Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2002-09-10
2004-02-10
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C327S359000
Reexamination Certificate
active
06690236
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gain variable amplifier for use in a communication device such as a portable telephone and, more particularly, to a gain variable amplifier with improved linearity of a power gain change.
2. Description of Related Art
In order to realize higher speech quality, a portable telephone in recent years is required to have the function of accurately controlling the strength of waves of transmission in accordance with the strength of waves received. In a gain variable amplifier used for each of transmitting and receiving units, it is therefore necessary to realize linearity of a power gain change over a wide wave strength range.
In a conventional gain variable amplifier, the linearity of a power gain change realized by an amplifier of one stage is limited. Consequently, as shown in
FIG. 1A
, amplifiers of multi-stages (two amplifiers Amp
1
and Amp
2
in
FIG. 1A
) are connected and gains of the amplifiers are added, thereby assuring the linearity of the power gain change in a wide range as a whole.
In another conventional technique, as an attempt to widen the linearity of the power gain change in the amplifier of one stage, as shown in an operating principle diagram of
FIG. 2
, in order to control the gain in a differential amplification circuit
10
in a gain variable amplifier
500
, a response of a gain control signal VAGC to a control input signal VCNT of a gain control circuit
50
is controlled so as to be either linear or to have a monotone increase characteristic of which curve opens upwards in a linear coordinate system.
FIG. 3
shows a specific example of such a circuit. In the circuit of
FIG. 3
, the gain control circuit
50
also has a differential input circuit configuration. An input voltage VCLS obtained by shifting the level of the control input signal VCNT so as to be linear by a level shifting circuit
51
is supplied to a differential pair in the gain control circuit
50
. A specific operation is as shown in FIG.
4
. The level of the input voltage VCLS obtained by the level shifting of the level shifting circuit
51
is shifted so that the input output characteristic of the gain control circuit
50
expresses a curve which opens upwards or is linear, thereby outputting the gain control signal VAGC having a response characteristic peculiar to the control input signal VCNT. By matching the characteristic with that of the differential amplification circuit
10
, a linear gain change characteristic is realized.
Specifically, when the input voltage VCLS shifts from VA to VB with respect to the control input signal VCNT, an output current ICNT of the gain control circuit
50
has a monotone increase characteristic of which curve opens upwards. At the time of converting the output current ICNT to a voltage by a load resistor and outputting the voltage as the gain control signal VAGC, the gain control signal VAGC is adjusted so as to match the linear characteristic of the differential amplification circuit
10
. Consequently, a monotone increase characteristic of which curve opens upwards can be obtained as an output characteristic of the differential amplification circuit
10
. When the characteristic is expressed in decibels as the power gain, a linear gain change characteristic can be obtained in response to the control input signal VCNT.
When the input voltage VCLS is shifted from VB to VC with respect to the control input signal VCNT, the output current ICNT characteristic of the gain control circuit
50
becomes linear. When the characteristic is adjusted so as to match the monotone increase characteristic of which curve opens upwards of the differential amplification circuit
10
, a monotone increase characteristic of which curve opens upwards is derived as an output characteristic of the differential amplification circuit
10
. Thus, a linear gain change characteristic can be similarly obtained.
In the conventional techniques, the technique of connecting amplifiers of multi-stages has a problem that, since the number of component circuits is large, a number of components are necessary so that circuit layout is rigidly limited. Since current consumption is high, there is also a problem such that continuous use time of a portable device such as a portable telephone is limited, and it is also difficult to realize high density packing due to heat generated in association with current consumption.
In the another conventional technique in which the gain control is adjusted to attain the linear characteristic or the monotone increase characteristic of which curve opens upwards, only a limited region of the input output characteristics of the differential amplification circuit
10
is used. When the differential amplification circuit
10
is used beyond the region, the linearity of the gain change is lost and the circuit cannot be used. The region in the input output characteristics of the differential amplification circuit
10
which can realize the linearity is therefore limited and the linearity over a wide range cannot be assured. There is also a problem such that, since the differential amplification circuit is used within the narrow operating range of the gain control signal VAGC, it is difficult to assure sufficient operating accuracy.
SUMMARY OF THE INVENTION
The invention has been achieved to solve the problems of the conventional techniques and its object is to provide a gain variable amplifier of one stage capable of realizing linearity of a power gain change over a wide range by properly changing a gain control characteristic in response to a control input signal VCNT.
In order to achieve the object, according to one aspect of the invention, there is provided a gain variable amplifier comprising: a gain control circuit having a gain control characteristic of outputting a predetermined gain control signal in accordance with a control input signal; and a differential amplification circuit having an input output characteristic of varying a gain by switching a differential output current passing through a differential pair by the gain control signal, wherein the gain control circuit variably controls the gain control characteristic in accordance with the input output characteristic.
Consequently, the gain control circuit can properly vary the gain control characteristic of the gain control signal in response to the control input signal in accordance with the input output characteristic of the differential amplification circuit. A predetermined gain change characteristic can be therefore given to the control input signal and the linear gain change over a wide input output range of the differential amplification circuit can be realized.
According to another aspect of the invention, in the gain variable amplifier, when the gain control signal is supplied to the differential amplification circuit and is in a first region in which the input output characteristic is a linear characteristic in a linear coordinate system, the gain control circuit variably controls the gain control characteristic so as to be a first monotone increase control characteristic of which curve opens upwards in a linear coordinate system, when the gain control signal is supplied to the differential amplification circuit and is in a second region in which the input output characteristic is a monotonous increase characteristic of which curve opens downwards in a linear coordinate system, the gain control circuit variably controls the gain control characteristic to a second monotone increase control characteristic of which curve is sharper than that of the first monotone increase control characteristic, and when the gain control signal is supplied to the differential amplification circuit and is in a third region in which the input output characteristic is a monotone increase characteristic of which curve opens upwards in a linear coordinate system, the gain control circuit variably controls the gain control characteristic to be a linear control characteristic.
In the first region in which the input output
Arent Fox Kintner Plotkin & Kahn
Fujitsu Limited
Nguyen Khanh Van
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