Variable gain amplifier circuit

Details Variable gain amplifier circuit Variable gain amplifier circuit

1999-05-26

2001-01-16

Mottola, Steven J. (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including gain control means

C330S288000

Reexamination Certificate

active

06175278

ABSTRACT:

This application is based on Japanese patent application No. 10-148036 filed on May 28, 1998, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to a variable gain amplifier circuit, and more particularly to a variable gain amplifier circuit suitable for use with a CMOS integrated circuit which processes an image signal.
b) Description of the Related Art
A variable gain amplifier circuit such as shown in
FIG. 11
has been proposed as cited in JP-B-2-34205 as a prior art. This variable amplifier uses an operational amplifier
100
as an inverter amplifier and can change an amplifier gain by changing a resistance ratio of an input resistor
101
to a feedback resistor
102
.
The input resistor
101
is made of a ladder resistor network of an eight-stage structure having resistors R and
2
R. A gain is determined by a combination of turned-on switches. Namely, an input voltage Vin is amplified by the operational amplifier
100
and output as an output voltage Vout at a gain determined by a combination of switches
103
whose contacts are connected to the inverting input terminal of the operational amplifier
100
(on-state switches
103
).
In the variable gain amplifier circuit shown in
FIG. 11
according to a related art, the operational amplifier
100
is required to have a very high gain-bandwidth product (GB product) if both the gain and bandwidth necessary for image signal processing are to be satisfied. It is therefore very difficult to practically realize a variable gain amplifier circuit for image signal processing by using CMOS integrated circuit techniques.
As apparent from the circuit shown in
FIG. 11
, if the input voltage Vin is replaced by a reference voltage Vref, this variable gain amplifier circuit is quite the same as a D/A converter which converts a digital signal constituted of bits S
1
to S
8
into an analog signal Vout.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a variable gain amplifier circuit having a wide bandwidth and a high amplification factor.
It is another object of the present invention to provide a variable gain amplifier circuit capable of switching between operating points at high speed and with high stability.
According to one aspect of the present invention, there is provided a variable gain amplifier circuit comprising: voltage-current conversion means for converting an input voltage into a current and outputting the current; current amplifying means for amplifying the current at a set gain; means for converting the amplified current into an output voltage; and operating point setting means for setting an operating point so that the output voltage converges into a predetermined range during a period while a reference voltage is input.
A variable gain amplifier circuit suitable for image signal processing can be obtained. This variable gain amplifier circuit can be easily fabricated with CMOS circuits and satisfies necessary operation bandwidth, maximum gain, and variable gain setting resolution.


REFERENCES:
patent: 5530404 (1996-06-01), Debroux
patent: 5917380 (1999-06-01), Darthenay et al.
patent: 2-34205 (1990-08-01), None

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