Abstract:
A high speed receiver circuit is disclosed with a high supply voltage and operable with other circuits operating at a low supply voltage. The receiver circuit comprises first and second differential input signals controlling first and second current switches. It also includes a top current supply connected to the high supply voltage for providing a current to be passed either through the first current switch and a first bottom current supply or the second current switch and a second bottom current supply. Further included are first and second resistors connected to the low supply voltage and in a series with the first or second bottom current supplies respectively. First and second differential output signals are produced at a point between each pair of the resistors and the bottom current supply. A common mode voltage of the first and second differential output signals is lower than the low supply voltage.

Description:
BACKGROUND 
   The present invention relates generally to semiconductor devices, and more particularly, to high-speed receiver semiconductor devices. Still more particularly, the present invention relates to the circuit and method of using thin and thick film gate oxide MOSFETs to construct wide common mode, high-speed differential receivers, in deep sub-micron technology semiconductor devices. 
   Differential signaling has been utilized for many years as a data transmission method. A differential receiver converts and amplifies a differential input signal (IN+ and IN−) to a differential output signal (VOUT+ and VOUT−). These receivers offer high data transmission speeds, low noise coupling, and low EMI (electromagnetic interference). For embodiment, teletype equipment was some of the first types of equipment to use differential signaling to communicate. Today, computers often communicate between ports by low voltage differential signal (LVDS) drivers and receiver pairs. In addition to the LVDS data transmission technology, emitter coupled logic (ECL), common mode logic (CML), and hyper-transport (high-bandwidth chip-to-chip technology) technology are utilized for data transmission methods. Typical differential signal transmission speeds are over 100 Mbps (mega bits per second). In each of these transmission methods, high speed, wide common mode, voltage differential receivers are necessary building blocks to attain the required data transmission speeds while meeting the low noise coupling, and the low EMI requirements. 
   Semiconductor technology is evolving into the deep sub-micron geometries of less than 100 nanometers (nm). This technology is needed to produce today&#39;s portable devices such as cellular telephones, laptops, and other portable electronic devices. The smaller geometry gates of less than 100 nm offer more complex functionality and higher performance, but not without a cost. As the nanometer gate geometry becomes smaller, its power dissipation increases dramatically, hence the battery power drain increases significantly. 
   Conventional wide common mode, high-speed differential receivers utilize thick gate oxide MOSFETs in deep sub-micron technology devices for all required series amplifier stages. Each of these stages is designed to operate from high voltage power supplies (VDDH) of approximately 3.3VDC to maximize data transmission speeds. A final translation stage must be implemented to convert the 3.3VDC (VDDH) supply voltage to 1.2VDC (VDDL) to interface with the subsequent digital logic devices using the 1.2VDC (VDDL) supply voltage. These multiple high-speed receiver amplifier stages that use the VDDH supply voltage dissipate much more power than receivers that use the VDDL supply voltage. 
   Desirable in the art of wide common mode, high-speed receiver designs are additional designs that reduce power dissipation while still meeting or exceeding the high-speed data transmission requirements. 
   SUMMARY 
   In view of the foregoing, this invention provides a circuit and method of using both thin and thick film gate oxide MOSFETs to construct a wide common mode, high-speed differential receiver in deep sub-micron technology semiconductor devices. 
   In one embodiment of this invention, a circuit and method comprised of a pre-amplifier stage utilizing both thick and thin film MOSFETS is presented to minimize the circuit&#39;s power dissipation while meeting the high-speed data transmission requirements. Such a circuit has a high supply voltage and is operable with additional circuits operating at a low supply voltage. The receiver circuit comprises a first and second differential input signals controlling a first and second current switches. It also includes a top current supply connected to the high supply voltage for providing a current to be passed either through the first current switch and a first bottom current supply or the second current switch and a second bottom current supply. Further included are a first and second resistors connected to the low supply voltage and in series with the first or second bottom current supplies respectively. A first and second differential output signals are produced at a point between each pair of the resistors and the bottom current supply. A common mode voltage of the first and second differential output signals is lower than the low supply voltage. 
   Although the invention is illustrated and described herein as embodied in a circuit and method of using both thin and thick film gate oxide MOSFETs to construct wide common mode high-speed differential receivers in deep sub-micron technology semiconductor devices, it is, nevertheless, not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention, and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  presents a wide common mode, high-speed differential receiver in accordance with one embodiment of the present invention. 
       FIG. 2  presents a diagram illustrating the current flow through the wide common mode, high-speed differential receiver in accordance with one embodiment of the present invention. 
       FIG. 3  presents a graph illustrating the relationship between the common mode input voltage, and the common mode output voltage, of the high-speed differential receiver, in accordance with one embodiment of the present invention. 
       FIG. 4  illustrates the amplifier stages of a conventional “low voltage differential signal” (LVDS) receiver and the common mode voltages of the input, intermediate, and output stages of the LVDS receiver. 
       FIG. 5  illustrates the amplifier stages of the wide common mode, high-speed differential receiver, in accordance with one embodiment of the present invention. 
   

