Amplifiers – With semiconductor amplifying device – Including gain control means
Reexamination Certificate
1999-03-05
2001-03-06
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including gain control means
C330S284000
Reexamination Certificate
active
06198349
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a variable gain amplifier and, more particularly, to a variable gain amplifier for adjusting a level of an analog image-signal component contained in a video signal supplied by, for example, a CCD linear image sensor so as to convert the image signal component which corresponds to an amount of an input light into digital data. Such a variable gain amplifier is used in a CCD analog-signal processing circuit which supplies an analog signal to an A/D converter converting an image signal into digital image data.
2. Description of the Related Art
FIG. 1
shows a first example of a conventional variable gain amplifier that comprises an amplifier
2
and an R-2R ladder
11
serving as a signal attenuator.
FIG. 2
is a circuit diagram of the R-2R ladder
11
. Additionally,
FIG. 3
shows a second example of a conventional variable gain amplifier that comprises an operational amplifier
12
and the R-2R ladder
11
as a signal attenuator provided in a feedback loop of the operational amplifier
12
.
FIG. 4
shows a third example of a conventional gain amplifier that comprises the amplifier
2
and a dB-attenuator
13
. The dB-attenuator
13
may be a &pgr;-type or a T-type.
FIG. 5
is a circuit diagram of the dB-amplifier
13
of a &pgr;-type. As shown in
FIG. 5
, the dB-amplifier
13
comprises a plurality of &pgr;-type amplifiers so that a necessary number of the &pgr;-type amplifiers through which a signal is passed are selected from among the plurality of &pgr;-type amplifiers.
FIG. 6
shows a fourth example of a conventional variable gain amplifier that comprises the operational amplifier
12
and the dB-attenuator
13
provided in a feedback loop of the operational amplifier
12
. Additionally, there is an amplifier as a fifth example of a conventional variable gain amplifier that comprises a voltage controlled variable gain amplifier and a D/A converter.
According to the above-mentioned first and second examples of the conventional variable gain amplifier, a variable range of a gain is very large and a number of variation steps is small. However, according to the second example, since a gain setting curve is inverse proportional to a setting data curve, a high-speed gain setting can be achieved by rendering an image signal after A/D conversion, that is, a peak value of image data to be the setting data.
According to the above-mentioned third and fourth examples of the conventional variable gain amplifier, a variable range of a gain and a number of variation steps can be freely set. Accordingly, a fine gain setting can be done with a minimum setting-data length, that is, a minimum number of bits. However, if the number of variation steps is increased and a width of each step is set to a small value, a resistance range of a resistor included in the attenuator
13
must be increased. That is, a high resistance is required for the resistor included in the attenuator
13
. This causes a problem when the variable gain amplifier is formed in a monolithic IC.
According to the fifth example of the conventional variable gain amplifier, a variable range of a gain, a number of variation steps and a setting curve can be freely set. However, it is difficult to achieve high accuracy fundamentally. If an R-2R ladder having a high accuracy is connected to an amplifier or an operational amplifier, an accurate IC process is required. Additionally, in this case, an ON resistance of each analog switch for selecting resistors included in the R-2R ladder must be decreased, that is, the size of the element constituting each analog switch must be increased. Accordingly, it is difficult to achieve a variable gain amplifier having a high accuracy, or such an amplifier must be very expensive if it can be made.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide an improved and useful variable gain amplifier in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a variable gain amplifier, which has a large variable range of a gain with respect to an analog signal having a level changing at a high-speed.
Another object of the present invention is to provide a variable gain amplifier in which an reverse proportional setting curve can be set for a fine gain adjustment and a high-speed gain setting.
A further object of the present invention is to provide a variable gain amplifier which can set a gain setting curve according to a logarithmic proportion (dB proportion) can be easily formed in a monolithic IC.
In order to achieve the above-mentioned problems, there is provided according to the present invention a variable gain amplifier comprising:
an amplifier; and
a digital-setting-type signal attenuator comprising:
a resistor string including a plurality of resistors connected in series;
a maximum attenuation-factor limiting resistor connected between the resistor string and a ground;
a plurality of analog switches each of which is connected to respective points of the resistor string; and
a decoder generating a control signal for selecting at least one of the analog switches, the control signal being generated based on setting data provided from an external device,
wherein a voltage across the resistor string is divided by the resistors included in the resistor string, and the divided voltage is output to the amplifier from at least one of the analog switches selected by the control signal generated by the decoder.
According to the present invention, a voltage across the resistor string is limited by the total resistance of the resistor string and a resistance of the maximum attenuation-factor limiting resistor. if the number of resistors included in the resistor strings is 2
I
, and the number of resistors selected by the selection of the one of the analog signal is DD1 which is represented by the setting data, a voltage output from the selected one of the analog switches is B times a voltage input to the resistor string. The ratio B is represented as follows.
(Ra+R×(2
I
−DD1)/2
I
)/(R+Ra)
where R is a total resistance of the resistor string, and Ra is a resistance of the maximum attenuation-factor limiting resistor.
If the setting data comprises 8 bits and I=8, and when Ra=R, the setting data represents a value from 0 to 255. Accordingly, the ratio B is equal to 1 when DD1=0, and the ratio B is equal to 257/512 when DD1=255. That is, the ratio B is set to a value ranging from 257/512 to 1 according to the value represented by the setting data. A variation in the ratio B when the value represented by the setting data is changed by a minimum value is equal to 1/512. That is, a step value of the ratio B is 1/512.
As mentioned above, the variable gain amplifier according to the present invention controls the maximum attenuation factor by the maximum attenuation-factor limiting resistor. Thereby, an unnecessary attenuation range can be eliminated, and also a fine step value can be achieved.
In the variable gain amplifier according to the present invention, the decoder may generate the control signal for selecting two adjacent ones of the analog switches so that a middle voltage between voltages output from the two selected analog switches is output from the digital-setting-type signal attenuator to the amplifier.
According to this invention, since a middle voltage between voltages of each pair of two adjacent analog switches can be obtained, the number of steps in the variation of the ratio B is twice the number of resistors included in the resistor string. Thus, a fine step value can be obtained without increasing the number of resistors included in the resistor string.
Additionally, the resistor string may include a compensation capacitance provided between an arbitrary point of the resistor string and an input point of the resistor string.
In this invention, since the compensation capacitor compensates for a limitation in a bandwidth due to a low-pass filter formed by a
Inage Osamu
Kanno Tohru
Nguyen Khanh Van
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pascal Robert
Ricoh & Company, Ltd.
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