Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2000-05-08
2001-12-18
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C360S046000
Reexamination Certificate
active
06331803
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multi-stage amplifier, and more particularly to a multi-stage amplifier that maintains a substantially constant bandwidth as the gain varies. The invention further relates to a magnetic storage medium that includes a preamplifier in which the multi-stage amplifier is embodied.
BACKGROUND OF THE INVENTION
Known variable voltage gain amplifiers include operational amplifiers of both single- and multi-stage designs. These conventional amplifiers often suffer from such performance limitations as limited dynamic range, distortion in the presence of heavy output loads, and a trade-off between the voltage gain available and the bandwidth over which the amplifier provides a substantially flat voltage gain profile.
FIG. 1A
is a schematic diagram of a conventional, single-stage, variable gain amplifier (VGA)
11
comprised of both n-channel and p-channel metal oxide semiconductor field effect transistors (MOSFETs), in accordance with conventional CMOS technology. The amplifier includes a pair of NMOS transistors N
12
and N
13
for receiving a differential input signal represented by the difference between input signals v
G1
and v
G2
. The input transistors are biased by a constant current source
14
, which is typically implemented with a current mirror configuration. Input transistors N
12
and N
13
are actively loaded with a pair of PMOS transistors P
15
and P
16
. The drains of P
15
and P
16
are respectively coupled to the drains of input transistors N
12
and N
13
to provide a variable load in response to a voltage control signal (v
c
) applied to the gates of each of the load transistors. A variable resistor R
19
, coupled between the drains of the input transistors, operates as a bandwidth limited gain control to adjust the gain while varying the bandwidth of the amplified differential output signals which are generated at opposite ends of the resistor.
FIG. 1B
illustrates another conventional, single-stage VGA design
21
. This design is similar to the design shown in
FIG. 1A
, except that this design employs a pair of variable resistors R
22
and R
23
instead of voltage-controlled load transistors. Also, there is no variable resistor coupled between the drains of the input transistors.
A basic problem with conventional amplifiers of the types shown in
FIGS. 1A and 1B
is that voltage gain is substantially inversely proportional to the bandwidth of the amplifier. In fact, the gain-bandwidth product in such amplifiers remains nearly constant in the operating range of the amplifier. Thus, an increase in gain decreases the bandwidth, and vice-versa. These conventional amplifiers lack the ability to provide a variable gain, while maintaining a substantially constant bandwidth. The amplifier illustrated in
FIG. 1B
also does not provide a constant common-mode output.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an amplifier that maintains a substantially constant bandwidth, as the gain is adjusted, and a method of forming such an amplifier.
It is another object of this invention to provide a multi-stage, constant bandwidth, variable gain amplifier, where the gain of each stage is individually controllable to control the overall gain of the amplifier without substantially degrading the bandwidth of the amplifier.
It is a further object of this invention to provide a magnetic storage medium including a pre-amplifier in which a variable gain amplifier capable of maintaining substantially constant bandwidth is embodied.
According to one aspect of the invention, an amplifier is provided which comprises a plurality of amplification stages having at least one common output node. Each stage includes a switch mechanism that controls the gain of that stage, wherein the switch mechanisms are selectively controllable to vary the gain of the amplifier while maintaining substantially constant bandwidth.
Preferably, each amplification stage includes a first pair of input transistors, and a first pair of cascoded transistors each coupled to a respective one of the corresponding input transistors. The transistors are preferably MOS transistors fabricated in accordance with CMOS technology.
Preferably, each switch mechanism comprises a plurality of switches including first and second switches each coupled between a drain and source of a respective one of the input transistors, and a third switch coupled between the drains of the input transistors.
In accordance with another aspect of the invention, a method of forming such an amplifier is provided.
In accordance with a further aspect of the invention, a magnetic storage medium is provided. Such a storage medium comprises a disk drive comprising a hard disk controller, and a read channel in communication with the hard disk controller. The output of a pre-amplifier is supplied to the read channel. The pre-amplifier comprises a plurality of amplification stages having at least one common output node, and a switch mechanism for each stage that controls the gain of that stage. The switch mechanisms are selectively controllable to vary the gain of the pre-amplifier while maintaining substantially constant bandwidth.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 5455816 (1995-10-01), Bitting
patent: 6011437 (2000-01-01), Sutardja et al.
patent: 6069866 (2000-05-01), Pietruszynski et al.
Lam Steven C.
Zheng Zhiliang
Janofsky Eric B.
Marvell International Ltd
Mottola Steven J.
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