   DESCRIPTION 
   In the present invention, a circuit and method comprised of a pre-amplifier stage utilizing both thick and thin film MOSFETs is presented to minimize the high-speed differential receiver circuit&#39;s power dissipation while meeting the high-speed data transmission requirements, and also provide supply voltage translation from a high supply voltage VDDH, to a low supply voltage VDDL, for subsequent amplifier stages and digital logic. 
     FIG. 1  presents a wide common mode high-speed differential receiver  100  using both thin and thick gate oxide MOSFETs in accordance with one embodiment of the present invention. Two PMOS transistors  102  and  104  are thick gate oxide transistors that form a source coupled differential transistor pair or input current switches that receive the differential input signals RX+ and RX−. A PMOS transistor  106  is also a thick gate oxide transistor, which is connected to the VDDH supply voltage and is a constant current source for both transistors  102  and  104 . Transistor  106  may be referred to as a top current supply. Transistors  102 ,  104 , and  106  are selected to be thick gate oxide transistors because they can operate at higher supply voltages (VDDH) while minimizing the leakage current, thereby reducing power dissipation. An NMOS transistor  108  is a thin gate oxide transistor that may be seen as a constant current source for the current path through transistors  106 ,  102 , and  108 , while an NMOS transistor  110  is also a thin gate oxide transistor that may be seen as another constant current source for the current path transistors  106 ,  104 , and  110 . The NMOS transistors  108  and  110  are referred to as bottom current supplies in view of the existence of the top current supply. Both current paths, as defined above and which will be further described in  FIG. 2 , are connected to VSS, or ground. 
   If the differential input signals RX+ and RX− are equal, then the current paths of  106 ,  102 ,  108  and  106 ,  104 ,  110  are equal, and the output signals OUT+ and OUT− are equal. If, however, the differential input signals RX+ and RX− are not equal, then the receiver  100  will amplify the difference and apply it to the output signals OUT+ and OUT−. It is noted that the output signals OUT+ and OUT− are pulled to the supply voltage VDDL by resistors  112  and  114 , respectively. Due to the function of these resistors, they are referred to as current-to-voltage converters. It is further understood any other circuit module that provides such a function can replace these resistors. Moreover, the change of output signal level provides the translation of the receiver supply voltage from the input stage VDDH to the output stage VDDL. Therefore, all subsequent amplifier stages can utilize thin gate oxide transistors that operate from the supply voltage VDDL. The thin gate oxide transistors provide higher speed and gain than the thick gate oxide transistors, but cannot operate from the higher supply voltage VDDH. Since the subsequent amplifier stages can utilize the VDDL supply voltage, the overall circuit power dissipation can be reduced significantly, and no additional supply voltage translation is necessary to interface with the subsequent digital logic. 
     FIG. 2  presents a diagram  200  that shows the current flow through the wide common mode, high-speed differential receiver  100  as presented in  FIG. 1 . In a static condition, with no differential input signal, the current I 202  flows from VDDH through the transistor  106  and divides equally between the transistors  102  and  104  (current I 204  and current I 206  respectively). The current I 208  is the sum of the current through the transistor  102  (current I 204 ) and the current through the resistor  112  (current I 212 ). Similarly, the current I 210  is the sum of the current through the transistor  104  (current I 206 ) and the current through the resistor  114  (current I 214 ). 
   When RX+ is lower than RX−, the transistor  102  has higher conductivity thereby allowing additional current to flow to the output signal OUT−, thereby avoiding the current path containing the transistor  104 . The current through the resistors  112  and  114 , as well as the voltages at the nodes OUT+ and OUT− are calculated as follows:
 
I 204 =I 202 
 
 I   212   =I   208   −I   202 
 
I 214 =I 210 
 
 V (OUT−)= VDDL−I   212   *R   112   =VDDL −( I   208   −I   202 )* R   112 
 
 V (OUT+)= VDDL−I   210   *R   114 
 
   When RX+ is higher than RX−, the transistor  104  has higher conductivity, thereby allowing additional current to flow through to the output signal OUT+. The current through the resistors  112  and  114 , as well as the voltages at OUT+ and OUT− are calculated as follows:
 
