Differential amplification circuit

Details Differential amplification circuit Differential amplification circuit

1999-07-09

2001-10-16

Shingleton, Michael B (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C327S307000, C327S011000, C327S069000, C327S302000

Reexamination Certificate

active

06304144

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to input offset compensation in a DC (Direct Current) feedback differential amplification circuit.
2. Description of the Related Art
Conventionally, a DC feedback circuit used to stabilize a DC component of an input voltage is utilized with a single input.
FIG. 1
exemplifies the configuration of a single-input differential amplification feedback circuit.
A DC component is removed by an operation of a capacitor
60
from a signal from an input terminal
68
, and only an AC (Alternating Current) component is extracted. A DC component generated by a voltage drop caused by resistors
61
and
62
is added to the signal which includes only the AC component, and the resultant signal is input to one of input terminals of a differential amplifier
63
. A normal signal obtained by amplifying the above described signal and its inverted signal are output from a differential amplifier
63
. Here, a feedback path is formed in order to stabilize the DC components. That is, in the configuration shown in
FIG. 1
, the normal output and the inverted output of the differential amplifier
63
are input to a low-pass filter
64
. After their DC components are extracted, these signals are input to a differential amplifier
65
. The differential amplifier
65
amplifies and outputs a difference between the DC component of the normal output of the differential amplifier
63
and that of the inverted output, and inputs the difference to the other of the input terminals of the differential amplifier
63
. By arranging the path for feeding back only the DC component, the DC components of the signals output from output terminals
66
and
67
are stabilized and output.
However, in the above described single-input amplification circuit, unless the gain of the differential amplifier
63
are sufficiently increased for a predetermined input voltage from the input terminal
68
, it is insufficient for amplifying, for example, a signal which is much attenuated within an SAW filter and output therefrom. Accordingly, a normal signal and its inverted signal are considered to be input to the differential amplifier
63
. In this way, the amplitude of a signal can be substantially doubled, and at the same time, an S/N ratio can be improved on the condition that the level of noise is approximate to that in the case of a single input.
FIGS. 2A and 2B
exemplify the configurations of a conventional DC feedback differential amplification circuit when a differential input is made.
Considered as the DC feedback configuration for compensating for the offset between DC components when the differential input is made is a configuration in which a low-pass filter
80
having a high-frequency cut-off frequency “fc1” in order to feed back a DC component to the input is added as shown in
FIG. 2A
or
2
B.
In
FIG. 2A
, a normal signal of a signal to be input is input from an input terminal
71
, while an inverted signal of the signal to be input is input from an input terminal
72
. A DC component is removed by a capacitor
73
from the signal from the input terminal
71
. The DC component set by resistors
75
and
76
is newly added to this signal, which is input to one of terminals of a differential amplifier
79
. A DC component is removed by a capacitor
74
also from the inverted signal from the input terminal
72
, and a DC component set by the resistors
77
and
78
is newly added. This signal then is input to the other of the terminals of the differential amplifier
79
. The differential amplifier
79
outputs to an output terminal
82
a normal signal obtained by amplifying the difference between the inputs from the two terminals, and outputs the inverted signal of the amplified signal to an output terminal
83
. The signals to be output to the output terminals
82
and
83
are input to a low-pass filter
80
via feedback paths. The low-pass filter extracts DC components, and the signals are input to a differential amplifier
81
. The differential amplifier amplifies the difference between the two DC voltage, and feeds back the amplified difference to the input side of the inverted signal of the differential amplifier
79
. In such a configuration, if there is a difference between the DC components of the signals to be fed to the output terminals
82
and
83
, the difference is amplified by the differential amplifier
81
and again input to the differential amplifier
79
. As a result, the DC components of the signals to be fed to the output terminals
82
and
83
can be matched, thereby compensating for the offset between the DC components.
FIG. 2B
shows a modification of the configuration shown in FIG.
2
A.
In the configuration shown in
FIG. 2A
, the positions at which the low-pass filter
80
and the differential amplifier
81
on the feedback paths are arranged are reversed from those shown in FIG.
2
A. However, their operations are the same as those of the configuration shown in FIG.
2
A.
That is, the signal input from the input terminal
72
is an inversion of the signal from the input terminal
71
. DC components are removed from the signals by the capacitors
73
and
74
from the signals from the input terminals
71
and
72
. The respective DC components set by the resistors
75
and
76
, and
77
and
78
are newly added, and the signals are input to the differential amplifier
79
. The normal and the inverted outputs of the differential amplifier
79
are fed to the differential amplifier
81
via feedback paths, and the difference between the normal and the inverted outputs is amplified, and the amplified difference is input to the low-pass filter
80
. The low-pass filter
80
extracts only a DC component, and feeds back the extracted component to the differential amplifier
79
. Also in this case, if the DC components of the outputs of the output terminals
82
and
83
are large, a corresponding DC voltage is input from the low-pass filter
80
to the differential amplifier
79
, so that the difference between the DC components of the normal and the inverted outputs of the differential amplifier
79
, is eliminated. Since the offset between the DC components is cancelled and output as described above, the normal and the inverted signals whose DC component values are matched are output.
In the conventional circuitry shown in
FIG. 2A
or
2
B, a high-frequency cut-off frequency “fc1” of the low-pass filter arranged for removing only a high frequency component and feeding back only a DC component exists within a DC feedback loop. In addition, a high-frequency cut-off frequency “fc2” is caused within a DC feedback loop by the output impedance of the capacitor
74
which is intended to remove a DC component of an input signal and to input only a high frequency component, and the differential amplifier
81
. The high-frequency cut-off frequency “fc1” must be sufficiently low for the frequency of the signal so as to perform a DC feedback operation. Also the high-frequency cut-off frequency “fc2” must be low so as to increase the value of the capacitor
74
in consideration of the same code succession of the signal. Furthermore, the gain of the DC feedback loop must be increased so as to reduce the compression residual of the offset. In this case, a sufficient phase margin cannot be secured for the gain of the feedback loop, which leads to oscillation of the feedback loop.
FIG. 3
explains a phase margin in the circuitry shown in
FIGS. 2A and 2B
.
As shown in the upper stage, the loop gain decreases as the frequency increases. Especially, the loop gain significantly decreases at the high-frequency cut-off frequencies “fc1” and “fc2”. The loop gain becomes smaller than 0 dB at a certain frequency. The phase of the loop gain starts to rotate at the high-frequency cut-off frequencies “fc1” and “fc2”. At the high-frequency cut-off frequency “fc1”, the phase shifts from 180° to 90°. When the phase exceeds the high-frequency cut-off frequency “fc2”, the phase starts to decrease from 90° and finally reaches 0°. For the

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