Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2002-03-08
2004-01-13
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S253000, C330S261000, C327S552000
Reexamination Certificate
active
06677822
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a filter circuit, more particularly relates to an active filter circuit called a transconductor-C (Gm-C), and a transconductor serving as a component of a filter circuit.
2. Description of the Related Art
In an integrated filter circuit including an active filter, for example, a Gm-C filter, it is desirable to enable easy, linear adjustment of a cut-off frequency f
c
while maintaining a Q factor of the filter.
A waveform equalizing technique as represented by partial response-maximum likelihood (PRML) is generally applied to a stored data reproduction system (read channel) for reproducing stored information from an information storage medium such as a magnetic or optical disk. Normally, in a signal waveform reproduced via an optical pickup or a magnetic head from a storage medium, large signal leakage occurs between adjacent bit data; namely, large inter-symbol interface (ISI), such that reproduction of data only by the signal level of the sampling time is difficult. The partial response (PR) technique and other techniques enable high density recording and reproduction combined together with a later stage Viterbi decoding algorithm etc. by permitting ISI for only two to five adjacent sampling times, while eliminating signal leakage in other sampling times.
By taking as an example a magnetic medium wherein a reproduced signal is inherently a differential system, the equalization method used is a differential series such as PR
4
(equalizing a write code 1 to three adjacent sample rows 1, 0, and −
1
), EPR
4
(similarly equalizing it to 1, 1, −1, and −1), and EEPR
4
(similarly equalizing it to 1, 2, 0, −2, and −1). Specifically, a high frequency enhanced analog low pass filter is used as an equalizer. For example, a 7-pole 2-zero filter comprising a Gm-C biquadratic filter is proposed by Geert A. De Veirman and Richard G. Yamasaki in “Design of a Bipolar 10-MHZ Programmable Continuous-Time 0.05° Equiripple Linear Phase Filter”,
IEEE Journal of Solid
-
State Circuits,
vol. 27, no. 3, March 1992. This filter configuration has the linearity of phase characteristic required by a digital read channel; that is, a good, constant group delay characteristic, and is generally used as an analog equalizing filter.
FIG. 5
is a block diagram of the configuration of the filter.
As shown in the figure, the filter comprises cascade-connected biquadratic filters/equalizers
101
(biquad
1
/equalizer),
102
(Biquad
2
), and
103
(Biquad
3
) and low pass filter (LPF)
104
. Note that the biquadratic filter/equalizer
101
has an equalizing function. In the filter configuration shown in
FIG. 5
, a reproduced signal S
in
is controlled in gain, then input to the first stage biquadratic filter/equalizer
101
and there adjusted in high frequency boost and equalized. Then, together with the biquadratic filters/equalizers
102
and
103
and the low pass filter
104
connected thereafter, a phase characteristic having a constant group delay is attained. According to Veirman and Yamasaki, the pole frequencies and Q factors of the filter components are as shown in FIG.
6
.
The pole frequencies in
FIG. 6
are scaled by the cut-off frequency of the equalizing filter. For example, in a read channel having a data rate of 400 Mbps, the cut-off frequency of the equalizing filter becomes about 100 MHZ. As a result, if the cut-off frequency of the equalizing filter is assumed to be 100 MHZ, from
FIG. 6
, for example, the pole frequency, that is, the cut-off frequency, of the third stage biquadratic filter
103
becomes 231.74 MHZ. Note that the combinations of the pole frequencies and Q factors in
FIG. 6
, that is, the pole arrangement, are those of “a linear phase filter having a 0.05° equiripple error” well known in filter design, but the invention is also applicable to other combinations of the pole frequencies and Q factors. The pole arrangement here is just an example.
The reproduced data rate of a disk medium differs by about 2.5 times between its inner track and outer track and is required to be adjustable to an optimal cut-off frequency by an external control means. At this time, all of the filter components, that is, the biquadratic filters and the low pass filter, have to have Q factors held at the values indicated in
FIG. 6
at all times. Further, the ratios of pole frequencies of the biquadratic filters and the low pass filter have to be the ratios indicated in
FIG. 6
regardless of the cut-off frequency of the equalizing filter as a whole. In other words, when adjusting the cut-off frequency of the equalizing filter as a whole in accordance with a change of the reproduced data rate, it is necessary that the component biquadratic filters and the low pass filter be monotonously increased or decreased in pole frequencies while maintaining constant Q factors.
Next, a method of designing the above cut-off frequency and Q factor will be explained by showing an example of the circuits of the components when configuring an equalizing filter by a Gm-C filter.
FIG. 7
shows the basic configuration of the biquadratic filters
102
and
103
, while
FIG. 8
shows a feed forward pulse slimming configuration used in the biquadratic filter/equalizer
101
. Furthermore,
FIG. 9
shows the configuration of a primary low pass filter
104
.
FIG. 7
shows an example of the configuration of a biquadratic filter having a differential configuration. As shown in the figure, two integrators comprised of Gm-C's are connected in cascade, while a negative feedback loop comprised of another Gm cell is connected to an output terminal thereof. Note that in
FIG. 7
, the load capacitance C is expressed as a differential capacitance, but generally
2
C capacitances are connected between positive and negative signal lines and a ground potential. This is done so that capacitance can easily be set considering the amount of parasitic capacitance, and so that a function of phase compensation capacitance can be easily combined in a common-mode feedback loop.
The transfer function of the biquadratic filters
102
and
103
having the configuration shown in FIG.
7
and used as an equalizing filter is given by the formula below:
Vlp
Vi
=
g
m1
g
m3
/
C
2
s
2
+
s
(
g
m2
/
C
)
+
(
g
m1
g
m3
/
C
2
)
(
1
)
Accordingly, the pole frequency &ohgr;
0
and Q (quality factor) of a filter are expressed by the formulas below:
ω
0
=
g
m1
g
m3
C
,
Q
=
g
m1
g
m3
g
m2
(
2
)
FIG. 8
shows an example of the configuration of an equalizing filter comprising an equalizer unit capable of adjusting a high pass boost by a feed forward amplifier K. The transfer function of the equalizing filter is given by the formula below.
Vlp
Vi
=
(
g
m1
g
m3
/
C
2
)
-
K
s
2
s
2
+
s
(
g
m2
/
C
)
+
(
g
m1
g
m3
/
C
2
)
(
3
)
Similarly, the pole frequency &ohgr;
0
and Q of the filter are expressed by the formulas below:
ω
0
=
g
m1
g
m3
C
,
Q
=
g
m1
g
m3
g
m2
(
4
)
Here, the reason for realizing the high pass boost by the biquadratic filter/equalizer
101
is, as will be understood from
FIG. 6
, so that high pass boosting can be attained by a relatively small K. Therefore, realization of high pass boosting is not limited to the biquadratic filter/equalizer
101
and can be attained by other biquadratic filters.
FIG. 9
is an example of the configuration of the low pass filter
104
. As shown in the figure, the transfer function of the filter is given by the formula below:
Vlp
Vi
=
(
g
m
/
C
)
s
+
(
g
m
/
C
)
(
5
)
The pole frequency &ohgr;
0
can be obtained as below.
ω
0
=
g
m
C
(
6
)
Realization of an equalizing filter having a constant group delay characteristic and a variable cut-off frequency is attained by setting the pole frequencies &ohgr;
0
and Q of the biquadratic filters so as to satisfy the ratios of the pole frequencies and Q factors shown in FIG.
6
. It is normally attained by controlling the g
m
of the biquadratic filters. According
Kananen, Esq. Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
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