U.S. patent application number 12/695773 was filed with the patent office on 2010-12-16 for active back-end termination circuit.
This patent application is currently assigned to National Tsing Hua University. Invention is credited to Yu-Hao HSU, Min-Sheng KAO, Jen-Ming WU.
Application Number | 20100315176 12/695773 |
Document ID | / |
Family ID | 43305920 |
Filed Date | 2010-12-16 |
United States Patent
Application |
20100315176 |
Kind Code |
A1 |
KAO; Min-Sheng ; et
al. |
December 16, 2010 |
ACTIVE BACK-END TERMINATION CIRCUIT
Abstract
The invention discloses an active back-end termination circuit,
which comprises a first resistor, a first transistor, a second
resistor, and a second transistor. The first resistor and the first
transistor are connected in series for forming a first impendence
unit. A first source of the first transistor is connected to a
working voltage with V.sub.TT. The second resistor and the second
transistor are connected in series for forming a second impendence
unit. A second gate and a second drain of the second transistor are
connected to the working voltage with V.sub.TT. Wherein, the first
impendence unit and the second impendence unit are connected in
parallel. The first transistor or the second transistor is switched
on through a power source, and the first transistor and the second
transistor change the impedance actively for matching a load
according to the voltage source.
Inventors: |
KAO; Min-Sheng; (Tucheng
City, TW) ; HSU; Yu-Hao; (Xinying City, TW) ;
WU; Jen-Ming; (Zhubei City, TW) |
Correspondence
Address: |
Tolpin & Partners, PC
11 S. LaSalle Street, Suite 2900
Chicago
IL
60603
US
|
Assignee: |
National Tsing Hua
University
Hsin-Chu
TW
|
Family ID: |
43305920 |
Appl. No.: |
12/695773 |
Filed: |
January 28, 2010 |
Current U.S.
Class: |
333/32 ;
333/22R |
Current CPC
Class: |
H01S 5/0427 20130101;
H03H 11/28 20130101 |
Class at
Publication: |
333/32 ;
333/22.R |
International
Class: |
H03H 7/38 20060101
H03H007/38 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 11, 2009 |
TW |
098119603 |
Claims
1. An active back-end termination circuit, comprising: a first
resistor including a first terminal and a second terminal; a first
transistor including a first source, a first gate and a first
drain, and the first gate and the first drain being connected to
the second terminal, and a matching unit being formed by connecting
the first resistor and the first transistor in series, and the
first source being connected to a working voltage V.sub.TT; a
second resistor including a third terminal and a fourth terminal;
and a second transistor including a second source, a second gate
and a second drain, and the second source being connected to the
fourth terminal, and a second matching unit being formed by
connecting the second reisitor and the second transistor in series,
and the second gate and the second drain being connected to the
working voltage V.sub.TT; wherein, the first matching unit and the
second matching unit are connected to a circuit in parallel by
connecting the first terminal and the third terminal to the circuit
in parallel; and wherein, the first transistor or the second
transistor is applied with a bias voltage by connecting the first
terminal and the third terminal to a voltage source, and an
impedance of the first matching unit and the second matching unit
are changed actively according to the voltage source for matching a
load.
2. The active back-end termination circuit of claim 1, wherein the
circuit is an output stage circuit.
3. The active back-end termination circuit of claim 1, wherein the
voltage source is a periodic wave voltage source.
4. The active back-end termination circuit of claim 3, wherein the
periodic wave voltage source is a sine wave voltage source, a
square wave voltage source or a triangle wave voltage source.
5. The active back-end termination circuit of claim 1, wherein the
voltage source drives the first matching unit if the voltage source
is situated at a period of a positive half cycle, and the voltage
source drives the second matching unit if the voltage source is
situated at a period of a negative half cycle.
6. The active back-end termination circuit of claim 1, wherein the
voltage source outputs a voltage with output light level (V.sub.OH)
or a voltage with output low level (V.sub.OL) by defining the
working voltage V.sub.TT as an amplitude origin.
7. The active back-end termination circuit of claim 1, wherein the
voltage source outputs the working voltage V.sub.TT without any
loss of DC current.
8. The actibve back-end termination circuit of claim 6, wherein the
first transistor or the second transistor provides an impedance
matching according to the bias voltage, if the voltage source
outputs the voltage with output high level (VOH) or the voltage
with output low level (VOL).
9. The active back-end termination circuit of claim 1, wherein the
load is a laser diode (LD) or an elector-absorption modulated laser
(EML).
