fig2 is a diagram of the first embodiment of the network transmitter according to the present invention . the transmitter 20 in fig2 is for transmitting differential transmission signals . as shown in fig2 , the transmitter 20 comprises a signal converter 21 , a voltage - mode driver 22 , a calibration circuit 23 , and a switch circuit 24 . the signal converter 21 converts an input signal s 1 into a differential current signal , and the voltage - mode driver 22 outputs a differential transmission signal t x + , t x − , via the matching resistors r m1 , r m2 and the pads p , according to the differential current signal . in a preferred embodiment , the values of the matching resistors r m1 , r m2 are adjusted in accordance with outside circuits . in an embodiment , please refer to the u . s . patent application ser . no . 10 / 978 , 628 , filed nov . 1 , 2004 , titled “ network transmitting unit with correction function ”, which has the same assignee with the present invention and is hereby incorporated by reference . the signal converter 21 switches between a first mode and a second mode , and outputs a differential current signal having a plurality of levels . when the signal converter 21 operates in the first mode ( i . e . the transmission rate is 1 gbps ), a direct level conversion is performed . that is , the differential current signal is directly changed from a level to a next level , without being changed to any intermediate level during the conversion . on the other hand , when the signal converter 21 operates in the second mode ( i . e . the transmission rate is 100 mbps or 10 mbps ), an indirect level conversion is performed . that is , the differential current signal is changed to at least one intermediate level during the conversion from a level to a next level . the number and the change sequence of the intermediate level are determined according to a rise time ( or fall time ) of the differential current signal , thereby conforming to the related ethernet specification . in one embodiment , the signal converter 21 is a digital - to - analog converter ( dac ). in another embodiment , the signal converter 21 comprises a current cell module including a plurality of current cells , and each level of the outputted differential current signal is corresponding to a possible configuration of the current cells . the current cells can be implemented by switched current cells , each provides a substantially equal current and is connected or disconnected by a switch . in the first mode ( i . e . direct level - converting mode ), 16 current cells are used since the signal converter 21 is required to provide a differential current signal having 17 levels . fig3 a and 3b are diagrams showing how the signal converter 21 operates in the first mode . in fig3 a , the 16 current cells provide 17 possible current cell configurations . a current signal with 17 levels can then be generated by calculating the total current for each of the 17 current cell configurations . in one embodiment , each current cell is coupled to a flip - flop , which generates a corresponding control signal according to the corresponding bit of a received input signal and a clock signal , as shown in fig3 b . fig3 c is a partial circuit diagram of the signal converter 21 . in fig3 c , q and { overscore ( q )} ( the inverse of q ) are the corresponding control signals outputted by the flip - flops in fig3 b . in sum , when the signal converter 21 operates in the first mode , the level of the outputted differential current signal changes once according to the input signal value during each clock period , without changing to any intermediate level . when the transmission rate is 1 gbps , the signal converter 21 does not need to perform the slew rate control since the voltage - mode driver 22 uses a feedback network with a specific configuration , as described later . on the other hand , the 100 mbps ethernet specification requires a three - level differential transmission signal . thus , the 16 current cells of the signal converter 21 are divided into two groups , each of which contains eight current cells , to provide three current cell configurations , as shown in fig4 a . in fig4 a , 1 and 0 represent that the corresponding group of current cells are all connected and disconnected , respectively . since the signal converter 21 operates in the second mode ( i . e . indirect level - converting mode ) at 100 mbps transmission rate , the current cells within each group become all connected or disconnected in a step - by - step manner . fig4 b shows that the current cells ( cell & lt ; 7 : 0 & gt ;) within one group become all connected or disconnected in four steps . in fig4 b , four clock signals with various phases , denoted as clock & lt ; 3 & gt ; . . . clock & lt ; 0 & gt ;, are applied in turn to control the eight current cells ( each clock signal for two cells ). another group of current cells ( cell & lt ; 15 : 8 & gt ;) operates in the same manner synchronously . thus , the differential current signal would be changed to three intermediate levels in turn during the conversion from a level to a next level . by adjusting the phase difference among the four clock signals , the time spent for the conversion can be controlled to conform to the related ethernet specification . the voltage - mode driver 22 receives the differential current signal from the signal converter 21 , and outputs the differential transmission signal . as shown in fig2 , the voltage - mode driver 22 comprises a differential amplifier 221 and a feedback network 