Amplifiers – Signal feedback – Variable impedance in feedback path varied by separate...
Reexamination Certificate
2003-06-19
2004-11-30
Nguyen, Patricia (Department: 2817)
Amplifiers
Signal feedback
Variable impedance in feedback path varied by separate...
C330S103000
Reexamination Certificate
active
06825718
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an impedance matching circuit and, more particularly, to an impedance matching circuit which is suitable to be applied on an IC chip for adjusting impedance variation caused by the transistors of the impedance matching circuit.
BACKGROUND OF THE INVENTION
In a cable communicating system, impedance matching circuit is necessary to be implemented in the cabling interface of a communicating device for reducing or eliminating signal reflections caused by unmatched impedances between the cable and the circuits inside the communicating device.
FIG. 1
is a schematic drawing illustrating the basic principle of the impedance matching of prior art. As shown, the output device
1
outputs a signal to drive a load unit
2
via a cable
3
. In order to accomplish the object of impedance matching, the characteristic impedance of the cable has to match the impedances of the inside circuits of both the output device
1
and the load unit
2
. Otherwise, signal reflections may occur. For example, if the output resistor
10
of the output device
1
is 50 &OHgr;, then the impedance of the cable
3
must be 50 &OHgr;, and the input resistor
20
of the load unit
2
must be 50 &OHgr; as well.
Generally, the values of resistances of the circuits built-in an IC (Integrated Circuit) chip are variable due to the inevitable deviations of semiconductor manufacturing processes. It is relatively difficult to precisely control the resistance of the IC chip for the purpose of impedance matching. Therefore, the conventional way is to apply a precision resistor outside the IC chip to achieve the object of impedance matching. However, the drawback of this prior art technique is the increases of the complexity of circuit design, the usage of electronic components, and the cost.
Because it is relatively difficult to design a linear resistor (for example, by using poly N-wells) with precise resistance inside an IC chip, another conventional way is to apply an adjustable resistor circuit inside the IC chip to compensate the variation of the resistance of the IC chip caused by semiconductor manufacturing processes.
FIG. 2
is a schematic diagram showing an on-chip impedance matching circuit of prior art. As shown in
FIG. 2
, the impedance matching circuit
4
includes a plurality of MOS transistors
40
,
41
,
42
. These MOS transistors
40
,
41
,
42
are controlled by providing switching voltages to the gates S
1
, S
2
, S
3
respectively so as to determinate whether these transistors are switched on or not. For example, if the gate S
2
is provided with a switching voltage, the MOS transistor
41
will be switched on, and the resistance value of the output resistor (Rout) will be equal to the sum of the values of the resistors from the resistor
44
through the resistor
45
. However, there is still a serious problem in such design.
FIG. 3
is a schematic diagram showing the circuits of the interior resistors
400
,
410
,
420
of the MOS transistors
40
,
41
,
42
of FIG.
2
. As previously described in
FIG. 2
, when one of the MOS transistors
40
,
41
,
42
is switched on, then a resistance will also be generated inside that MOS transistor. These inevitable resistances caused by switching on these MOS transistors
40
,
41
,
42
can be schematically represented by the interior resistors
400
,
410
,
420
respectively. That is, When the MOS transistor
41
is switched on, i.e. S
2
is short, the output resistor value (Rout) is actually equal to the sum of resistors
44
through
45
, and
410
. Therefore, when designing an on-chip impedance matching circuit, not only the resistors
43
,
44
,
45
should be calculated, but also the interior resistors
400
,
410
,
420
must be considered. Moreover, the values of such interior resistors
400
,
410
,
420
are variable by applying different values of input voltage V
i
, which will distort the output signals and impede the functions of the IC chip.
SUMMARY OF THE INVENTION
The first object of the present invention is to provide an impedance matching circuit which is suitable to be applied on an IC chip for adjusting impedance variation of the transistors formed on the IC chip.
The second object of the present invention is to provide an impedance matching circuit which comprises a resistor unit and an operational amplifier. By utilizing the ideal characteristic of the operational amplifier, the variations of the interior resistors caused by the transistors can be eliminated, and thus the resistance of the impedance matching circuit can be precisely controlled and selected.
In an embodiment of the impedance matching circuit in accordance with the present invention, the impedance matching circuit comprises a resistor unit, an OP amplifier circuit connected with the resistor unit, a feedback selecting circuit connected in parallel with the OP amplifier circuit, and a resistor selecting circuit connected with both the OP amplifier circuit and the feedback selecting circuit. The feedback selecting circuit further includes a plurality of switching circuits for enabling some of the resistors furnished in the resistor selecting circuit. By selecting and actuating one of the switching circuits, some certain resistors will be enabled so as to adjust the resistance value of the resistor selecting circuit. The resistor unit and the switching circuits are designed in such a manner that the resistor unit is able to compensate an equivalent resistance of the switching circuit which is actuated. As a result, the impedance of the whole impedance matching circuit is precisely matching with the adjusted resistance value of the resistor selecting circuit and will not be influenced by any variation of the input signals.
Other and her features, advantages and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings are incorporated in and constitute a part of this application and, together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.
REFERENCES:
patent: 3562552 (1971-02-01), Baudino et al.
patent: 5585741 (1996-12-01), Jordan
patent: 6278339 (2001-08-01), Abadeer et al.
patent: 6621346 (2003-09-01), Nabicht et al.
patent: 6665525 (2003-12-01), Dent et al.
Kang Tzung-Hung
Lee Chao-Cheng
Nguyen Patricia
Realtek Semiconductor Corp.
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