Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Exponential
Reexamination Certificate
2001-09-13
2004-08-17
Nguyen, Long (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Exponential
C327S359000, C327S563000
Reexamination Certificate
active
06777999
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-280829, filed Sep. 14, 2000, the entire contents of which are incorporated herein by reference,
BACKGROUND
The present invention relates to an exponential conversion circuit having a function of exponentially changing a gain of a variable gain amplifier on the basis of a gain control signal.
In recent years, mobile communication devices represented by a portable phone or the like are briskly developed It is important that these mobile communication devices are small in size and light in weight because these devices are based on a presupposition that the devices can be carried by a user.
Consequently, at present, it rarely happens that these mobile communication devices comprise a combination of a plurality of individual components (functions). The mobile communication devices comprise ASIC's mixedly providing the plurality of functions. As a consequence, size reduction and weight reduction of the mobile communication devices are realized.
By the way, such mobile communication devices naturally have a sending and receiving circuit for sending and receiving electronic waves for the exchange of information by wire (electric waves). In the IF (intermediate frequency) portion of the sending and receiving circuit, a variable gain amplifier is arranged, and this variable gain amplifier has a function of adjusting an IF signal to an appropriate level.
For example, there is available a code division multiple access (CDMA) method as one of the mobile communication methods. In the CDMA method, the control of the sent electric power in a mobile station becomes indispensable, a wide scope gain control of 70 dB or more is demanded for the variable gain amplifier used in the IF portion.
Generally, in order to perform such wide scope gain control with the variable gain amplifier, it is necessary to exponentially adjust a signal level with respect to the gain control signal, Furthermore, in order to facilitate the gain control, it becomes important that the relation between the control input signal and the decibel display output signal has a linear configuration over a wide scope.
Furthermore, portable phones are based on a presupposition that the phones are carried by a user. Consequently, it is desired that the gain of the variable gain amplifier used therein has a small dependency on a temperature change resulting from a change in the environment in which the phones are used. Furthermore, a gain error must be suppressed which is caused by a disparity in a threshold value of a MOS transistor resulting from a manufacturing process of an integrated circuit.
However, for example, the constant maintenance of the characteristic for exponentially changing the gain of the variable gain amplifier with respect to the gain control signal, and the change of the decibel display output signal with respect to the control input signal become very difficult for the following reasons.
In the beginning, the variable gain amplifier will be explained.
As shown in
FIG. 1
, a variable gain amplifier and a gain control circuit thereof comprise a MOS transistor (CMOS circuit).
Here, the MOS transistor is generally used in the double characteristic area (strong inversion area) but can be used in a sub threshold area (weak inversion area). In this case, an exponential operation is conducted and the transmission characteristic can be approximately described in the following manner.
I
D
=
kx
W
L
exp
V
GS
nV
T
(
1
)
Incidentally, in the expression (1), symbol I
p
denotes a drain current of a MOS transistor, symbol W denotes a channel width of the MOS transistor, symbol L denotes a channel length of the MOS transistor, symbol V
GS
denotes a voltage between the gate and the source of the MOS transistor, and symbol V
T
denotes a thermal voltage. Symbol n is a constant. Furthermore, symbol K
x
has a value associated with a conductance of the MOS transistor. Symbol Kx depends on the manufacturing process of the integrated circuit together with the constant n.
By the way, in
FIG. 1
, a variable gain amplifier
702
can vary the gain with a bias current Ibias. Furthermore, the bias current Ibias becomes equal to a drain current I
D
of a MOS transistor M
701
with the current mirror circuits M
702
and M
703
inside of a gain control circuit
701
.
On the other, when the MOS transistor M
701
in the variable gain control circuit
701
is allowed to be operated in a weak inversion area to give a gain control signal Vc to a gate of the MOS transistor M
701
, the drain current I
D
of the MOS transistor M
701
changes exponentially with the change in the gain control circuit Vc.
That is, as a consequence, the gain of the variable gain amplifier
702
changes exponentially with the change of the gain control signal Vc.
However, the following problem is generated in order to directly use the characteristic of the expression (1) in the circuit of FIG.
1
.
That is, when logarithm on both sides of the equation (1) is taken, the following equation is provided.
log
I
D
=
log
kx
+
log
W
L
+
(
1
nV
T
)
V
GS
(
2
)
Here, as described above, in expression (2), symbol K
x
is affected by the influence of the manufacturing process of the integrated circuit, the transmission characteristic (expression (2)) of the MOS transistor, namely, the exponential conversion characteristic changes with the manufacturing process, specifically, a disparity in the thickness and the processing generated at the time of the manufacturing process.
Furthermore, a third item on the right side of the expression (2) determines the exponential conversion characteristic (characteristic of the exponential correlation). However, with respect to a heat voltage V
T
, in order to maintain the temperature dependency, the exponential conversion characteristic also changes depending on the temperature change in the case where a temperature change is generated in the MOS transistors M
701
, M
702
and M
703
in the gain control circuit. As a consequence, the variable scope (gain characteristic) of the gain of the variable gain amplifier
702
changes.
Incidentally, in the gain control circuit
701
of
FIG. 1
, even when a bipolar transistor is used instead of the MOS transistor
701
, the exponential conversion characteristic (characteristic of the exponential correlation) comes to have a temperature dependency for the same reasons as described above.
Consequently, in the case where the exponential conversion characteristic of an active device is directly used in the variable gain amplifier
702
, an error is generated in the exponential conversion characteristic resulting from a change in the environment (temperature change) and a manufacturing process of the integrated circuit so that a desired exponential conversion characteristic cannot be obtained.
Furthermore, with respect to the variable gain amplifier
702
used in a wireless receiver, it is important to linearly change a decibel display output signal with respect to the control input signal. Furthermore, it is required that the temperature dependency of the exponential conversion circuit is small, and the exponential conversion characteristic is not affected by the characteristic change of the active device resulting from the manufacturing process of the integrated circuit.
In this manner, in the case where exponential operation (characteristic) of the active device (MOS transistor) is directly used in the control of the variable gain amplifier
702
, the variable gain amplifier
702
cannot maintain at a constant level a variable scope (gain characteristic) of the gain of the variable gain amplifier
702
with respect to the variable scope of the gain control signal Vc because of the change of the temperature environment of the active device, the characteristic change of the active device due to the manufacturing process of the integrated circuit, or the like.
From this, it
Itakura Tetsuro
Kanou Nobuo
Yamaji Takafumi
Gray Cary Ware & Freidenrich LLP
Nguyen Long
LandOfFree
Exponential conversion circuit and variable gain circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Exponential conversion circuit and variable gain circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Exponential conversion circuit and variable gain circuit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3333132