Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2003-04-16
2004-08-31
Nguyen, Khanh Van (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C327S374000
Reexamination Certificate
active
06784735
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a differential amplifier, and more particularly to a high switching speed differential amplifier and the amplification method thereof.
BACKGROUND OF THE INVENTION
In the driver circuit design for laser diodes, it is required a gain stage capable of providing large current driving and high switching speed.
FIG. 1
shows a basic scheme
10
of a driver circuit for laser diodes, which includes a front-end gain stage
14
and a modulation stage
16
. The gain stage
14
is inputted a pseudo emitter coupling logic (PECL) signal
12
, and its output is connected to the modulation stage
16
. For the large capacitive delay effect in the modulation stage
16
, the gain stage
14
is employed to amplify the pseudo emitter coupling logic signal
12
to improve the system.
Current gain stage
14
usually utilizes traditional differential amplifier.
FIG. 2
shows a traditional differential amplifier
34
, which comprises three parts, a first and a second passive loads
70
a
and
70
b,
a differential pair
72
, and a current source
74
. The first and second passive loads
70
a
and
70
b
are constructed with resistors
18
and
20
connected to a first and a second supply voltages
46
and
48
, respectively. The differential pair
72
is composed of a pair of NMOS transistors
26
and
28
with their gates to receive a pair of differential input signals
36
and
38
, respectively, also shown by V
in+
and V
in−
in FIG.
2
. The outputs of the NMOS transistors
26
and
28
are connected to the resistors
18
and
20
through nodes
22
and
24
, respectively. The current source
74
is composed of an NMOS transistor
20
with its gate biased, source connected to ground
32
, and drain connected to the differential pair
72
. The operation of the differential amplifier
34
is briefly described in the following. Once the gates of the NMOS transistors
26
and
28
receive the differential voltage signals
36
and
38
, the differential pair
72
transforms them to a current flowing through the resistor
18
from the drain current of the NMOS transistor
26
. Resulted from the small signal module, it is obtained a voltage gain G
m
×R
d
, where G
m
is the transconductance of the differential pair
72
and R
d
is the resistance of the resistor
18
. To improve the switching speed of the differential amplifier
34
, the resistance R
d
of the resistor
18
must be low, while that also reduces the voltage gain G
m
×R
d
at the same time. If the resistance R
d
of the resistor
18
is increased to obtain larger voltage gain G
m
×R
d
, the larger resistor
18
will lower the frequency response and slow down the switching speed. In other words, the traditional differential amplifier
34
cannot provide high voltage gain and high switching speed at the same time. Hence, it is desired a differential amplifier with high amplification gain and high switching speed.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to disclose a differential amplifier to provide high gain and high switching speed at the same time.
In a differential amplifier, according to the present invention, there is included a differential pair, a current source connected to the differential pair, and two active loads connected to the differential pair. The differential pair is composed of a pair of MOS transistors to receive a pair of differential signals from two inputs. Each of the active loads includes a pair of MOS transistors, among which one active load is connected to an output of one MOS transistor in the differential pair and forms two paths switched therebetween in response to one of the inputs of the differential pair, and another active load is connected to an output of the other MOS transistor in the differential pair and forms third and fourth paths switched therebetween in response to another input of the differential pair.
REFERENCES:
patent: 5821809 (1998-10-01), Boerstler et al.
patent: RE36013 (1998-12-01), Lee
patent: 6369652 (2002-04-01), Nguyen et al.
Frontend Analog and Digital Technology Corporation
Nguyen Khanh Van
Rosenberg , Klein & Lee
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