such a gain control circuit therein.
The above and other objects of the present invention are achieved in accordance with the present invention by a variable-gain amplifier circuit comprising;
an adjustable impedance circuit part including a bipolar transistor having a base connected to a control terminal for receiving a controlled bias voltage, one of a collector and an emitter of the bipolar trans
Hayes & Soloway P.C.
Mottola Steven J.
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