10. The active back-end termination circuit of claim 1, wherein the
first transistor or the second transistor is an N-type metal oxide
semiconductor (NMOS), a P-type metal oxide semiconductor (PMOS) or
a bipolar junction transistor (BJT).
11. The active back-end termination circuit of claim 1, wherein the
active back-end termination circuit and another active back-end
termination circuit are connected in parallel to change an
equivalent impedance for impedance matching.
12. An active back-end termination circuit, comprising: a first
resistor including a first terminal and a second terminal; a first
transistor including a first source, a first gate and a first
drain, and the first drain being connected to the second terminal,
and a matching unit being formed by connecting the first resistor
and the first transistor in series, and the first source being
connected to a working voltage V.sub.TT, and the first gate being
connected to an external voltage source; and a second resistor
including a third terminal and a fourth terminal; a second
transistor including a second source, a second gate and a second
drain, and the second source being connected to the fourth
terminal, and a second matching unti being formed by connecting the
second resistor and the second transistor in series, and the second
drain being connected to the working voltage V.sub.TT, and the
second gate being connected to the external voltage source;
wherein, the first matching unit and the second matching unit are
connected to a circuit in parallel by connecting the first terminal
and the third terminal to the circuit in paralled; and wherein, the
first transistor or the second transistor is applied with a bias
voltage by connecting the first terminal and the third terminal a
voltage source, and an impedance of the first matching unit and the
second matching unit are actively changed according to the voltage
source for matching a load.
13. The active back-end termination circuit of claim 12, wherein
the circuit is an output stage circuit.
14. The active back-end termination circuit of claim 12, wherein
the voltage source is a periodic wave voltage source.
15. The active back-end termination circuit of claim 14, wherein
the periodic wave voltage source is a sine wave voltage source, a
square wave voltage source or a triangle wave voltage source.
16. The active back-end termination circuit of claim 12, wherein
the voltage source drives the first matching unit if the voltage
source is situated at a period of a positive half cycle, and the
voltage source drives the second matching unit if the voltage
source is situated at a period of a negative half cycle.
17. The active back-end termination circuit of claim 12, wherein
the voltage source outputs a voltage with output high level (VOH)
or a voltage with output low level (VOL) by defining the working
voltage V.sub.TT as an amplitude origin.
18. The active back-end termination circuit of claim 12, wherein
the voltage source outputs the working voltage V.sub.TT without any
loss of DC current.
19. The active back-end termination circuit of claim 17, wherein
the first transistor and the second transistor will provide an
impedance matching according to the bias voltage, if the voltage
source outputs the voltage with output high level (VOH) or the
voltage with output low level (V.sub.OL).
20. The active back-end termination circuit of claim 12, wherein
the load is a laser diode (LD) or an elector-absorption modulated
laser (EML).
21. The active back-end termination circuit of claim 12, wherein
the first transistor or the second transistor is an N-type metal
oxide semiconductor (NMOS), a P-type metal oxide semiconductor
(PMOS) or a bipolar junction transistor (BJT).
22. The active back-end termination circuit of claim 12, wherein
the active back-end termination circuit and another active back-end
termination circuit are connected in parallel to change an
equivalent impedance for impendance matching.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an active back-end
termination circuit, in particular to a DC-coupled active back-end
termination circuit for matching the impedance of the load.
[0003] 2. Description of the Related Art
[0004] At present, optical communication technology is the
mainstream of the high-speed communication network. A direct
modulation with distributed feedback (DFB) laser is generally
adopted to a middle-distance of the optical communication
technology, and an external modulation with DFB laser is adopted to
a long-distance of the optical communication technology. As to the
transmission rate, present manufacturers adopt a mature network
technology in system carrier, namely a coarse wavelength division
multiplexing (CWDM) technology having a primary technical
specification of OC48(2.488 Gb/s).about. OC192(9.953 Gb/s), and the
research works will be headed to OC768(39.812 Gb/s)
continually.
[0005] The traditional laser driver is designed with an open
collector architecture, and uses an external matching network for
driving laser diode. With the increasing operation speed of the
optical communication network, the signal reflection between the
laser driver and the laser diode is getting worse due to the
impedance mismatch and it impacts the performance of the
transmission. To solve this problem, the passive back-end
termination circuit is used, but it costs 50% modulation current
due to the internal resistor (Rint). In another prior art, the
internal AC-coupled active back-end termination circuit disclosed
in U.S. Pat. No. 6,667,661 is used, but it is very diffucult to
design a high quality capacitor in chip process and it occupies
large area.
