Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
1999-09-23
2001-08-14
Ton, My-Trang Nu (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C330S253000
Reexamination Certificate
active
06275073
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a differential input circuit, and particularly to a differential input circuit having CMOS circuits for high-speed signal transmission.
2. Description of the Related Art
Small-amplitude signal transmission has been widely used in CMOS circuits with an increase in a speed of signal transmission. The small-amplitude signal transmission is a technique which transmits signals so that the transmitted signals are not saturated, that is, switching transistors are not saturated. In the small-amplitude signal transmission, since transmitted signals are easily affected by a noise, differential signals are generally used.
In an interface system called LVDS (Low Voltage Differential Signals), small-amplitude signals having an amplitude of 100 mV at minimum are inputted and a logic level of the inputted signals must be determined. Additionally, the inputted signals have a wide voltage range ranging from 0 V to 2.4 V.
FIG.1
shows an example of a conventional differential input circuit. Transistors Tr
11
and Tr
12
constitute a current mirror circuit with their gates being connected to each other, and supply currents having the same amplitude to transistors Tr
13
and Tr
14
. Precisely, the amplitudes of the currents supplied to the transistors Tr
13
and Tr
14
are proportional to widths of gates of the transistors Tr
11
and Tr
12
, respectively. Thus, if the transistors Tr
11
and Tr
12
are designed to have the same physical dimension, currents having the same amplitude are supplied to the transistors Tr
13
and Tr
14
.
An output voltage V
out
at an output terminal Out is changed when ON-state resistances between a drain and a source of the transistors Tr
13
and Tr
14
are changed in accordance with gate voltages of the transistors Tr
13
and Tr
14
, which gate voltages correspond to input voltages of the circuit. When an input voltage V
P
at a P
−in
terminal connected to a gate of the transistor Tr
13
is larger than an input voltage V
N
at an N
−in
terminal connected to a gate of the transistor Tr
14
, the output voltage V
out
is of a high level, and when V
P
is smaller than V
N
, the output voltage V
out
is of a low level. A transistor
15
which is connected to sources of the transistors Tr
13
and Tr
14
functions to define a total current flowing through the circuit. That is, the transistor
15
functions as a constant current source.
However, the above-mentioned conventional circuit can operate only when a common voltage (a center value of differential input signals) is within a limited range.
FIG. 2
is a diagram for illustrating the common voltage. The common voltage is defined as a center value V
ic
of the positive input voltage V
P
and the negative input voltage V
N
of a differential input circuit U.
For example, when both the input voltages V
P
and V
N
are so high that the transistor Tr
15
is saturated, both the transistors Tr
13
, Tr
14
are turned on. In this case, the output voltage V
out
is fixed to be of the low level. On the other hand, if both the input voltages V
P
and V
N
are lower than a threshold voltage of the transistors Tr
13
and Tr
14
, both the transistors Tr
13
and Tr
14
are turned off. In this case, the output voltage V
out
is indefinite.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a differential input circuit which can positively operate over a wide range of the common voltage.
FIG. 3
is a diagram showing a principle of the present invention. As shown in
FIG. 3
, a current switch
20
is connected to a positive signal input P
−in
and a negative signal input N
−in
. A first constant current source
21
of a current mirror type generating a positive current and a second constant current source
22
of a current mirror type generating a negative current are connected symmetrically to each other to constitute a differential amplifier circuit. The current switch
20
is connected to the first and second constant current sources so that an operating point of the differential amplifier circuit can be changed.
According to the invention, since the current switch
20
is provided so that the operating point of the differential amplifier circuit can be changed, as mentioned above, the differential input circuit can positively operate over a wide range of the common voltage.
The current switch may comprise a first switching part which controls a current derived from said first constant current source and a second switch part which controls a current derived from the second constant current source.
In this case, each of the first and second switch parts may comprise a first switch element connected to the positive input and a second switch element connected to the negative input. Further, the first and second switch parts may be constructed so that the first switch elements of said first and second switch parts are complementary to each other and the second switch elements of said first and second switch parts are complementary to each other.
In this invention, two complementary switch elements are connected to each of the positive input P
−in
and the negative input N
−in
. Here, the term “complementary switch elements” means that whenever one of the switch elements is not operating, the other switch element is operating. Thus, according to the invention, when the common voltage is high, at least one of the complementary switch elements can operate, and when the common voltage is low, at least the other switch element can operate. Thus, the differential input circuit can operate over a wide range of the common voltage.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.
REFERENCES:
patent: 4047059 (1977-09-01), Rosenthal
patent: 5036219 (1991-07-01), Dingwall et al.
patent: 5477170 (1995-12-01), Yotsuyanagi
patent: 5729178 (1998-03-01), Park et al.
patent: 5754076 (1998-05-01), Kimura
patent: 5844433 (1998-12-01), Nishimura
patent: 5990741 (1999-11-01), Yamamoto et al.
patent: 5994962 (1999-11-01), Watanabe et al.
Armstrong Westerman Hattori McLeland & Naughton LLP
Fujitsu Limited
Nu Ton My-Trang
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