Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1998-03-16
2001-01-16
Shingleton, Michael B. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S294000, C330S085000, C330S258000
Reexamination Certificate
active
06175276
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to amplifiers in general; and, in particular, to preamplifiers for read heads in magnetic disk drives.
2. Discussion of the Related Art
The preamplifier in this application processes a differential signal from a magneto-resistive read head for a disk drive. In addition to signal amplification, the preamplifier contributes noise. If the preamplifier has a high gain, noise introduced by subsequent stages can be rendered, as compared to noise introduced by the preamplifier, relatively insignificant. Thus, noise in a preamplifier should be minimized, while the preamplifier must also maximize gain and bandwidth.
FIG. 1
shows a prior art preamplifier
100
receiving a differential signal across terminals
110
and
111
, which are amplified by differential stages
100
a
and
100
b
. Differential stages
100
a
and
100
b
include emitter-coupled transistor pairs
101
and
102
, and
105
and
106
. Differential stages
100
a
and
100
b
are used because of the high gain required. For example, a bias of several hundred millivolts in the differential signal across terminals
110
and
111
(i.e., the signal provided by the read head) can exceed the rail-to-rail voltage. However, preamplifier
100
of
FIG. 1
is undesirable because of the noise in four transistors.
As shown in
FIG. 1
, each of differential stages
100
a
and
100
b
is used as a single-ended amplifier. The signal at terminal
110
is coupled to the base terminal of transistor
101
and the signal at terminal
111
is coupled to base terminal of transistor
105
. Current sources
113
and
114
provide bias currents to differential stages
100
a
and
100
b
, respectively. Transistors
115
-
118
, which are each biased by a common reference voltage source
119
, form cascodes to provide an AC ground. Differential amplifiers
120
and
121
drive differential stages
100
a
and
100
b
respectively to provide an output differential voltage at terminals
122
and
123
. Capacitors
124
and
125
filters out high frequency AC noise signals in input terminals
110
and
111
, respectively. Resistors
126
-
129
are typically given the same resistance.
Preamplifier
100
's gain can be set by selecting a suitable resistance value for resistors
126
-
129
. Roughly, the gain is determined by the ratio of this selected resistance to the emitter resistance in each of transistors
101
,
102
,
105
and
106
. Gain is increased by a higher resistance or a larger current in each of transistors
115
-
118
.
Since differential stages
100
a
and
100
b
are essentially two single-ended amplifiers, preamplifier
100
does not have a good common mode rejection characteristics.
FIG. 2
shows a preamplifier
150
in the prior art, which is a variation of preamplifier
100
, for use with multiple read heads. (To simplify discussion, like elements in the figures of the application are provided like reference numerals.) In preamplifier
150
, multiple differential signals from multiple read heads share differential stages
150
a
and
150
b
. For example, as shown in
FIG. 2
, the differential output signal from a first read head is provided at terminals
110
and
111
and coupled into differential stages
150
a
and
150
b
via the base terminals of transistors
101
and
105
, respectively. In like manner, the differential output signal of a second read head is provided at terminals
151
and
152
and coupled into differential stages
150
a
and
150
b
via the base terminals of transistors
153
and
154
, respectively. A mechanism (not shown) is provided to select which one of the output differential signals from the multiple read head is coupled into preamplifier
150
.
To minimize shot noise, transistors
101
,
102
,
115
and
116
are made as large as practical. At the same time, however, without reducing the resistances in resistors
126
-
129
, increasing the currents in transistors
101
,
102
,
115
and
116
limits the dynamic range of the output signals. Reducing the resistances of resistors
126
-
129
reduces the gain of the preamplifier.
SUMMARY OF THE INVENTION
In accordance with the present invention, a preamplifier includes (i) an indirectly coupled transistor pair receiving an input differential signal and (ii) an AC coupling circuit indirectly coupling the transistors of the pair to each other.
In one embodiment, a current bypass circuit is also included for decreasing the emitter resistance and raising the gain while observing voltage overhead restrictions.
Thus, the present invention provides indirect AC coupling of a transistor pair in a preamplifier, allowing the preamplifier to function with fewer primary noise-contributing transistors and to perform common mode rejection.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.
REFERENCES:
patent: 3541465 (1970-11-01), Nagata et al.
patent: 4528517 (1985-07-01), Schlotzhauer
patent: 4752744 (1988-06-01), Aoki
patent: 4833418 (1989-05-01), Quintus et al.
patent: 5635874 (1997-06-01), Perrot
patent: 5729178 (1998-03-01), Park et al.
Smith et al. “Distributed Components in Printed Circuit” Proceedings Electronics Components Syposuim 1956 Washington, DC.
Cook Carmen C.
Kwok Edward C.
National Semiconductor Corporation
Shingleton Michael B.
Skjerven Morrill & MacPherson LLP
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