Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
2000-02-29
2001-05-01
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S072000, C327S309000
Reexamination Certificate
active
06225835
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an amplifier, and particularly relates to an amplifier which can substantially avoid a duty-ratio error.
2. Description of the Related Art
As technology makes further progress in size reduction and lowering of power consumption of communication equipment, there is an increasing demand for circuits used in communication equipment to operate with low voltages. An IF (intermediate frequency) amplifier unit, which is generally provided at a stage prior to an FM demodulation unit, is required to provide a sufficient gain and a small waveform distortion under low-voltage conditions.
FIG. 1
is a circuit diagram of a related-art IF amplifier unit.
The IF amplifier unit of
FIG. 1
includes amplifiers A
1
and A
2
, a comparator C
1
, and resistors R
1
through R
5
.
The amplifier A
1
receives an input signal via resistor R
1
at one input node when the input signal is supplied to an input terminal, and receives a reference voltage VSG at the other input node. The amplifier A
1
amplifies the input signal with reference to the reference voltage VSG. The input node of the amplifier A
1
that is connected to the resistor R
1
is connected to an output node of the amplifier A
1
via the resistor R
2
.
The signal amplified by the amplifier A
1
takes two different routes to be input to the amplifier A
2
, one being through the resistor R
3
and the other being through the resistor R
4
. The amplifier A
2
serves to cancel an offset of the input signal relative to the reference voltage VSG. The input node of the amplifier A
2
that is connected to the resistor R
3
is connected to the output node of the amplifier A
2
via the resistor R
5
. The other input node of the amplifier A
2
that is connected to the resistor R
4
is connected to the ground via a capacitor.
A signal output from the amplifier A
2
is supplied to the comparator C
1
. The comparator C
1
receives the reference voltage VSG in addition to the signal from the amplifier A
2
, and compares the signal with the reference voltage VSG to output a result of the comparison at its output node.
The IF amplifier unit of
FIG. 1
exhibits problems when the input signal supplied to the input terminal exceeds a certain threshold voltage level.
FIGS. 2A through 2C
are illustrative drawings of waveforms for explaining problems that arise when an input signal exceeds a threshold voltage level.
FIG. 2A
shows an input signal together with a power voltage level and a ground voltage level where the input signal is supplied to the input terminal. As long as the input signal stays below a threshold voltage level, no problem is observed.
When the input signal exceeds the threshold voltage level, however, a signal output from the amplifier A
1
behaves as shown in FIG.
2
B. As shown in
FIG. 2B
, the output signal of the amplifier A
1
has an average voltage level V
2
that is different from the reference voltage VSG. This is because clamping occurs when the input signal to the input terminal exceeds the threshold voltage level. When the signal as shown in
FIG. 2B
is supplied to the amplifier A
2
, an output signal of the amplifier A
2
behaves as shown in FIG.
2
C.
Operating with reference to the reference voltage VSG, the comparator C
1
produces an output signal that suffers a phase displacement amounting to a temporal displacement dT in comparison with an output signal that would be obtained when a difference between the average voltage level V
2
and the reference voltage VSG is insignificant. This phase displacement leads to an error in a duty ratio.
Accordingly, there is a need for an IF amplifier which can reduce a difference between a reference voltage and an average voltage so as to suppress an error of a duty ratio.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide an amplifier which can satisfy the need described above.
It is another and more specific object of the present invention to provide an amplifier which can reduce a difference between a reference voltage and an average voltage so as to suppress an error of a duty ratio.
In order to achieve the above objects according to the present invention, a device for amplifying a signal includes a first amplifier which amplifies an input signal with reference to a reference voltage level to generate an amplified signal, a limiter circuit which limits an upper end of the amplified signal to a level twice as high as the reference voltage level so as to output a limited amplified signal, a second amplifier which amplifies the limited amplified signal with reference to an average of the limited amplified signal, and a comparator which compares an output of said second amplifier with the reference voltage level to output a result of the comparison.
In the amplifying device described above, the limiter circuit is situated between the first amplifier and the second amplifier so as to limit the amplified signal of the first amplifier to the level twice as high as the reference voltage level. This makes it possible to make an average voltage level of the signal input to the second amplifier substantially equal to the reference voltage level. That is, the limiter circuit serves to reduce a difference between the average voltage level and the reference voltage level. Such a reduction is useful in the communication technology since it prevents a displacement of signal phase.
According to one aspect of the present invention, the device as described above is such that the reference voltage level is lower than half of a power voltage level which drives the device.
In the configuration described above, use of the reference voltage level that is lower than half the power voltage level makes it possible to insure reliable operation even when the power voltage is lowered.
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: 4782290 (1988-11-01), Sakai et al.
patent: 5012207 (1991-04-01), Edwards
patent: 5408529 (1995-04-01), Greaves
patent: 6-29796 (1994-02-01), None
patent: 6-310967 (1994-11-01), None
Arent Fox Kintner & Plotkin & Kahn, PLLC
Fujitsu Limited
Lam Tuan T.
Nguyen Hiep
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