SUMMARY OF THE INVENTION
[0006] In view of the aforementioned problems of the prior art, the
object of the present invention is to provide an active back-end
termination circuit for matching the impedance of the load.
[0007] According to another object of the present invention, an
active back-end termination circuit is provided, comprising a first
resistor, a first transistor, a second resistor and a second
transistor. The first resistor comprises a first terminal and a
second terminal The first transistor comprises a first source, a
first gate and a first drain. The first gate and the first drain
are connected to the second terminal, and a first matching unit is
formed by connecting the first transistor and the first resistor in
series. The first source is connected to a working voltage
V.sub.TT. The second resistor comprises a third terminal and a
fourth terminal. The second transistor comprises a second source, a
second gate and a second drain. The second source is connected to
the fourth terminal, and a second matching unit is formed by
connecting the second resistor and the second transistor in series.
The second gate and the second drain are connected to the working
voltage V.sub.TT. The first matching unit and the second matching
unit are connected in parallel by connecting the first terminal and
the third terminal are connected to a circuit. The first transistor
or the second transistor is applied with a bias voltage by
connecting the first terminal and the third terminal to the voltage
source. According to the bias voltage supplied by the voltage
source is different, so that the first matching unit and the second
matching unit actively change an impedance to match with a load
according to the voltage source. In other word, the first
transistor and the second transistor may be switchon or switch off
according to the voltage source.
[0008] If the voltage source is situated at a period of a positive
half cycle, the voltage source drives the first matching unit, and
if the voltage source is situated at a period of a negative half
cycle, the voltage source drives the second matching unit.
[0009] The voltage source outputs a voltage with output high level
(VOH) or a voltage with output low level (VOL) by defining the
working voltage V.sub.TT as an amplitude origin.
[0010] The voltage source outputs the working voltage V.sub.TT
without any loss of DC current.
[0011] The load is a laser diode (LD) or an elector-absorption
modulated laser (EML).
[0012] When the voltage source outputs a voltage with output high
level (VOH) or a voltage with output low level (VOL), the first
transistor or second transistorprovides an impedance matching
according to the bias voltage.
[0013] Another objective of the present invention is to provide an
active back-end termination circuit, comprising: a first resistor,
a first transistor, a second resistor and a second transistor. The
first resistor comprises a first terminal and a second terminal.
The first transistor comprises a first source, a first gate and a
first drain, and the first drain is connected to the second
terminal, and a first matching unit is formed by connecting the
first resistor and the first transistor in series, and the first
source is connected to the working voltage V.sub.TT, and the first
gate is connected to an external voltage source. The second
resistor comprises a third terminal and a fourth terminal. The
second transistor comprises a second source, a second gate and a
second drain, and the second source is connected to the fourth
terminal, and a second matching unit is formed by connecting the
second resistor and the second transistor in series. The second
drain is connected to the working voltage V.sub.TT, and the second
gate is connected to an external voltage source. The first terminal
and the third terminal are connected to a circuit, such that the
first matching unit and the second matching unit are connected in
parallel to the circuit. The first terminal and the third terminal
are connected to the voltage source, first transistor or second
transistor, and a bias voltage is provided according to the voltage
source and the external voltage source. Now, the first transistor
and the second transistor are variable resistors for actively
matching an impedance of a load.
[0014] If the voltage source is situated at a period of a positive
half cycle, the voltage source drives the first matching unit, and
if the voltage source is situated at a period of a negative half
cycle, the voltage source drives the second matching unit.
[0015] The voltage source outputs a voltage with output high level
(VOH) or a voltage with output low level (VOL) by using the working
voltage V.sub.TT as an amplitude origin.
[0016] The voltage source outputs the working voltage V.sub.TT
without any loss of DC current.
[0017] The load is a laser diode (LD) or an elector-absorption
modulated laser (EML).
[0018] When the voltage source outputs a voltage with output high
level (VOH) or a voltage with output low level (VOL), the first
transistor or second transistor provides an impedance matching
according to the bias voltage.
[0019] In summation of the description above, the active back-end
termination circuit of the present invention has one or more of the
following advantages:
[0020] (1) The active back-end termination circuit has a driving
efficiency higher than the passive back-end termination
circuit.
[0021] (2) The active back-end termination circuit provides the
impedance matching without designing a capacitor in a chip process,
and thus no large chip area is occupied by the capacitor.
