# Patent

## Title
Output circuit with overshoot-reducing function

## Abstract

Output circuit with reduced overshoot includes input end, output end, a circuit composed of PMOS and NMOS, rising and falling edge trigger bias circuits. The rising and falling edge trigger bias circuits output biasing voltages to the output end for clamping the voltage of the output signals respectively according to the rising edge and the falling edge of the input signal. In this way, the overshoot of the output signal is reduced.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an output circuit, and more particularly, to an output circuit with overshoot-reducing function.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional output circuit 100. The output circuit 100 comprises an input end, an output end, an inverter INV1, delay circuits 110 and 120, PMOS QP1, and NMOS QN1.
The input end of the output circuit 100 receives an input signal DIN. The output end of the output circuit 100 outputs an output signal DOUT. It is assumed that the output end of the output circuit 100 is equivalently coupled to a capacitor CL.
The PMOS QP1 comprises a first end, a second end, and a control end. The NMOS QN1 comprises a first end, a second end, and a control end. The inverter INV1 is coupled between the input end of the output circuit 100, and the delay circuits 110 and 120. The delay circuit 110 is coupled between the inverter INV1 and the control end of the PMOS QP1. The delay circuit 120 is coupled between the inverter INV1 and the control end of the NMOS QN1. The first end of the PMOS QP1 is coupled to a voltage source VDD (supplying a voltage VDD), the second end of the PMOS QP1 is coupled to the output end of the output circuit 100, and the control end of the PMOS QP1 is coupled to the delay circuit 110. The first end of the NMOS QN1 is coupled to a voltage source VSS (supply a voltage VSS), the second of the NMOS QN1 is coupled to the output end of the output circuit 100, and the control end of the NMOS QN1 is coupled to the delay circuit 120.
The inverter INV1 is disposed for receiving the input signal DIN, inverting the received input signal DIN, and outputting the inverted input signal DIN.
The delay circuit 110 is coupled between the inverter INV1 and the control end of the PMOS QP1 for receiving the inverted input signal DIN, delaying the received inverted input signal DIN for a predetermined period DL1, and then inputting the delayed input signal DIN to the control end of the PMOS QP1 (the node DP). The delay circuit 110 can be realized with an even number of the inverters coupled in series (the even number is “2m” as shown in FIG. 1) for delaying the predetermined period DL1.
The delay circuit 120 is coupled between the inverter INV1 and the control end of the NMOS QN1 for receiving the inverted input signal DIN, delaying the received inverted input signal DIN for a predetermined period DL2, and then inputting the delayed input signal DIN to the control end of the NMOS QN1 (the node DN). The delay circuit 120 can be realized with an even number of the inverters coupled in series (the even number is “2n” as shown in FIG. 1) for delaying the predetermined period DL2.
The voltage VDD can be a high voltage, and the voltage VSS can be ground.
Additionally, the delay periods DL1 and DL2 are different, which means the numbers “2m” and “2n” of the inverters are different. The difference between the delay periods DL1 and DL2 avoids the PMOS QP1 and the NMOS QN1 being both turned on at the same time, which generates the current flows from the voltage source VDD directly to the voltage source VSS.
Please refer to FIG. 2. FIG. 2 is a timing diagram illustrating the conventional output circuit 100. As shown in FIG. 2, after the input signal DIN is inputted, on the node DP, the signal inverted from the input signal DIN and delayed for the predetermined period DL1 is generated and inputted to the PMOS QP1. Meanwhile, on the node DN, the signal inverted from the input signal DIN and delayed for the predetermined period DL2 is generated and inputted to the NMOS QN1. In this way, the output signal DOUT is generated by the PMOS QP1 and the NMOS QN1. When the input signal DIN is in a transient status (for example, in the period of the voltage of the signal DIN changing from a high voltage to a low voltage, or vice versa), the voltage of the output signal DOUT possibly rises over the voltage VDD or falls over the voltage VSS, which is so-called overshoot, and damages the components in the circuit.