222 . in the feedback network 222 , c f1 and c f2 are adjustable capacitors adjusted by the calibration circuit 23 before the transmitter 20 starts transmission , as described later . the feedback network 222 also includes switches sw 1 and sw 2 , coupled to the feedback capacitors c f1 and c f2 respectively . the feedback network 222 switches between a first configuration and a second configuration , so as to provide two various transfer characteristics . the first configuration is a resistor - capacitor network ( rc network ). that is , the switches sw 1 and sw 2 are kept connected such that c f1 and c f2 are parallel connected with r f1 and r f2 respectively . the second configuration is a resistor network . that is , the switches sw 1 and sw 2 are kept disconnected such that c f1 and c f2 are also disconnected from the feedback network 222 . when the transmitter 20 operates in 1 gbps , the signal converter 21 switches to the first mode ( direct level - converting mode ) and the feedback network 222 switches to the first configuration ( i . e . rc network ). since the charge / discharge characteristic of the feedback capacitors c f1 and c f2 , the level of the differential transmission signal would change smoothly , not drastically ( the degree of smoothness is determined by the rc time constant of the rc network ). thus , by adjusting the capacitor values of c f1 and c f2 , the rise / fall time of the differential transmission signal can be controlled to meet the related specification . on the other hand , when the transmitter 20 operates in 100 mbps , the signal converter 21 switches to the second mode ( indirect level - converting mode ) and the feedback network 222 switches to the second configuration ( i . e . resistor network ). as mentioned earlier , by adjusting the phase difference among the clock signals used in the indirect level conversion , the time spent for the conversion from a level to a next level can be controlled to meet the related specification . the calibration circuit 23 is coupled to the feedback network 222 , and adjusts the rc time constant of the feedback network 222 before the transmitter 20 starts transmission . fig5 is a block diagram of one embodiment of the calibration circuit 23 . the calibration circuit 23 comprises : a first simulation circuit 231 for outputting a first simulation signal corresponding to the feedback resistor r f1 ; a second simulation circuit 232 for outputting a second simulation signal corresponding to the feedback capacitor c f1 ; a comparator 233 for outputting a comparison result according the first and second simulation signals ; and a control circuit 234 for generating a calibration signal according to the comparison result . the calibration signal is provided to the feedback network 222 to adjust the rc time constant thereof . in the embodiment of fig5 , the resistor r s is for simulating the values of the feedback resistors r f1 and r f2 of the feedback network 222 , and the capacitor c s is for simulating the values of the feedback capacitors c f1 and c f2 of the feedback network 222 . the first simulation signal is the voltage level at point a , i . e . v a = i 1 * r s ; the second simulation signal is the voltage level at point b , i . e . v b = i 2 / c s * t , wherein t is the time spent for the charging of the capacitor c s . in order to generate a specification - conforming rc time constant , the capacitor c s needs to cause the voltage levels of the points a and b to be equal after being charged for a predetermined time t . that is , r s * c s = i 2 / i 1 * t eq . ( 1 - 2 ) if the comparison result of the comparator 233 shows that eq . ( 1 - 1 ) is not met , the rc time constant needs an adjustment . for instance , if the result shows that v a is larger than v b , it means the charging of c s is not fast enough . thus , the capacitor value of c s should be decreased . the control circuit 234 adjusts the capacitor value of c s according to the comparison result ( the control circuit 234 is also coupled to c f1 and c f2 in fig5 ). when the capacitor value of c s is adjusted to meet eq . ( 1 - 1 ), eq . ( 1 - 2 ) represents the rc time constant required for the feedback network 222 . it is also applicable to adjust r f1 and r f2 only , or adjust r f1 , r f2 , c f1 , and c f2 in the meantime . in fig2 , the switch circuit 24 is for controlling the feedback network 222 to switch between the first and second configurations , and controlling the signal converter 21 to switch between the first and second modes , according to the transmission rate of the transmitter 20 . fig6 is a diagram of the second embodiment of the transmitter according to the present invention . as shown in fig6 , the second embodiment is a combination of the first embodiment in fig2 and a current - mode driver 61 . as to the principle and operation of an embodiment of the current - mode driver 61 , please refer to the u . s . patent application ser . no . 10 / 909 , 811 , filed aug . 2 , 2004 , titled “ network device with hybrid - mode transmitter ”, which has the same assignee with the present invention and is hereby incorporated by reference . while the present invention has been shown and described with reference to the preferred embodiments thereof and in terms of the illustrative drawings , it should not be considered as limited thereby . various possible modifications and alterations could be conceived of by one skilled in the art to the form and the content of any particular embodiment , without departing from the scope and the spirit of the present invention .