[0022] (3) The active back-end termination circuit consumes no DC
current when the voltage source outputs the working voltage with
V.sub.TT.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic diagram of an active back-end
termination circuit in accordance with the present invention;
[0024] FIG. 2 is a schematic view of a sine-wave voltage source of
the present invention;
[0025] FIG. 3 is a schematic diagram of an active back-end
termination circuit in accordance with a first preferred embodiment
of the present invention;
[0026] FIG. 4 is a Smith chart diagram of a first matching unit in
accordance with a first preferred embodiment of the present
invention;
[0027] FIG. 5 is a Smith chart diagram of a second matching unit in
accordance with a first preferred embodiment of the present
invention;
[0028] FIG. 6 is a schematic diagram of an active back-end
termination circuit in accordance with a second preferred
embodiment of the present invention;
[0029] FIG. 7 is a Smith chart diagram of a first matching unit in
accordance with a second preferred embodiment of the present
invention;
[0030] FIG. 8 is a Smith chart diagram of a second matching unit in
accordance with a second preferred embodiment of the present
invention;
[0031] FIG. 9 is a schematic diagram of an active back-end
termination circuit in accordance with a third preferred embodiment
of the present invention;
[0032] FIG. 10 is a Smith chart diagram of an active back-end
termination circuit in accordance with a third preferred embodiment
of the present invention;
[0033] FIG. 11 is a schematic diagram of an active back-end
termination circuit in accordance with a fourth preferred
embodiment of the present invention;
[0034] FIG. 12 is a Smith chart diagram of an active back-end
termination circuit in accordance with a fourth preferred
embodiment of the present invention;
[0035] FIG. 13 is a schematic diagram of another active back-end
termination circuit in accordance with the present invention;
[0036] FIG. 14 is an eye diagram of a conventional passive back-end
termination connected to an output stage circuit in 25 ohm driving
system;
[0037] FIG. 15 is an eye diagram of a DC-coupled active back-end
termination connected to an output stage circuit in 25 ohm driving
system in accordance with the present invention;
[0038] FIG. 16 is an eye diagram of a conventional passive back-end
termination circuit connected to an output stage in 50 ohm driving
system; and
[0039] FIG. 17 is an eye diagram of a DC-coupled active back-end
termination circuit connected to an output stage in 50 ohm driving
system in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] With reference to FIG. 1 for a schematic diagram of an
active back-end termination circuit in accordance with the present
invention, the active back-end termination circuit 1 comprises a
first matching unit 11 and a second matching unit 12. The first
matching unit 11 comprises a first resistor 111 and a first
transistor 112. The first resistor 111 comprises a first terminal
1111 and a second terminal 1112. The first transistor 112 comprises
a first source, a first gate and a first drain, and the first gate
and the first drain are connected to the second terminal 1112, such
that the first resistor 111 and the first transistor 112 are
connected in series, and the first source is connected to a working
voltage V.sub.TT.
[0041] The second matching unit 12 comprises a second resistor 121
and a second transistor 122. The second resistor 121 comprises a
third terminal 1211 and a fourth terminal 1212. The second
transistor 122 comprises a second source, a second gate and a
second drain, and the second source is connected to the fourth
terminal 1212, such that the second resistor 121 and the second
transistor 122 are connected in series, and the second gate and the
second drain are connected to the working voltage V.sub.TT.
[0042] The first terminal 1111 and the third terminal 1211 are
connected to a circuit, such that the first matching unit 11 and
the second matching unit 12 are connected in parallel with a
circuit. If the first terminal 1111 and the third terminal 1211 are
connected to a voltage source, the first transistor 112 or the
second transistor 122 generates a bias voltage according to the
voltage source. If the first transistor 112 and the second
transistor 122 are switched on according to the bias voltage, the
resistance will be different, so that the first transistor 112 and
the second transistor 122 may be considered as variable resistors
for actively matching an impedance of a load. The first transistor
112 or second transistor 122 may be an N-type metal oxide
semiconductor (NMOS), a P-type metal oxide semiconductor (PMOS) or
a bipolar junction transistor (BJT), but the present invention is
not limited to such arrangements only.
[0043] The voltage source may be a periodic wave voltage source,
such as a sine wave voltage source, a square wave voltage source or
a triangle wave voltage source. If the voltage source is a sine
wave voltage source as shown in FIG. 2, the voltage source may
define the working voltage V.sub.TT as an amplitude origin to
output a voltage with output high level (VOH) or a voltage with
output low level (VOL). If the voltage source is situated at a
period of a positive half cycle, the voltage source will drive the
first matching unit 11. If the voltage source is situated at a
period of a negative half cycle, the voltage source will drive the
second matching unit 12.