A conventional method for reducing the overshoot problem is to couple a capacitor to the output end of the output circuit 100. However, the capacitor on the output end lowers the slew rate of the output signal DOUT, and consequently the access speed of the output circuit 100 is also lowered. As the speed of internal components of systems increases, the demand for the speed of the memory is also increased. It is not satisfying the demand with just purely raising the frequency of the clock signal of the memories, and therefore the technologies of the Synchronous Dynamic Random Access Memory (SRAM), the Double Data Rate (DDR), and the second generation of the DDR (DDR2) have to be utilized to meet the demand. The frequencies of dies of the conventional memories equal to the frequencies of the input/output buffers (I/O buffers. However, the frequencies of the I/O buffers of the memories of the DDR2 technology is doubled than the frequencies of the cores of the memories. The method with adding capacitors to the output ends for reducing overshoots lowers the speed of the output circuit of the memory.
Therefore, it is important to provide an innovative technology of Off-Chip Driver (OCD) with voltage-regulating circuits to reduce overshoots when the output circuit charges/discharges.
SUMMARY OF THE INVENTION
The present invention provides an output circuit with overshoot-reducing function. The output circuit comprises an input end for receiving an input signal; an output end; a PMOS comprising a first end coupled to a first voltage source supplying a first voltage; a control end coupled to the input end; and a second end coupled to the output end; an NMOS comprising a first end coupled to a second voltage source supplying a second voltage; a control end coupled to the input end; and a second end coupled to the output end; a rising-edge trigger biasing circuit coupled between the input end and the output end for outputting a third voltage to the output end so as to reduce overshoot on the output end according to a rising edge of the input signal; and a falling-edge trigger biasing circuit coupled between the input end and the output end for outputting a fourth voltage to the output end so as to reduce overshoot on the output end according to a falling edge of the input signal; wherein the third voltage is higher than the second voltage, and the fourth voltage is lower than the first voltage.
The present invention further provides an output circuit with overshoot-reducing function. The output circuit comprises an input end for receiving an input signal; an output end; a first switch coupled between a first voltage source supplying a first voltage and the output end for coupling the first voltage source to the output end; a second switch coupled between a second voltage source supplying a second voltage and the output end for coupling the second voltage source to the output end; a first trigger biasing circuit coupled between the input end, the output end and a third voltage source supplying a third voltage for outputting the third voltage to the output end according to a first status of the input signal; and a second trigger biasing circuit coupled between the input end, the output end and a fourth voltage source supplying a fourth voltage for outputting the fourth voltage to the output end according to a second status of the input signal; wherein the second switch is turned for coupling the second voltage source to the output end only after the first trigger biasing circuit outputs the third voltage to the output end for a first predetermined period, and the third voltage is different from the second voltage.
The present invention further provides an output circuit with overshoot-reducing function. The output circuit comprises an input end for receiving an input signal; an output end; a first control device coupled between a first voltage source supplying a first predetermined voltage and the output end for coupling the first voltage source to the output end; and a second control device coupled between a second voltage source supplying a second predetermined voltage and the output end for coupling the second voltage source to the output end; wherein the first control device couples the first voltage source to the output end only after the second control device outputs the second predetermined voltage to the output end for a first predetermined period, and the first predetermined voltage is different from the second predetermined voltage.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating a conventional output circuit.
FIG. 2 is a timing diagram illustrating the conventional output circuit.
FIG. 3 is a diagram illustrating the output circuit of the present invention.
FIG. 4 is a timing diagram illustrating the output circuit of the present invention.
FIG. 5 is a diagram illustrating one switch of the present invention.
FIG. 6 is a diagram illustrating another switch of the present invention.
FIG. 7 is a diagram illustrating the output circuit with a terminal resistor of the present invention.



