Abstract:
A transmitter for transmitting a transmission signal is disclosed. The transmitter includes: a gain stage, for receiving an input signal and amplifying the input signal according to a gain to generate an amplified signal; and an output stage, coupled to the gain stage, for receiving a first reference voltage signal and the amplified signal and utilizing the first reference voltage signal to perform a predetermined operation on the amplified signal to generate the output signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transmitter, and more particularly, to a transmitter having low power consumption. 
     2. Description of the Related Art 
     In the field of signal transmissions, in order to allow the signal to be successfully transferred from a transmitter to a receiver, the impedance matching should be considered when the transmitter and the receiver are being designed. In other words, the impedances of the transmitter and the receiver are designed to match the characteristic impedance of the transmission medium (ex: cable). 
       FIG. 1  shows a block diagram of a transmission system according to the prior art. As shown in  FIG. 1 , assume that the equivalent impedance of the transmission medium  110  is R. Therefore, the impedances of the transmitter  100  and the receiver  120 , which correspond to the transmission medium side, are substantially equal to the impedance R of the transmission medium. This helps prevent the transmission signal from reflection, and optimizes the transmission efficiency (that is, the voltage level Va of the node A is substantially equal to the voltage level Vb of the node B). 
     As is known, a conventional transmitter can be divided into two types of transmitters, the current-mode transmitter and the voltage-mode transmitter. Please refer to  FIG. 2 , which is a diagram showing a current-mode transmitter  200  and a receiver  220 . Here, assume that the impedance of the transmission medium  210  is R. Therefore, for the purpose of impedance matching, the equivalent input impedance Rb of the receiver  220  is equal to R. When the signal is being transferred, the voltage level Va of the node A is equal to the voltage level Vb of the node B. At this time, for the transmitter  200 , the impedance of the input impedance Ra is also equal to R. Therefore, the current outputted from the transmitter  200  is 2Vb(t)/R. Furthermore, in order to make sure that the current can be definitely outputted, the working voltage Vdd of the transmitter  200  must be larger or equal to the maximum of Vb(t). Therefore, the power consumption of the entire transmitter  200  can be represented by the following equation (1):
 
Power consumption≧ Vb ( t ) max *2 Vb ( t )/ R   equation (1)
 
     Please refer to  FIG. 3 , which is a diagram showing a voltage-mode transmitter  300  and a receiver  320 . Similarly, the equivalent input impedance Rb of the receiver  320  is equal to the impedance R of the transmission medium. The voltage level Va of the node A is equal to the voltage level Vb of the node B. At this time, for the transmitter  300 , the impedance of the input impedance Ra is equal to R. Therefore, the current outputted form the transmitter is Vb(t)/R. For the node C, the voltage level Vc(t) of the node C is equal to 2Vb(t). Therefore, in order to make sure that the current of the transmitter  300  can be outputted. The working voltage Vdd of the transmitter  300  should be larger or equal to the maximum of 2Vb(t). And the power consumption of the transmitter  300  can be represented by the following equation (2):
 
Power consumption≧2 Vb ( t ) max   *Vb ( t )/ R   equation (2)
 
     Obviously, when the current-mode transmitter is utilized, the needed current is larger, but needed working voltage is lower. On the other hand, when the voltage-mode transmitter is utilized, the needed working voltage is larger, but the needed current is lower. Please refer to equations (1) and (2), it is easily seen that regardless that the of whether above-mentioned current-mode transmitter and voltage-mode transmitter are utilized, the lowest power consumptions of the current-mode transmitter and voltage-mode transmitter are both equal to 2Vb(t) max *Vb(t)/R, which needs to be reduced. 
     SUMMARY OF THE INVENTION 
     It is therefore one of the objectives of the claimed invention to provide a transmitter to solve the above-mentioned problems. 
     It is therefore one of the objectives of the claimed invention to provide a transmitter to reduce the power consumption. 
     According to an exemplary embodiment of the present invention, a transmitter for transferring a transmission signal through a transmission medium is disclosed. The transmitter comprises: a gain stage, for receiving an input signal and generating a first signal according to the input signal; and an output stage for receiving the first signal and outputting the transmission signal according to the first signal, wherein the output stage operates according to a first working voltage; wherein a voltage level of the first working voltage is less than twice of the maximum of the transmission signal. 
     These and other objectives of the claimed 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 block diagram of a transmission system according to the prior art. 
         FIG. 2  is a diagram of a current-mode transmitter according to the prior art. 
         FIG. 3  is a diagram of a voltage-mode transmitter according to the prior art. 
         FIG. 4  is a simplified diagram of a transconductance circuit. 
         FIG. 5  is a diagram of a transmitter according to the present invention. 
         FIG. 6  is a diagram showing an embodiment of the transmitter shown in  FIG. 5 . 
         FIG. 7  is a diagram showing another embodiment of a transmitter according to the present invention. 
         FIG. 8  is a diagram showing the other embodiment of a transmitter according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before the structure of the present invention transmitter is disclosed, please refer to  FIG. 4  first.  FIG. 4  is a simplified diagram of a transconductance cell  400 . For the transconductance circuit  400 , the relationship between the output current and the input voltage can be represented as the following equation (3):
 