[0044] With reference to FIG. 3 for a schematic diagram of an
active back-end termination circuit in accordance with a first
preferred embodiment of the present invention, FIG. 4 for a Smith
chart diagram of a first matching unit in accordance with a first
preferred embodiment of the present invention, and FIG. 5 for a
Smith chart diagram of a second matching unit in accordance with a
first preferred embodiment of the present invention respectively,
if the voltage source outputs a voltage with output high level,
such as a voltage value of 5V, and the operating frequency is
approximately equal to 1 GHz, the first transistor 112 of the first
matching unit 11 or the second transistor 122 of the second
matching unit 12 will be affected by a bias voltage to obtain an
impedance of 82 ohms of the first matching unit, and an impedance
of 356 ohms of the second matching unit.
[0045] With reference to FIG. 6 for a schematic diagram of an
active back-end termination circuit in accordance with a second
preferred embodiment of the present invention, FIG. 7 for a Smith
chart diagram of a first matching unit in accordance with a second
preferred embodiment of the present invention, and FIG. 8 for a
Smith chart diagram of a second matching unit in accordance with a
second preferred embodiment of the present invention respectively,
if the voltage source outputs a voltage with output low level such
as a voltage value of 2V, and the operating frequency is
approximately equal to l GHz, the first transistor 112 of the first
matching unit 11 or the transistor 122 of the second matching unit
12 is affected by a bias voltage to obtain an impedance of 363 ohms
for the first matching unit, and an impedance of 83 ohms for the
second matching unit.
[0046] In the aforementioned two preferred embodiments, if the
voltage source is a voltage with output high level, an impedance of
82 ohms for the first matching unit and an impedance of 356 ohms
for the second matching unit can be obtained. If the voltage source
is a voltage with output low level, an impedance of 363 ohms for
the first matching unit and an impedance of 83 ohms for the second
matching unit can be achieved. The impendances of the whole set of
matching units at a voltage with output high level or a voltage
with output low level are very close and equal to 66.65 ohms and
67.56 ohms respectively.
[0047] If the voltage source is at a working voltage V.sub.TT such
as a voltage of 3.5V, and the first source of the first transistor
112 of the first matching unit 11 is connected to the working
voltage V.sub.TT, and both terminals have an equal electric
potential, there will have no loss of DC current. Similarly, the
second transistor 122 of the second matching unit 12 has no loss of
DC current.
[0048] With reference to FIG. 9 for a schematic diagram of an
active back-end termination circuit in accordance with a third
preferred embodiment of the present invention, the active back-end
termination circuit 1 is connected to an output stage circuit 2 to
match a load. If the load is a laser diode (LD) in 25 ohm driving
system, and four sets of active back-end termination circuits may
be used to match the laser diode.
[0049] With reference to FIG. 10 for a Smith chart diagram of an
active back-end termination circuit in accordance with a third
preferred embodiment of the present invention, different operating
frequencies such as 1 GHz, 5 GHz, 6.04 GHz, 7.09 GHz, 8.07 GHz,
9.05 GHz and 10.39 GHz give the impedance values of 30.6945 ohms,
29.7445 ohms, 29.2975 ohms, 28.8262 ohms, 28.3755 ohms, 27.9182
ohms and 27.2871 ohms respectively, and their impedance values are
very close. Therefore, the active back-end termination circuit of
the present invention may provide an impedance matching.
[0050] With reference to FIG. 11 for a schematic diagram of an
active back-end termination circuit in accordance with a fourth
preferred embodiment of the present invention, the active back-end
termination circuit 1 is connected to the output stage circuit 2 to
match a load. If the load is an elector-absorption modulated laser
(EML) in 50 ohm driving system, and two sets of active back-end
termination circuits may be used for matching the
elector-absorption modulated laser.
[0051] With reference to FIG. 12 for a Smith chart diagram of an
active back-end termination circuit in accordance with a fourth
preferred embodiment of the present invention, different operating
frequencies such as 1 GHz, 5 GHz, 6.04 GHz, 7.09 GHz, 8.07 GHz,
9.05 GHz and 10.39 GHz give impedance values of 61.2828 ohms,
56.7781 ohms, 54.8778 ohms, 52.9571 ohms, 51.1932 ohms, 49.4708
ohms and 47.1961 ohms respectively, and the values are very close
at different frequencies, and thus the active back-end termination
circuit of the present invention may provide an impedance
matching.
