certain terms are used throughout the description and following claims to refer to particular components . as one skilled in the art will appreciate , manufacturers may refer to a component by different names . this document does not intend to distinguish between components that differ in name but not function . in the following description and in the claims , the terms “ include ” and “ comprise ” are used in an open - ended fashion , and thus should be interpreted to mean “ include , but not limited to . . . ”. also , the term “ couple ” is intended to mean either an indirect or direct electrical connection . accordingly , if one device is electrically connected to another device , that connection may be through a direct electrical connection , or through an indirect electrical connection via other devices and connections . please refer to fig1 , which is a diagram illustrating a constant current generating circuit 100 in accordance with an embodiment of the present invention . as shown in fig1 , the constant current generating circuit 100 is used to generate a constant current ic , and includes an operational amplifier 102 , a first current generating circuit 110 , a second current generating circuit 120 , a current mirror 130 , a switch module ( in this embodiment , the switch module includes switches sw 1 _ 1 , sw 1 _ 2 , sw 1 _ 3 , and sw 1 _ 4 ), and a calibration circuit 140 , wherein the first current generating circuit 110 includes a transistor m 1 , the second current generating circuit 120 includes a transistor m 2 and an adjustable resistor rc , the calibration circuit 140 includes a transmitting circuit 142 , a receiving circuit 144 , and a digital signal processor 146 . the digital signal processor 146 contains a plurality of electronic fuses ( efuses ) 148 . in this embodiment , the constant current generating circuit 100 is disposed in a chip , and a contact n 1 shown in fig1 is a contact of the chip . in a chip testing phase , the contact n 1 is used to connect an external resistor rext such that the first current generating circuit 110 generates a first current correspondingly . in addition , a contact n 2 shown in fig1 is a signal output contact of the chip for transmitting the signal outputted by the transmitting circuit 142 to the outside of the chip . in an embodiment of the present invention , the chip employing the constant current generating circuit 100 may be a network control chip , and the transmitting circuit 142 and the receiving circuit 144 may be part of an analog front end ( afe ) circuit of the chip . in addition , the transmitting circuit 142 , which is used to receive network data from the digital signal processor 146 , and transmit the received and processed network data to a transmission line outside the chip via the contact n 2 , may be implemented by a digital - to - analog converter ( dac ) ; besides , the receiving circuit 144 , which is used to receive network data from the contact n 2 and transmit the received and analog - to - digital converted network data to the digital signal processor 146 for subsequent processing , may be implemented by an analog - to - digital converter ( adc ). regarding a chip testing phase , please refer to fig2 . first , the constant current generating circuit 100 is connected to the external resistor rext via the contact n 1 , the switches sw 1 _ 1 and sw 1 _ 2 are turned on based on the control of the control signal vc 1 , and the switches sw 2 _ 1 and sw 2 _ 2 remain turned off based on the control of the control signal vc 2 , wherein the control signals vc 1 and vc 2 may be generated by the digital signal processor 146 or other signal sources . at this time , since the positive / non - inverting input node of the operational amplifier 102 is connected to a bandgap voltage vbg , the first current generating circuit 110 will generate a first current i1 with a current value equal to vbg / rext , and the current mirror 130 will duplicate the first current i1 to produce a mirrored current ibx . thereafter , the transmitting circuit 142 will convert the mirrored current ibx into a first voltage vox in accordance with a reference data di obtained from the digital signal processor 146 , wherein the reference data di is used to determine the ratio of the conversion from the mirrored current ibx to the first voltage vox that is performed by the transmitting circuit 142 . after that , the receiving circuit 144 will convert the first voltage vox into a first digital code dox , and then the first digital code dox is sent to the digital signal processor 146 and stored in the digital signal processor 146 . please refer to fig3 . after the first digit code dox is stored in the digital signal processor 146 , the switches sw 1 _ 1 and sw 1 _ 2 remain turned off based on the control of the control signal v c1 , the switches sw 2 _ 1 and sw 2 _ 2 are turned on based on the control of the control signal v c2 . at this time , since the positive / non - inverting input node of the operational amplifier 102 is connected to a bandgap voltage vbg , the second current generating circuit 120 will generate a second current i 2 with a current value equal to vbg / rc , and the current mirror 130 will duplicate the second current i 2 to generate a mirrored current ibc . thereafter , the transmitting circuit 142 will convert the mirrored current ibc into a second voltage voc in accordance with the reference data di obtained from the