I 206 =I 202 
 
I 212 =I 208 
 
 I   214   =I   210   −I   202 
 
 V (OUT−)= VDDL−I   212   *R   112   =VDDL−I   208   *R   112 
 
 V (OUT+)= VDDL −( I   210   −I   202 )* R   114 
 
     FIG. 3  presents a graph  300  illustrating the relationship between the common mode input voltage, and the common mode output voltage of the high-speed differential receiver  100 . In this embodiment, it is assumed the current going through the bottom current sources  108  and  110  are the same, and the two resistors are of the same value too. This relationship can be mathematically represented as:
 I 208 =I 210 =INCS R 112 =R 114 =R 
   It can be seen from the graph  300  that the input differential signals RX+ and RX− have a common mode voltage higher than VDDL and is amplified to produce the OUT+ and OUT− signals whose common mode voltage is lower than VDDL. A range  302  illustrates the input common mode voltage swing, while a range  304  illustrates the output common mode voltage swing. The maximum output voltage is calculated as Vmax=VDDL−INCS*R while the minimum output voltage is calculated as Vmin=VDDL−(INCS−I 202 )*R. Since the signals OUT+ and OUT− are lower than VDDL, the subsequent amplifier stages may utilize thin gate oxide transistor amplifiers. This results in significant power savings when compared to conventional common mode high-speed differential receivers. 
     FIG. 4  illustrates the amplifier modules/stages of a conventional “low voltage differential signal” (LVDS) receiver  400  using thick gate oxide transistors. The common mode voltages of a pre-amplifier stage/module  402 , intermediate gain stages/modules  404  and  406 , and an output translation stage/module  408  of the conventional LVDS receiver  400  are also shown. Below the conventional LVDS receiver  400  is a graph  410  of the various amplifier stage signals and a comparison of their associated voltage levels. It is noted that the stages  402 ,  404 , and  406  of the conventional LVDS receiver  400  utilize the supply voltage VDDH as well as thick gate oxide transistors. All stages utilize the same ground/VSS. The input differential signals RX+ and RX−, illustrated as signal  412 , have a higher common mode voltage than VDDL, thereby requiring thick gate oxide transistors. The output signals O 1 + and O 1 − of the pre-amplifier stage  402  are amplified as shown in signal  414 , but since it also has a common mode voltage higher than VDDL, thick gate oxide transistors must still be used. The intermediate gain stages  404  and  406  further amplify and produce the differential signals O 2 + and O 2 −, as well as O 3 + and O 3 −, respectively. The two differential pairs are further shown as signals  416  and  418  in the graph  410 . The output translation stage  408  utilizes the VDDL supply voltage to interface the conventional LVDS receiver  400  to the subsequent digital logic operating at the VDDL supply voltage. It is noted that only the output differential signals OUT+ and OUT−, which are products of the output translation stage  408  and are illustrated as signal  420  in the graph  410 , have a common mode differential voltage less that VDDL. Since the stages  402 ,  404  and  406  utilize the supply voltage VDDH, power dissipation may be significant. 
     FIG. 5  illustrates the amplifier stages of a wide common mode high-speed differential receiver  500  using both thin and thick gate oxide MOSFETs in a pre-amplifier stage  502 , while using thin gate oxide transistors in all subsequent amplifier stages (e.g., intermediate gain stages  504  and  506 , and output translation stage  508 ) in accordance with one embodiment of the present invention. The common mode voltages of the pre-amplifier stage  502 , the intermediate gain stages  504  and  506 , and the output translation stage  508  of the receiver  500  are also shown. Below the receiver  500  is a graph  510  of the various amplifier stage signals and a comparison of their associated voltage levels. It is noted that all stages (e.g., stages  502 ,  504 ,  506 , and  508 ) of the receiver  500  utilize the supply voltage VDDL. The pre-amplifier  502 , however, also utilizes the supply voltage VDDH. This circuit provides both the amplification of the differential signals RX+ and RX− and voltage translation from the VDDH to the VDDL supply voltage. All stages utilize the same ground VSS. Due to the fact that the voltage translation to VDDL occurs in the pre-amplifier stage  502 , all subsequent stages may utilize the reduced VDDL supply voltage and hence, utilize thin gate oxide transistors. The receiver  500  significantly reduces the power dissipation when compared to the conventional design, because VDDL supply voltage is used in more stages of the receiver  500 . 
   The input differential signals RX+ and RX− into the pre-amplifier stage  502  have a higher common mode voltage as shown by signal  512  than VDDL, which requires thick gate oxide transistors in this input stage. The output signals O 1 + and O 1 − of the pre-amplifier stage  502  are minimally amplified as shown in signal  514 , but now have a common mode voltage equal to, or lower than, VDDL, which permits the use of thin gate oxide transistors in subsequent amplifier stages. The intermediate gain stages  504  and  506  further amplify the differential signals O 2 + and O 2 − as well as O 3 + and O 3 −, as shown by the increasing differential signals  516  and  518  respectively. It is noted that the common mode voltage is lower than VDDL, which permits the use of thin gate oxide transistors in these amplifier stages. The output translation stage  508  of the receiver  500  may then be utilized as an additional amplifier stage before connection to the digital logic operating at the VDDL supply voltage. It is further noted that the outputs OUT+ and OUT− of the output translation stage  508  have a differential voltage that operates between 0 V and VDDL, as illustrated by signal  520 . 
   The above invention provides many different embodiments, or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments, and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in a design and method for, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein, without departing from the spirit of the invention, and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly, and in a manner consistent with the scope of the invention, as set forth in the following claims.