Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2000-12-21
2002-04-09
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S118000, C341S119000, C341S120000, C341S127000, C341S136000, C341S155000, C341S156000, C341S158000, C341S160000, C341S166000
Reexamination Certificate
active
06369743
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an A/D converter that operates at high speed necessary for digitizing a reproduction signal of a hard disk, a comparator that accomplishes such an A/D converter, and a differential amplifier that accomplishes such a comparator.
2. Description of the Related Art
As the speed of a signal process increases, a high speed A/D converter is desired. For example, a hard disk drive has an A/D converter that digitizes a reproduction signal of a head is disposed so as to perform an equalizing process and a Viterbi decoding process. As the speed of a hard disk drive increases, an A/D converter having 6 to 8 quantizing bits having a sampling clock signal frequency of several 100 MHz (for example, 400 MHz) is desired.
An A/D converter compares an input voltage with a reference voltage and encodes the compared voltage so as to convert an analog signal into a digital signal. As was described above, to accomplish an A/D converter that operates at high speed, a comparator should be composed of a differential amplifier having a high gain and a wide frequency band.
Conventionally, an analog circuit that operates at high speed is composed of bipolar transistors. Thus, a differential amplifier having a high gain and a wide frequency band may be composed of bipolar transistors.
However, the power consumption of a bipolar transistor is large. In addition, a bipolar transistor cannot be integrated with another signal processing circuit as an integrated circuit. Thus, it is strongly desired to accomplish a differential amplifier having a high gain and a wide frequency band with CMOS transistors.
Parameters that allow a differential amplifier composed of CMOS transistors to have a high gain and a wide frequency band are the current and the size because gm (mutual conductance) of an MOS transistor depends on the current that flows therein and the size thereof. Thus, to accomplish a differential amplifier having a high gain, it is necessary to set a high current value or increase the size of each MOS transistor.
When the current that flows in a MOS transistor is increased, the power consumption is adversely increased. When the size of a MOS transistor is increased, the parasitic capacitance is increased. Thus, a wide frequency band cannot be accomplished.
In a differential circuit composed of bipolar transistors, a compensating circuit is disposed. The compensating circuit generates a compensation current that flows in the reverse direction of a current that flows in a capacitance C
BC
formed between the base and the collector of each bipolar transistor. The compensation current cancels a current that flows between the base and the collector of the bipolar transistor. As a result, the problem of the band limitation due to the parasitic capacitance can be solved. Thus, a differential amplifier having a wide frequency band is accomplished. Such a technique is proposed in “A Low-Power Wide-Band Amplifier Using a New Parasitic Capacitance Compensation Technique”, IEEE Journal of Solid-State Circuit, Vol. 121, No. 1, February 1990.
When a differential amplifier having a high gain and a wide frequency band is accomplished using CMOS transistors, such a technique may be used. As was described above, when the size of each MOS transistor is increased, a high gain can be obtained. In that case, the parasitic capacitance is increased. When the technique for canceling the current that flows in the parasitic capacitance with the compensation current is applied to a CMOS structure, a CMOS differential amplifier having a high gain and a wide frequency band is accomplished. Thus, using such a differential amplifier, a high speed A/D converter can be accomplished.
As shown in
FIG. 1
, when an amplifier is considered as a model of which a network of a resistor R and a capacitor C is driven by a signal source V
i
having a signal source resistor R
S
, the following formula can be obtained.
G
0
=
R
s
//
R
R
S
(
1
)
f
3
dB
=
1
2
piC
(
R
//
R
S
)
(
2
)
G
0
B
=
G
0
f
3
dB
=
1
2
PiCR
S
(
3
)
where G
0
is a DC gain; f
3 dB
is a frequency band that lowers by 3 dB; pi is &dgr; (ratio of circumference of circle to its diameter); and G
0
B is a gain bandwidth.
As expressed in Formula (3), the frequency band depends on the capacitance C and the resistance R
S
of the signal source. In the case of a bipolar transistor, the capacitance C that limits the frequency band is equivalent to the capacitance C
BC
formed between the base and the collector of the bipolar transistor. Since the capacitance C
BC
formed between the base and the collector of the bipolar transistor is amplified by the mirror effect. Thus, the capacitance C
BC
largely affects the decrease of the frequency band.
As shown in
FIG. 2
, to solve such a problem, a current source sC
C
V
O
(where s: Laplace operator) that varies corresponding to the output voltage V
O
is disposed on the output side. The current sC
C
V
O
cancels the current that flows in the capacitance C. In that case, the following formulas can be obtained.
G
0
=
R
s
//
R
R
S
(
4
)
f
3
dB
=
1
2
Pi
(
C
-
C
c
)
(
R
//
R
s
)
(
5
)
G
0
B
=
G
0
f
3
dB
=
1
2
Pi
(
C
-
C
c
)
R
s
(
6
)
Assuming that C=Cs, the denominator becomes 0. Thus, it is clear that the frequency band is not limited.
FIG. 3
shows an example of the structure of a differential amplifier using bipolar transistors, each of which having a current that flows in a capacitance formed between the base and the collector that is canceled with a compensation current corresponding to an output voltage so as to widen the frequency band.
In
FIG. 3
, the emitters of NPN transistors
201
and
202
are connected. The emitters of the transistors
201
and
202
are connected to a ground line
204
through a current source
203
. Input terminals
221
and
222
are connected to the bases of the transistors
201
and
202
.
The collectors of the transistors
201
and
202
are connected to a power line
207
through resistors
205
and
206
, respectively. In addition, the collectors of the transistors
201
and
202
are connected to the bases of transistors
208
and
209
, respectively. The collectors of the transistors
208
and
209
are connected to the power line
207
. The emitters of transistors
208
and
209
are connected to the ground line
204
through current sources
210
and
211
, respectively. In addition, the emitters of the transistors
208
and
209
are connected to output terminals
223
and
224
, respectively.
In addition, the emitters of the transistors
208
and
209
are connected to the bases of transistors
212
and
213
, respectively. The collectors of the transistors
212
and
213
are connected to the collectors of the transistors
202
and
201
, respectively. The emitters of the transistors
212
and
213
are connected to the ground line
204
through current sources
214
and
215
, respectively. In addition, a capacitor
216
is connected between the emitter of the transistor
212
and the emitter of the transistor
213
.
In
FIG. 3
, a difference input voltage that is input from the input terminals
221
and
222
is amplified by the transistors
201
and
202
. The amplified voltage is output from the output terminals
223
and
224
through the emitter follower transistors
208
and
209
, respectively.
In addition, the output voltage takes place between the emitters of the transistors
212
and
213
through an emitter follower circuit composed of the transistors
212
and
213
. A current corresponding to the output voltage flows in the capacitor
216
connected between the emitters of the transistors
212
and
213
.
As shown in
FIG. 4
, when the capacitor
216
is composed of transistors
231
and
232
that are similar to the transistors
201
and
202
that compose the differential pair, the capacitance C
C
of the capacitor
216
becomes almost the same as the capacitance C
CB
between the base and the collector of each of t
Kananen, Esq. Ronald P.
Mai Lam T.
Rader & Fishman & Grauer, PLLC
Sony Corporation
Tokar Michael
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