[0052] In the present invention, the number of active back-end
termination circuits may be adjusted according to the load, and the
load may be a 25 ohm, 50 ohm, 75 ohm or 100 ohm driving system, but
the present invention is not limited to such arrangements only.
[0053] With reference to FIG. 13 for a schematic diagram of another
active back-end termination circuit in accordance with the present
invention, the active back-end termination circuit 3 comprises a
first matching unit 31 and a second matching unit 32. The first
matching unit 31 comprises a first resistor 311 and a first
transistor 312. The first resistor 311 comprises a first terminal
3111 and a second terminal 3112. The first transistor 312 comprises
a first source, a first gate and a first drain, and the first drain
is connected to the second terminal 3112, such that the first
resistor 311 and the first transistor 312 are connected in series,
and the first source is connected to a working voltage V.sub.TT,
and the first gate is connected to an external voltage source.
[0054] The second matching unit 32 comprises a second resistor 321
and a second transistor 322. The second resistor 321 comprises a
third terminal 3211 and a fourth terminal 3212. The second
transistor 322 comprises a second source, a second gate and a
second drain, and the second source is connected to the fourth
terminal 3212, such that the second resistor 321 and the second
transistor 322 are connected in series, and the second drain is
connected to a working voltage V.sub.TT, and the second gate is
connected to an external voltage source.
[0055] The first terminal 3111 and the third terminal 3211 are
connected to a circuit, such that the first matching unit 31 and
the second matching unit 32 are connected in parallel to the
circuit. If the first terminal 3111 and the third terminal 3211 are
connected to a voltage source, the first transistor 312 or the
second transistor 322 is swithed on according to the voltage source
and the external voltage source. If the first transistor 312 and
second transistor 322 are switched on within different bias
voltages, the resistance values will be different, and thus the
first transistor 312 and the second transistor 322 may be
considered as variable resistors for actively matching an impedance
of a load, and the load may be a laser diode (LD) or an
elector-absorption modulated laser (EML).
[0056] The voltage source may be a periodic wave voltage source,
such as a sine wave voltage source, a square wave voltage source or
a triangle wave voltage source for outputting a voltage with output
high level (VOH) or a voltage with output low level (VOL) by
defining the working voltage V.sub.TT as an amplitude origin. If
the voltage source is situated at a period of a positive half
cycle, the voltage source will drive the first matching unit 31. If
the voltage source is situated at a period of a negative half
cycle, the voltage source will drive the second matching unit 32.
If the voltage source is situated at a working voltage V.sub.TT,
one end of the circuit has the same electric potential of the
working voltage V.sub.TT, and thus there will be no loss of DC
current.
[0057] The active back-end termination circuit 3 may also be
connected to the output stage circuit 2 as shown in FIGS. 9 and 11.
Now, the active back-end termination circuit 3 replaces the active
back-end termination unit 1. The external voltage source is used
and connected to the first gate of the first transistor 312 and the
second gate of the second transistor 322, and the first transistor
312 and the second transistor 322 generate corresponding impedance
values at different bias voltages for actively matching the
impedance of the load.
[0058] With reference to FIG. 14 for an eye diagram of a
conventional passive back-end termination circuit in 25-ohm driving
system and FIG. 15 for an eye diagram of a DC-coupled active
back-end termination circuit in 25 ohm driving system in accordance
with the present invention, the amplitude of the prior art ranges
from -0.55 to 0.55, and the amplitude of the present invention
ranges from -0.84 to 0.83. Obviously, the range of amplitudes of
the present invention is greater than the range of amplitudes of
the prior art, indicating that the present invention has a wider
range of operating voltages to achieve a better driving efficiency
and provide a better waveform quality.
[0059] With reference to FIG. 16 for an eye diagram of a
conventional passive back-end termination circuit in 50 ohm driving
system, and FIG. 17 for an eye diagram of an active DC-coupled
back-end termination circuit in 50 ohm driving system in accordance
with the present invention respectively, the range of amplitudes of
the prior art is from -1.159 to 1.053, and the range of amplitudes
of the present invention is from -1.429 to 1.481. Obviously, the
range of amplitudes of the present invention is greater than the
range of amplitudes of the prior art, indicating that the invention
has a wider range of operating voltages to achieve a better driving
efficiency and provide a better waveform quality.
[0060] While the invention has been described by means of specific
embodiments, numerous modifications and variations could be made
thereto by those skilled in the art without departing from the
scope and spirit of the invention set forth in the claims.
* * * * *