 I (out)= Gm *( Vp−Vn )  equation (3)
 
     Therefore, as shown in  FIG. 4 , when the output end of the transconductance cell  400  is coupled to the negative input end of the transconductance cell  400 , a feedback circuit is formed. The impedance of transconductance cell  400 , which corresponds to the input end is equal to 1/Gm. 
     Please refer to  FIG. 5 , which is a diagram of a transmitter  500  according to the present invention. In this embodiment, the impedance of the transmission medium  510  is also assumed as R. Furthermore, in order to achieve the purpose of impedance matching, the input impedance of the receiver  520  is also adjusted as R. Please note that, as shown in  FIG. 5 , the present invention transmitter  500  is implemented as the above-mentioned transconductance cell structure. 
     Obviously, in order to achieve the purpose of impedance matching, the transconductance Gm of the transmitter  500  should be equal to 1/R. Furthermore, in this embodiment, assume that the signal transferred to the receiver  520  is Vb(t). For the node B, the current flowing through the node B is equal to Vb(t)/R. Obviously, the output current of the transmitter  500  is also equal to Vb(t)/R. In addition, as mentioned previously, the voltage level Va of the node A is equal to the voltage level Vb(t). According to the equation (3), it can be figured out that the needed input signal Vp of the transmitter  500  substantially corresponds to 2Vb(t). 
     In one embodiment, the present invention transmitter  500  utilizes two working voltages (shown as a high working voltage Vdd_h and a low working voltage Vdd_l) to reduce to the power consumption of the transmitter  500 . For example, please refer to  FIG. 6 , which is a diagram of an embodiment of the transmitter  500  shown in  FIG. 5 . As shown in  FIG. 6 , the transmitter  500 , which is implemented by utilizing transconductance circuit, can be divided into a gain stage  610  and an output stage  620 . The gain stage  610  utilizes the working voltage Vdd_h to operate such that the gain stage  610  can perform a gain operation (for example, to amplify) on the input signal 2Vb(t), and the processed signal is outputted to the node C and the node D. In addition, the output stage  620  operates by utilizing another working voltage Vdd_l. The output stage  620  comprises two transistors Mp and Mn for receiving the signal from the nodes C and D outputted by the gain stage  610 . Obviously, because the gate voltage of the transistors Mp and Mn are controlled by the output signal of the gain stage  610 , the current outputted by the output stage  620  is also controlled by the output signal of the gain stage  610  according to the characteristics of the transistors Mp and Mn. As known by those skill in the art, as long as the parameters (here, the parameter can include the characteristics of the transistors Mp and Mn and the output voltage of the gain stage) are well defined, the current outputted by the transmitter  500  can be under control as Vb(t)/R. In this way, the voltage level Vb(t) can be established in the receiver  510  such that the purpose of transmitting signals can be achieved. 
     Please note that, the gain stage  610  and the output stage  620  utilize different working voltages (it means that the working voltages have different voltage levels, for example, as mentioned previously, they can correspond to a high working voltage and a low working voltage). As is known, the output stage  620  should output the transmission signal and therefore expends more power. On the other hand, because the current of the gain stage  610  can be designed as a very small current value, the power consumption of the gain stage  610  is much smaller than that of the output stage  620 . In other words, the main power consumption of the transmitter  500  is spent by the output stage  620 . However, the present invention output stage  620  utilizes a lower working voltage Vdd_l to operate. Therefore, the power consumption of the entire transmitter is also smaller. 
     In this embodiment, for the working voltage Vdd_h, the working voltage Vdd_h should be large enough to allow the gain stage to work normally. But on the other hand, the voltage level of the output stage  620  is Vb(t). Therefore, the working voltage Vdd_l should be larger than or equal to the maximum voltage level Vb(t) max  of the node A. In addition, the current outputted by the output stage  620  is Vb(t)/R. That is, the working voltage Vdd_l should be as low as possible but cannot be lower than the maximum voltage Vb(t)max of the node A such that the power consumption can be reduced. Therefore, in this embodiment, the working voltages Vdd_h and Vdd_l are not the same. 
     The power consumption of the output stage  620  can be represented as the following equation (4):
 