digital signal processor 146 . next , the receiving circuit 144 will convert the second voltage voc into a second digital code doc , and then the second digital code doc is sent to the digital signal processor 146 and stored in the digital signal processor 146 . then , since the first digital code dox and the second digital code doc stored in the digital signal processor 146 represent the current values of the first current i 1 and the second current i 2 respectively , the digital signal processor 146 can generate a correction code dcc according to the first digital code dox and the second digital code doc to adjust the resistance value of the adjustable resistor rc , thereby allowing the current generated by the second current generating circuit 120 to be close to the current generated by the first current generating circuit 110 as much as possible . for example , the digital signal processor 146 may utilize the code values or the code difference of the first digit code dox and the second digital code doc to search a look - up table for the correction code dcc used to adjust the adjustable resistor rc ; or the digital signal processor 146 may generate different correction codes dcc ( which have different code values ) continuously to adjust the resistance value of the adjustable resistor rc , such that the current i 2 generated by the second current generating circuit 120 and the corresponding second digital code doc would change continuously until the second digital code doc is very close to the first digital code dox . through the above - described adjustment , the resistance value of the adjustable resistor rc will be very close to the resistance value of the external resistor rext . therefore , the current i 2 generated by the second current generating circuit 120 will be very close to the current i 1 generated by the first current generating circuit 110 . at this point , the digital signal processor 146 may utilize the electronic fuse 148 to record the current correction code dcc . therefore , in the subsequent use of the chip , the resistance value of the adjustable resistor rc is fixed since the correction code dcc is fixed by the electronic fuse 148 . in this way , the chip can utilize the second current generating circuit 120 to generate a desired constant current ic . since the external resistor is no longer needed in the subsequent use of the chip , the subsequent production cost is reduced . in addition , due to the fact that the calibration circuit 140 of the constant current generating circuit 100 is implemented using the transmitting circuit 142 and the receiving circuit 144 of the chip per se , there is no need to add additional calibration circuits in the chip , thus reducing the cost of the chip design and manufacture . however , it should be noted that although the calibration circuit 140 is implemented using the transmitting circuit 142 and the receiving circuit 144 of the chip per se according to the embodiment in fig2 , the present invention is not limited thereto . in other embodiments of the present invention , the calibration circuit 140 may be an independent calibration circuit in a chip and may have other types of calibration circuit design . to put it another way , the calibration circuit 140 may be implemented without using the transmitting circuit 142 and the receiving circuit 144 of the chip per se . these design changes should also belong to the scope of the present invention . please refer to fig4 , which is a flowchart illustrating a method of generating the constant current according to an embodiment of the present invention . referring to fig1 - 4 and the disclosed contents directed to fig1 - 3 , the flow is described as below : step 400 : provide a chip , wherein the chip includes a first current generating circuit and a second current generating circuit , and the second current generating circuit includes a transistor and an adjustable resistor ; step 402 : connect an external resistor to the first current generating circuit such that the first current generating circuit may utilize the external resistor to generate a first current ; step 404 : utilize the second current generating circuit to generate a second current ; step 406 : adjust the resistance value of the adjustable resistor in accordance with the first current and the second current , such that the second current is substantially equal to the first current , and the second current is used as a constant current in the chip . in summary , the constant current generating circuit and associated method of the present invention can adjust the resistance value of an adjustable resistor in a chip to be close to the resistance value of an external resistor . in this way , the chip can use the calibrated internal resistor to produce a reliable constant current . as there is no need for an external resistor , the proposed design does reduce the following production cost . in addition , the calibration circuit of the constant current generating circuit of the present invention can be implemented using the transmitting circuit and the receiving circuit of the chip per se . therefore , additional hardware of the calibration circuit is not needed at all , which further reduces the cost of the chip design and manufacture . 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 . accordingly , the above disclosure should be construed as limited only by the metes and bounds of the appended claims .