Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2002-03-21
2003-11-04
Shingleton, Michael B. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S288000
Reexamination Certificate
active
06642792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a signal amplification device and, more particularly, relates to an amplification device which can be driven at a low voltage.
2. Description of the Related Art
A conventional amplifier may have a circuit constitution such as shown in FIG.
1
. In such an amplifier, a charge signal obtained by a CCD (Charge Coupled Device) image sensor is converted into a voltage signal through a capacitance in a floating diffusion layer. The voltage signal is amplified by a source follower amplifier on a sensor chip and is then output from the sensor. A reset level of the floating diffusion layer periodically appears in output signals from the sensor. Accordingly, a correlated double sampling (CDS) circuit extracts the portion corresponding to a pixel signal among the output signals. The output signal extracted by the CDS circuit is further amplified by an amplifier using a differential amplifier circuit. In other words, such an amplifier includes a differential amplifier circuit
2
using a current mirror as a load. A reference voltage V
STD
is supplied to the gate of an MOS transistor MN
02
as one input terminal. A voltage signal V
SIG
as a CCD output signal is supplied to the gate of an MOS transistor MN
01
as the other input terminal. The differential amplifier circuit
2
generates an output signal according to a difference between the two supplied voltages. Because the differential amplifier circuit
2
uses a current mirror circuit as a load, the circuit
2
functions as a transconductance type amplifier. That is, a voltage signal is supplied as an input signal but a current signal is obtained as an output signal. The generated current signal is converted into a voltage signal V
OUT
by a resistor R
L
coupled to an output terminal
4
of the differential amplifier circuit
2
. The operating point of the signal V
OUT
is determined in accordance with a constant current which is supplied from a transistor MN
04
(gate bias voltage V
B2
) as a constant-current source to the resistor R
L
.
In the conventional amplifier shown in
FIG. 1
, the potential of the output terminal
4
of the differential amplifier circuit
2
, namely, a node between the drain of the transistor MN
02
and the drain of a transistor MP
02
is equal to that of the voltage signal V
OUT
. The potential is varied in accordance with the signal V
OUT
. In the differential amplifier circuit
2
, however, a plurality of transistors are arranged in series between a power supply voltage V
DD
and ground potential GND. Specifically, three MOS transistors of the transistor MP
02
of the current mirror circuit, the transistor MN
02
of the differential amplifier circuit, and a transistor MN
03
(gate bias voltage V
B1
) as a constant-current source are connected in series. To operate, each transistor requires a predetermined potential difference between the source and the drain. The remainder obtained by subtracting the potential difference from the V
DD
-GND voltage is a voltage fluctuation allowance &Dgr;V
OUT
which is allowable at the output terminal
4
. In the example conventional amplifier, the voltage V
DD
is set to a relatively large value, for example, +5V. In accordance therewith, the voltage fluctuation allowance &Dgr;V
OUT
is held enough.
In order to realize a reduction in the size of various portable devices, it is preferable that batteries of smaller size are used. Accordingly, power consumption by devices has been reduced so that the devices can operate for a sufficient time even when equipped with a small battery having a small capacity. In a digital still camera or video camera, such reduced power consumption and size reduction are desired. Meanwhile, as communication capacity has increased, it has become possible for portable telephones and portable terminals to receive and transmit images. As a result, there is a growing demand for mounting a photographing function on such devices. Because the portable telephones is small, a reduction in power consumption is further required in association with the small size. As one way of reducing power consumption, the CCD output signal processing circuit can be driven at a low voltage, namely, the power supply voltage V
DD
is reduced. When this is done, however, it is difficult to reduce the potential difference between the source and the drain of a transistor. The amount of reduction in the voltage V
DD
is fundamentally passed to the allowance &Dgr;V
OUT
. Conversely, such a conventional amplifier has certain problems including that it is difficult to improve low-voltage driving while maintaining the amplitude of an output voltage signal and that power consumption cannot always be reduced as desired.
SUMMARY OF THE INVENTION
The present invention is made to solve the above problems. It is an object of the present invention to provide a signal amplification device in which low-voltage drive is realized while maintaining the amplitude of an output voltage signal, such that power consumption is reduced.
According to the present invention, there is provided a signal amplification device including a differential amplifier circuit which generates a first output according to an input signal and a current mirror circuit which receives the first output of the differential amplifier circuit to an input-side current path to generate a mirror current through an output-side current path in accordance with a current flowing through the input-side current path, wherein the mirror current is converted into an output voltage by a resistive load in the output-side current path of the current mirror circuit.
According to the present invention, the first output generated from the differential amplifier circuit is not converted into a voltage signal in principle at an output terminal of the differential amplifier circuit but is normally generated as a current signal, so that any fluctuation in potential at the output terminal is suppressed. Accordingly, the first output generated from the differential amplifier circuit is supplied to the input-side current path of the current mirror circuit. The current mirror circuit generates a mirror current through the output-side current path in accordance with the input-side current flowing through the input-side current path. In other words, the mirror current also fluctuates in accordance with the current fluctuation of the first output from the differential amplifier circuit in the input-side current path. As noted above, the current signal is transmitted to the output-side current path of the current mirror circuit. To suppress the potential fluctuation at the output terminal of the differential amplifier circuit, fundamentally, a resistor is not disposed in the path through which the current signal flows, in the input-side current path. Rather, a resistor is provided as a load in the output-side current path. A fluctuation in mirror current signal is converted into a voltage fluctuation across the resistor. The voltage fluctuation is extracted as an output voltage signal of the present voltage signal amplification device. A driving voltage for the differential amplifier circuit and the input-side current path can be reduced as much as the suppressed amount of voltage fluctuation at the output terminal of the differential amplifier circuit. Only a single transistor in the output-side current path is required for controlling a mirror current signal in accordance with a signal from the input-side current path. On the other hand, a plurality of transistors may be serially connected between power supply potential and ground potential in the differential amplifier circuit or the input-side current path. That is, the amplitude of the output voltage signal generated in the output-side current path can be large because there are fewer transistors in the output-side current path than in the differential amplifier circuit or the input-side current path. Accordingly, the driving voltage in the output-side current path can be reduced while the amplitude of the
Takahashi Tatsuya
Takeuchi Seiji
Hogan & Hartson LLP
Sanyo Electric Co,. Ltd.
Shingleton Michael B.
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