Power consumption= Vb ( t ) max *( Vb ( t )/ R )  equation (4)
 
     As mentioned previously, most of the power consumption is consumed by the output stage  520 . Therefore, the power consumption of the transmitter  500  is about Vb(t).sub.max*(Vb(t)/R). In contrast to the prior art transmitter, as shown in equation (4), the transmitter  500  only has half power consumption. 
     Please note that, the present invention does not limit the voltage level of the working voltage Vdd_h, and the working voltage Vdd_h can be set as low as possible as long as the gain stage  610  can operate normally. Moreover, the present invention does not limit the voltage level of the working voltage Vdd_l. As mentioned previously, the working voltage Vdd_l only needs to be larger than or equal to the maximum Vb(t) max  of the transmission signal such that the power consumption of the transmitter  500  can be reduced. 
     In addition, please note that, the present invention does not limit the implementations of the gain stage  610  and the output stage  620 , The circuit designer can change the inner circuits of the transmitter  500  according to different demands. For example, in the above-mentioned embodiment, the transmitter  500  is a single-ended circuit, but in the actual implementation, the transmitter  500  can also be a differential circuit. 
     Please refer to  FIG. 7 , which is a diagram showing detailed circuits of a transmitter  500  of another embodiment according to the present invention. As shown in  FIG. 7 , the transmitter  500  is implemented as a differential circuit, which comprises a gain stage  710  and an output stage  720 , also. The gain stage  710  comprises a transconductance cell  711  and an impedance device  712 . As mentioned previously, the transconductance cell  711  can perform a transconductance operation on the received input signal Vip and Vin. In addition, the differential circuit, which is in the right side of the transconductance cell  711 , corresponds to the feedback circuit shown in  FIG. 4 . In this embodiment, the impedance device  712  transforms the current signal outputted by the transconductance cell  711  into a voltage signal, which is used to control the gates of the transistors of the output stage  720 . Therefore, needed output current Vb(t)/R can be outputted. Please note, the impedance device  712  can be implemented by a transistor. 
     In this embodiment, the needed working voltage of the output stage  720  only has to be larger than or equal to Vb(t) max  such that the transmitter  500  can have low power consumption. 
     Please note that, the impedance device  712  is an optional device. As is known, as long as the working voltage can be set appropriately, the impedance device  712  is no longer utilized. That is, the output of the transconductance cell  711  can be directly utilized to drive the output stage  720 . This change also follows the spirit of the present invention. 
     Please refer to  FIG. 8 , which is a diagram showing detailed circuits of a transmitter  500  of the other embodiment according to the present invention. Please note that, different from the embodiment shown in  FIG. 7 , in the transconductance cell  811 , one resistor is implemented by a 0.5 R resistor. Therefore, the differential input signal only has to correspond to Vb(t). Obviously, because the input signal of the gain stage  810  only corresponds to Vb(t), the working voltage of the gain stage  810  only needs to correspondingly correspond to Vb(t). In other words, the gain stage  810  and the output stage  820  can share the same working voltage Vdd_l. In this embodiment, because the above-mentioned structure only needs the same working voltage, the transforming circuit for changing the voltage level of the working voltage is not utilized. This can further reduce the complexity of the entire circuit. 
     Please note that, similar to the embodiment shown in  FIG. 7 , in this embodiment, the transistor  812  is also an optional device. As mentioned previously, if parameters (e.g: the working voltage) can be set appropriately, the impedance device  812  is no longer utilized. And, the output of the transconductance cell  811  can be directly utilized to drive the output stage  820 . This change is also obeys consistent with the spirit of the present invention. 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention should not be limited to the specific construction and arrangement shown and described, since various other modifications may occur to those ordinarily skilled in the art.