DETAILED DESCRIPTION
Please refer to FIG. 3. FIG. 3 is a diagram illustrating the output circuit 300 of the present invention. The output circuit 300 comprises an input end, an output end, an inverter INV1, delay circuits 110 and 120, PMOS QP1, NMOS QN1, a falling-edge trigger biasing circuit 310, and a rising-edge trigger biasing circuit 320.
The input end of the output circuit 300 receives an input signal DIN. The output end of the output circuit 300 outputs an output signal DOUT. It is assumed that the output end of the output circuit 300 is equivalently coupled to a capacitor CL.
The PMOS QP1 comprises a first end, a second end, and a control end. The NMOS QN1 comprises a first end, a second end, and a control end. The inverter INV1 is coupled between the input end of the output circuit 100, and the delay circuits 110 and 120. The delay circuit 110 is coupled between the inverter INV1 and the control end of the PMOS QP1. The delay circuit 120 is coupled between the inverter INV1 and the control end of the NMOS QN1. The first end of the PMOS QP1 is coupled to a voltage source VDD (supplying a voltage VDD), the second end of the PMOS QP1 is coupled to the output end of the output circuit 300, and the control end of the PMOS QP1 is coupled to the delay circuit 110. The first end of the NMOS QN1 is coupled to a voltage source VSS (supply a voltage VSS), the second of the NMOS QN1 is coupled to the output end of the output circuit 300, and the control end of the NMOS QN1 is coupled to the delay circuit 120. The falling-edge trigger biasing circuit comprises a falling-edge trigger circuit 301 and a biasing circuit 311. The falling-edge trigger circuit 301 is coupled between the input end of the output circuit 300 and the biasing circuit 311. The biasing circuit 311 is coupled between the falling-edge trigger circuit 301 and the output end of the output circuit 300. The rising-edge trigger biasing circuit comprises a rising-edge trigger circuit 302 and a biasing circuit 321. The rising-edge trigger circuit 302 is coupled between the input end of the output circuit 300 and the biasing circuit 321. The biasing circuit 321 is coupled between the rising-edge trigger circuit 302 and the output end of the output circuit 300. The biasing circuit 311 comprises a switch SW1 and a voltage source V1 supplying a voltage V1. The switch SW1 comprises a first end 1, a second end 2, and a control end C. The first end 1 of the switch SW1 is coupled to the voltage source V1, the second end 2 of the switch SW1 is coupled to the output end of the output circuit 300, and the control end C of the switch SW1 is coupled to the falling-edge trigger circuit 301. The biasing circuit 321 comprises a switch SW2 and a voltage source V2 supplying a voltage V2. The switch SW2 comprises a first end 1, a second end 2, and a control end C. The first end 1 of the switch SW2 is coupled to the voltage source V2, the second end 2 of the switch SW2 is coupled to the output end of the output circuit 300, and the control end C of the switch SW2 is coupled to the rising-edge trigger circuit 302.
The inverter INV1 is disposed for receiving the input signal DIN, inverting the received input signal DIN, and outputting the inverted input signal DIN.
The delay circuit 110 is coupled between the inverter INV1 and the control end of the PMOS QP1 for receiving the inverted input signal DIN, delaying the received inverted input signal DIN for a predetermined period DL1, and then inputting the delayed input signal DIN to the control end of the PMOS QP1 (the node DP). The delay circuit 110 can be realized with an even number of the inverters coupled in series (the even number is “2m” as shown in FIG. 1) for delaying the predetermined period DL1.
The delay circuit 120 is coupled between the inverter INV1 and the control end of the NMOS QN1 for receiving the inverted input signal DIN, delaying the received inverted input signal DIN for a predetermined period DL2, and then inputting the delayed input signal DIN to the control end of the NMOS QN1 (the node DN). The delay circuit 120 can be realized with an even number of the inverters coupled in series (the even number is “2n” as shown in FIG. 1) for delaying the predetermined period DL2.
The voltage VDD can be a high voltage, and the voltage VSS can be ground. The voltage V1 is lower than the voltage VDD in principle, for example, the voltage V1 can be in the range between the average of the voltages VDD and VSS and the voltage VDD. The voltage V2 is higher than the voltage VSS in principle, for example, the voltage V2 can be in the range between the average of the voltages VDD and VSS and the voltage VSS.
