Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1999-10-28
2001-06-26
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S253000
Reexamination Certificate
active
06252458
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a differential amplifier, and in particular to a differential amplifier having a resistance load type differential pair in an integrated circuit.
The differential amplifier is utilized in a large number of electric circuits regardless of technical fields, and many of these electric circuits are utilized in an integrated form such as an IC or LSI.
As for circuit parts such as transistors, only discrete parts which had fulfilled the specification have been shipped. Therefore, circuit designs for the discrete parts have been made possible without much consideration of manufacturing process conditions because of little individual differences between the parts. Only consideration focused on circumferential variations such as temperature in use had to be made. However, as demands for speedup and space saving increase in the market, the integration of the circuits has been urged.
Integrated circuit designs, considering a manufacturing yield, require circuit designs allowing individual differences between lots depending on manufacturing process conditions.
Furthermore, minute machining for IC's (or LSI's) has been energetically advanced in recent years for the purpose of the speedup and cost reduction. Along with this, source voltages of electric circuits tend to decrease. This is because a malfunction caused by an occurrence of a leak current or the like due to an insulator destruction with a high voltage is prevented since the thickness of an oxide film of a gate or the like is thinned by the minute machining, taking a CMOS transistor as an example.
Therefore, a direct current design at a low voltage considering variations of circumferential and manufacturing process conditions has become more and more difficult. Particularly, it is required for a differential amplifier in many cases, especially in such a case as performing a feed forward control, that the gain variation is small against the variations of the circumferential and manufacturing process conditions.
2. Description of the Related Art
For a bias current of a differential amplifier, a current source (constant current source) {circle around (
1
)} as shown in
FIG. 8
for suppressing a current amount variation caused by variations of circumferential conditions, a current source {circle around (
2
)} as shown in
FIG. 9
for suppressing an internal resistance variation in an amplifier having a resistance load type differential pair and suppressing a limiter amplitude variation, or. a current source {circle around (
3
)} as shown in
FIG. 10
for suppressing a small signal gain variation in an amplifier having a resistance load type differential pair has been so far used individually according to the object of use. Hereinafter, the respective operations will be described taking a CMOS transistor as an example.
Firstly, the current source (constant current source) {circle around (
1
)} shown in
FIG. 8
will be described. It is often the case that an integrated circuit IC
1
internally has a reference constant voltage source S
1
utilizing PN semiconductor junctions with an output voltage V
BGR
to the extent of 1.2V which is called a band gap reference (BGR), having an extremely little variation dependency on the variations of the circumferential and manufacturing process conditions.
This reference voltage V
BGR
is dropped by a voltage divider rl to an allowable input range of an operational amplifier OA
1
to make a reference potential V
ref
thereof. The operational amplifier OA
1
controls a transistor M
11
so that a potential R
c
×I
c
by a current I
c
which flows through a resistor r
2
(resistance R
c
) which is externally connected to the integrated circuit IC
1
is equal to the reference potential V
ref
. Thus, the current source {circle around (
1
)} of
FIG. 8
outputs a current determined by the following equation:
I
c
=V
ref
/R
c
Eq. (1)
It is to be noted that the reference voltage V
ref
has a little dependency on condition variations since it is generated from the voltage V
BGR
which has little dependency on the condition variations by a voltage division ratio of the resistor r
1
, which is constant even if the resistance varies. Also, the reference resistor r
2
has a very little temperature dependency of less than±several 100 ppm/° C. because of an externally connected component.
Since Eq. (1) includes no element depending on the source, no dependency on a source voltage arises as long as the operational amplifier OA
1
or the like has a range of source voltage where a normal operation is enabled. Therefore, a constant current I
c
determined by Eq. (1) which has a very little dependency on the condition variations can be obtained. The current I
c
is reproduced as a current I
d
by a current mirror CM
1
which is composed of transistors M
12
, M
14
, M
13
, and M
15
so as to be provided as a bias current I
s
to a differential pair of an amplifier (not shown) of a resistance load type.
On the other hand, it is generally known that a small signal gain G of an amplifier having a resistance load type differential pair is expressed by the following equation:
G=R×{square root over ( )}&bgr;×{square root over ( )}I
s
Eq. (2)
[R: load resistance, &bgr;: gain coefficient of MOS-FET, I
s
: bias current value]
[&bgr;=&mgr;·C
ox
·W/L (&mgr;: electron mobility, C
ox
: gate oxide film capacity, W: gate width, L: gate length)]
Namely, supposing that an input potential difference of the differential pair is v
in
, an output signal amplitude of v
in
×G is generated between a load resistor (not shown) and the differential pair.
Also, a limiter amplitude V
lm
(V
in
×G≦V
lm
) which is a maximum output amplitude generated between the load resistor and the differential pair is expressed by the following equation:
V
lm
=R×I
s
Eq. (3)
In view of an integrated circuit having a resistance element in the manufacturing process, the load resistance R and the gain coefficient &bgr; vary as the circumferential and manufacturing process conditions vary as given by the following equations:
R=R
typ
(1±&Dgr;
r
) Eq. (4)
&bgr;=&bgr;
typ
(1±&Dgr;
B
) Eq. (5)
where R
typ
and &bgr;
typ
indicate design values in a circumferential condition which is most frequently used and in a manufacturing process condition (typical condition) which is best achieved, and &Dgr;
r
and &Dgr;
B
indicate variation amounts when the values are off the circumferential and manufacturing conditions.
It is to be noted that upon a circuit design, &Dgr;
r
and &Dgr;
B
are preliminary given to each manufacturing process used from conditions such as a working temperature range and a manufacturing yield according to the design specification. In addition, although &Dgr;
r
and &Dgr;
B
have a correlation with the temperature variation in some manufacturing processes, it may be generally considered that they vary independently of the condition variations.
Accordingly, the small signal gain G and the limiter amplitude V
lm
of the amplifier having the resistance load type differential pair given by Eqs. (2) and (3) are given by the following equations in consideration of the condition variations.
G=R
typ
×{square root over ( )}&bgr;
typ
×{square root over ( )}I
s
×(1±&Dgr;
r
)×(1±&Dgr;
B
)
0.5
Eq. (6)
V
lm
=R
typ
×I
s
×(1±&Dgr;
r
) Eq. (7)
Therefore, when the current I
d
generated by the constant current source {circumflex over (1)} in
FIG. 8
is provided as the bias current I
s
(=constant) to the differential pair, Eqs. (6) and (7) can be respectively rewritten as the following equations:
G=G
typ
×(1±&Dgr;
r
)×(1&Dgr;
B
)
0.5
Eq. (8)
[G
typ
=R
typ
×{square root over ( )}&bgr;
typ
×{square root over ( )}I
s
=constant&rsqb
Choe Henry
Fujitsu Limited
Helfgott & Karas P.C.
Pascal Robert
LandOfFree
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