Amplifiers – With control of power supply or bias voltage – With control of input electrode or gain control electrode bias
Reexamination Certificate
2000-04-06
2001-12-25
Shingleton, Michael B. (Department: 2817)
Amplifiers
With control of power supply or bias voltage
With control of input electrode or gain control electrode bias
C330S134000, C455S241100, C455S251100, C455S253100, C455S253200
Reexamination Certificate
active
06333675
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a variable gain amplifier, and more particularly, to a variable gain amplifier preferably used in communication devices, such as portable telephones.
A variable gain amplifier is used in an automatic gain control (AGC) circuit of a send and receive unit of a portable telephone.
FIG. 1
is a schematic block diagram of a conventional variable gain amplifier
10
. The variable gain amplifier
10
amplifies an input signal Pin while changing its gain in response to a gain control signal (a control input voltage Vc) and generates an output signal Pout. The variable gain amplifier
10
includes a plurality of (three in this case) unit variable gain amplifiers (hereinafter referred to as “unit amplifiers”)
11
,
12
and
13
, a direct current (DC) amplifier
14
for producing a signal used to control respective gains in response to the control input voltage Vc, level shift circuits
15
,
16
and
17
and voltage-limiting circuits
18
,
19
and
20
.
The DC amplifier
14
provides an amplified signal generated by amplifying the control input voltage Vc to each of the level shift circuits
15
,
16
and
17
. The level shift circuits
15
,
16
and
17
level-shift the amplified signal by a predetermined DC voltage and generate level shifted voltages Vb
1
, Vb
2
and Vb
3
, respectively. The voltage-limiting circuits
18
,
19
and
20
limit the level shifted voltages Vb
1
, Vb
2
and Vb
3
from the level shift circuits
15
,
16
and
17
to a constant DC voltage range and provide the limited control voltages Vc
1
, Vc
2
and Vc
3
to the unit amplifiers
11
,
12
and
13
, respectively.
FIG.
2
(
a
) is a graph showing the relationships between the control input voltage Vc and the voltages Vb
1
, Vb
2
and Vb
3
of the level shift circuits
15
,
16
and
17
, and FIG.
2
(
b
) is a graph showing the relationships between the control input voltage Vc and the control voltages Vc
1
, Vc
2
and Vc
3
of the voltage-limiting circuits
18
,
19
and
20
.
When the control voltages Vc
1
, Vc
2
and Vc
3
are generated, the unit amplifiers
11
,
12
and
13
sequentially perform amplification operations together with the increase of the control input voltage Vc. As shown in FIG.
2
(
c
), the variable gain amplifier
10
outputs the amplified signal having a substantially linear gain characteristic against the change of the control input voltage Vc.
However, in the high gain region (region where the control input voltage Vc approaches the maximum value), the noise factor of the conventional variable gain amplifier
10
increases. This is because the gains of the unit amplifiers
11
to
13
are limited in order to obtain the substantially linear gain characteristic of the variable gain amplifier
10
. Thus, when the gain of the first unit amplifier
11
is increased to reduce the noise factor, the gain linearity deteriorates, and the noise factor deteriorates when the gain of the unit amplifier
11
is suppressed to improve the linearity.
Further, because each of the level shift circuits
15
to
17
level-shifts the input signal separately and generates the voltages Vb
1
to Vb
3
, the switching timing of the unit amplifiers
11
to
13
is shifted as shown in FIG.
3
(
a
) or FIG.
3
(
c
) if the shift amount of each of the level shift circuits
15
to
17
is changed by process irregularities. FIG.
3
(
b
) shows ideal switching timing. This impedes the linearity of gain characteristic. Further, when the limit values of the voltage-limiting circuits
18
to
20
have dispersion due to the process irregularities, the linearity of the gain characteristic is not adequate.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a variable gain amplifier having an improved linear gain characteristic and a low noise factor.
In one aspect of the present invention, a variable gain amplifier includes a plurality of series connected unit variable gain amplifiers including a front stage amplifier and a back stage amplifier, for amplifying an input signal in accordance with respective gains and generating a multi-stage amplified signal. A gain control circuit is connected to the plurality of unit variable gain amplifiers to generate control signals which control the respective gains of the unit variable gain amplifiers in accordance with a gain control input signal so that the gain of the front stage amplifier is greater than that of the back stage amplifier.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
REFERENCES:
patent: 5841320 (1998-11-01), Ichihara
patent: 6011437 (2000-01-01), Sutardja et al.
patent: 54100644 (1979-08-01), None
patent: 9-36679 (1997-02-01), None
Arent Fox Kintner & Plotkin & Kahn, PLLC
Fujitsu Limited
Shingleton Michael B.
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