The falling-edge trigger circuit 301 triggers a pulse signal PF with a predetermined period on the node SF according to the input signal DIN. More particularly, the falling-edge trigger circuit 301 triggers a pulse signal with the predetermined period PF on the node SF when the input signal DIN in the transient status that the voltage of the input signal DIN falls from the high voltage to the low voltage (falling edges). The pulse signal PF is transmitted to the control end C of the switch SW1. When the switch SW1 does not receive the pulse signal PF, the first end 1 of the switch SW1 is not coupled to the second end 2 of the switch SW1. That is, the voltage source V1 does not transmit the voltage V1 to the output end of the output circuit 300, and thus the output signal DOUT is not affected. When the switch SW1 receives the pulse signal PF, the first end 1 of the switch SW1 is coupled to the second end 2 of the switch SW1. That is, the voltage source V1 transmits the voltage V1 to the output end of the output circuit 300, and thus the output signal DOUT is affected. More particularly, at the time, the output signal DOUT is clamped at the voltage V1 for reducing the overshoots. In the conventional output circuit 100, the output signal DOUT, at the time, steps down from the high voltage to the low voltage, which generates the overshoots. However, in the output circuit 300 of the present invention, the output signal DOUT, at the time, is clamped at the voltage V1 by the falling-edge trigger circuit 310, and the overshoots are reduced. The period of the pulse signal PF can be designed according to the periods DL1 and DL2. In one embodiment, the period of the pulse signal PF can be designed to be shorter than the period of the transient status of the input signal DIN for avoid generating the output signal DOUT incorrectly. In another embodiment, the period of the pulse signal PF can be designed to be longer than the entire period that the unstable overshoots are generated on the output signal DOUT, for example, the entire period of the transient status of the input signal DIN changing from the high voltage to the low voltage. In this way, the overshoots on the output signal can be completely reduced.
The rising-edge trigger circuit 302 triggers a pulse signal with a predetermined period PR on the node SR according to the input signal DIN. More particularly, the rising-edge trigger circuit 302 triggers a pulse signal PR with the predetermined period on the node SF when the input signal DIN in the transient status that the voltage of the input signal DIN rises from the low voltage to the high voltage (rising edges). The pulse signal PF is transmitted to the control end C of the switch SW2. When the switch SW2 does not receive the pulse signal PR, the first end 1 of the switch SW2 is not coupled to the second end 2 of the switch SW2. That is, the voltage source V2 does not transmit the voltage V2 to the output end of the output circuit 300, and thus the output signal DOUT is not affected. When the switch SW2 receives the pulse signal PR, the first end 1 of the switch SW2 is coupled to the second end 2 of the switch SW2. That is, the voltage source V2 transmits the voltage V2 to the output end of the output circuit 300, and thus the output signal DOUT is affected. More particularly, at the time, the output signal DOUT is clamped at the voltage V2 for reducing the overshoots. In the conventional output circuit 100, the output signal DOUT, at the time, steps up from the low voltage to the high voltage, which generates the overshoots. However, in the output circuit 300 of the present invention, the output signal DOUT, at the time, is clamped at the voltage V2 by the rising-edge trigger circuit 320, and the overshoots are reduced. The period of the pulse signal PR can be designed according to the periods DL1 and DL2. In one embodiment, the period of the pulse signal PR can be designed to be shorter than the period of the transient status of the input signal DIN for avoid generating the output signal DOUT incorrectly. In another embodiment, the period of the pulse signal PR can be designed to be longer than the entire period that the unstable overshoots are generated on the output signal DOUT, for example, the entire period of the transient status of the input signal DIN changing from the low voltage to the high voltage. In this way, the overshoots on the output signal can be completely reduced.
Additionally, the delay periods DL1 and DL2 are different, which means the numbers “2m” and “2n” of the inverters are different. The difference between the delay periods DL1 and DL2 avoids the PMOS QP1 and the NMOS QN1 being both turned on at the same time, which generates the current flows from the voltage source VDD directly to the voltage source VSS.
Please refer to FIG. 4. FIG. 4 is a timing diagram illustrating the output circuit 300 of the present invention. As shown in FIG. 4, after the input signal DIN is inputted, on the node DP, the signal inverted from the input signal DIN and delayed for the predetermined period DL1 is generated and inputted to the PMOS QP1. Meanwhile, on the node DN, the signal inverted from the input signal DIN and delayed for the predetermined period DL2 is generated and inputted to the NMOS QN1.
In one embodiment of the present invention, when the input signal DIN falls from the high voltage to the low voltage, the falling-edge trigger circuit 301 generates the pulse signal PF on the node SF for turning on the switch SW1 so as to allow the output signal DOUT to receive the voltage V1. After the switch SW1 is turned on, the inverted inputted signal DIN turns on the NMOS QN1. In this way, the voltage difference between the first end and the second end of the NMOS QN1 reduces to (V1—VSS) and the overshoot is consequently reduced. When the input signal DIN rises from the low voltage to the high voltage, the rising-edge trigger circuit 302 generates the pulse signal PR on the node SR for turning on the switch SW2 so as to allow the output signal DOUT to receive the voltage V2. After the switch SW2 is turned on, the inverted inputted signal DIN turns on the PMOS QP1. In this way, the voltage difference between the first end and the second end of the PMOS QP1 reduces to (VDD−V2) and the overshoot is consequently reduced.
Please refer to FIG. 5. FIG. 5 is a diagram illustrating the switch SW1 of the present invention. As shown in FIG. 5, the switch SW1 comprises an inverter INV2, a PMOS QP2 and an NMOS QN2. The PMOS QP2 comprises a first end, a second end, and a third end. The NMOS QN2 comprises a first end, a second end, and a third end. The inverter INV2 comprises an input end and an output end. The input end of the inverter INV2 is coupled to the control end C of the switch SW1 for receiving the pulse signal PF transmitted from the falling-edge trigger circuit 301, and the inverter INV2 accordingly generates the inverted pulse signal PF. The first end of the PMOS QP2 is coupled to the first end of the switch SW1 for coupling to the voltage source V1. The second end of the PMOS QP2 is coupled to the second end of the switch SW1 for coupling to the output end of the output circuit 300. The control end of the PMOS QP2 is coupled to the output end of the inverter INV2 for receiving the inverted pulse signal PF. When the PMOS QP2 receives the inverted pulse signal PF, the first end of the PMOS QP2 is coupled to the second end of the PMOS QP2 for transmitting the voltage V1 to the output end of the output circuit 300. The first end of the NMOS QN2 is coupled to the first end of the switch SW1 for coupling to the voltage source V1. The second end of the NMOS QN2 is coupled to the second end of the switch SW1 for coupling to the output end of the output circuit 300. The control end of the NMOS QN2 is coupled to the control end C of the switch SW1 for receiving the pulse signal PF. When the NMOS QN2 receives the pulse signal PF, the first end of the NMOS QN2 is coupled to the second end of the NMOS QN2 for transmitting the voltage V1 to the output end of the output circuit 300. In another embodiment of the present invention as shown in FIG. 7, a terminated resistor can be disposed between the first end of the switch SW1 and the voltage source V1 for increasing the integrity of the signal. The terminated resistor can be a normal resistor or a MOS resistor; for instances, FIG. 7 illustrates the output circuit of the present invention for which a resistor R is utilized as the terminal resistor and the resistor R is coupled between the first end of the switch SW1 and the voltage source V1 .
Please refer to FIG. 6. FIG. 6 is a diagram illustrating the switch SW2 of the present invention. As shown in FIG. 6, the switch SW2 comprises an inverter INV3, a PMOS QP3 and an NMOS QN3. The PMOS QP3 comprises a first end, a second end, and a third end. The NMOS QN3 comprises a first end, a second end, and a third end. The inverter INV3 comprises an input end and an output end. The input end of the inverter INV3 is coupled to the control end C of the switch SW2 for receiving the pulse signal PR transmitted from the rising-edge trigger circuit 302, and the inverter INV3 accordingly generates the inverted pulse signal PR. The first end of the PMOS QP3 is coupled to the first end of the switch SW2 for coupling to the voltage source V2. The second end of the PMOS QP3 is coupled to the second end of the switch SW2 for coupling to the output end of the output circuit 300. The control end of the PMOS QP3 is coupled to the output end of the inverter INV3 for receiving the inverted pulse signal PR. When the PMOS QP3 receives the inverted pulse signal PR, the first end of the PMOS QP3 is coupled to the second end of the PMOS QP3 for transmitting the voltage V2 to the output end of the output circuit 300. The first end of the NMOS QN3 is coupled to the first end of the switch SW2 for coupling to the voltage source V2. The second end of the NMOS QN3 is coupled to the second end of the switch SW2 for coupling to the output end of the output circuit 300. The control end of the NMOS QN3 is coupled to the control end C of the switch SW2 for receiving the pulse signal PR. When the NMOS QN3 receives the pulse signal PR, the first end of the NMOS QN3 is coupled to the second end of the NMOS QN3 for transmitting the voltage V2 to the output end of the output circuit 300. In another embodiment of the present invention, a terminated resistor can be disposed between the first end of the switch SW2 and the voltage source V2 for increasing the integrity of the signal. The terminated resistor can be a normal resistor or a MOS resistor.
To sum up, the output circuit provided by the present invention, with the falling-edge trigger circuit, rising-edge trigger circuit, and voltage sources, reduces the overshoot problem on the output signal so as to reduce the damage to the components, which provides great convenience.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.

## Claims

1. An output circuit with overshoot-reducing function,
the output circuit comprising:
an input end for receiving an input signal;
an output end;
a PMOS comprising:
a first end coupled to a first voltage source supplying a first voltage;
a control end coupled to the input end; and
a second end directly coupled to the output end;

an NMOS comprising:
a first end coupled to a second voltage source supplying a second voltage;
a control end coupled to the input end; and
a second end directly coupled to the output end;

a rising-edge trigger biasing circuit coupled between the input end and the output end for outputting a third voltage to the output end so as to reduce overshoot on the output end according to a rising edge of the input signal, the rising-edge trigger biasing circuit comprising:
a rising-edge trigger circuit coupled to the input end for generating a pulse signal with a predetermined period according to the rising edge of the input signal; and
a biasing circuit coupled between the output end and the rising-edge trigger circuit for providing the third voltage to the output end according to the pulse signal generated by the rising-edge trigger circuit, the biasing circuit of the rising-edge trigger biasing circuit comprising:
a third voltage source for supplying the third voltage; and
a second switch comprising a first end directly coupled to the third voltage source, a control end coupled to the rising-edge trigger circuit for receiving the pulse signal generated by the rising-edge trigger circuit, and a second end directly coupled to the output end for coupling the third voltage source to the output end according to the pulse signal generated by the rising-edge trigger circuit;


a falling-edge trigger biasing circuit coupled between the input end and the output end for outputting a fourth voltage to the output end so as to reduce overshoot on the output end according to a falling edge of the input signal, the falling-edge trigger biasing circuit comprising:
a falling-edge trigger circuit coupled to the input end for generating a pulse signal with a predetermined period according to the falling edge of the input signal; and
a biasing circuit coupled between the output end and the falling-edge trigger circuit for providing the fourth voltage to the output end according to the pulse signal generated by the falling-edge trigger circuit, the biasing circuit of the falling-edge trigger biasing circuit comprising:
a fourth voltage source for supplying the fourth voltage; and
a first switch comprising a first end directly coupled to the fourth voltage source, a control end coupled to the falling-edge trigger circuit for receiving the pulse signal generated by the falling-edge trigger circuit, and a second end directly coupled to the output end for coupling the fourth voltage source to the output end according to the pulse signal generated by the falling-edge trigger circuit;


an inverter coupled to the input end for generating an inverted input signal according to the input signal;
a first delay circuit coupled between the inverter and the control end of the PMOS for delaying the inverted input signal for a first predetermined period; and
a second delay circuit coupled between the inverter and the control end of the NMOS for delaying the inverted input signal for a second predetermined period;
wherein the second predetermined period is different from the first predetermined period;
wherein the third voltage is higher than the second voltage, and the fourth voltage is lower than the first voltage.

2. The output circuit of claim 1, wherein the third voltage is between average of the first voltage and the second voltage and the second voltage, and the fourth voltage is between average of the first voltage and the second voltage and the first voltage.

3. The output circuit of claim 1, wherein the PMOS is turned on for enabling the output end to couple to the first voltage source only after the switch is turned on for a predetermined period.

4. The output circuit of claim 1, wherein the biasing circuit of the rising-edge trigger biasing circuit further comprises a terminated resistor coupled between the third voltage source and the second switch.

5. The output circuit of claim 1, wherein the second switch is an NMOS, the biasing circuit of the rising-edge trigger biasing circuit further comprising:
an inverter coupled to the rising-edge trigger circuit for receiving the pulse signal generated by the rising-edge trigger circuit and accordingly generating an inverted pulse signal; and
a PMOS comprising:
a first end coupled to the third voltage source;
a control end coupled to the inverter for receiving the inverted pulse signal; and
a second end coupled to the output end for coupling the third voltage source to the output end according to the inverted pulse signal.

6. The output circuit of claim 1, wherein the NMOS is turned on for enabling the output end to couple to the second voltage source only after the switch is turned on for a predetermined period.

7. The output circuit of claim 1, wherein the biasing circuit of the falling-edge trigger biasing circuit further comprises a terminated resistor coupled between the fourth voltage source and the first switch.

8. The output circuit of claim 1, wherein the first switch is an NMOS, the biasing circuit of the falling-edge trigger biasing circuit further comprising:
an inverter coupled to the falling-edge trigger circuit for receiving the pulse signal generated by the falling-edge trigger circuit and accordingly generating an inverted pulse signal; and
a PMOS comprising:
a first end coupled to the fourth voltage source;
a control end coupled to the inverter for receiving the inverted pulse signal; and
a second end coupled to the output end for coupling the fourth voltage source to the output end according to the inverted pulse signal.

9. The output circuit of claim 1, wherein the first delay circuit comprises N inverters, the second delay circuit comprises M inverters, N and M are even numbers, and N is different from M.