Abstract:
A low voltage differential signal (LVDS) driver circuit with reduced power consumption. A pre-driver stage, implemented as a differential current mode amplifier, is driven by the differential input signal and provides a corresponding differential drive signal, which drives the output stage, implemented as a differential voltage mode amplifier, which, in turn, provides the differential output signal for the load. Total current consumption equals the load current, which is provided by the output stage, plus a much smaller current used by the pre-driver stage.

Description:
RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 12/049,045, filed Mar. 14, 2008 now abandoned, entitled “Low Voltage Differential Signal Driver with Reduced Power Consumption”. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to differential signal drivers, and in particular, to low voltage differential signal (LVDS) driver circuits. 
     2. Related Art 
     Many integrated circuits (“chips”) drive signals to and receive signals from other chips across a variety of signal media and media lengths. Typically, the signal media, e.g., cables, are designed to have a transmission line impedance of 50 ohms, which is typically matched to the output impedance of the transmitter and input impedance of the receiver. One example of the circuit used to drive such signals is an LVDS circuit, which is primarily used in short range applications, e.g., inter-chip signals on printed circuit boards. As is well known, advantages of LVDS circuits include high bandwidth, low power, reduced EMI (electromagnetic interference) effects, and better immunity to common mode noise. 
     Referring to  FIG. 1 , a typical LVDS circuit  10  includes a pre-driver stage  12  and output stage  14  for driving current through the load impedance  16  (typically a resistance of 100 ohms), the voltage across which is sensed by a receiver circuit  18  in accordance with well known principles. The data to be transferred arrives in the form of a differential signal  11   c  having opposing positive  11   p  and negative  11   n  signal phases. In response to this signal  11   c , the pre-driver stage  12  produces a corresponding differential drive signal  13 , also having opposing signal phases  13   p ,  13   n , which the output stage  14  uses to produce the LVDS output signal  15  with its own opposing signal phases  15   p ,  15   n . This signal  15  produces the load current  15   i  for conduction by the load impedance  16 . 
     The power supply VDD provides a supply current  11   a  for the pre-driver stage  12  and a supply current  11   b  for the output stage  14 . The output stage supply current  11   b  includes supply current Id for the output stage  14 , as well as the current Iload needed for driving the load impedance  16 . The amount of supply current consumed by the pre-driver stage  12  and output stage  14  can be, and often is, significant relative to the load current Iload. 
     Referring to  FIGS. 2A and 2B , two forms of output stages  14  are typically used: voltage mode ( FIG. 2A ), and current mode ( FIG. 2B ). As is well known, the voltage mode driver  14   a  has the advantage of low power, but often provides poor line impedance matching. In contrast, the current mode diver  14   b  has the advantage of good line impedance matching, but generally consumes higher power. 
     Referring to  FIG. 3 , one example of a conventional current mode output stage  14   a  includes NPN bipolar junction transistors Q 1 , Q 2 , resistances R 1 , R 2 , and a current source  20   a , interconnected substantially as shown. Each of the resistances R 1 , R 2  is substantially equal to 50 ohms so as to be half of the 100 ohm impedance of the load  16 . In such a circuit  14   a , current I 1  is equal to the load current  15   i  (I 1 =Iload), while current I 2  is equal to three times the load current  15   i  (12=3*Iload). As a result, the current source  20   a  must provide four times the amount of the load current so as to provide sufficient current for operation of the output stage  14   a  and sufficient current  15   i  for the load  16 . Additionally, the signals  13   p ,  13   n  provided by the pre-driver stage  12  (not shown) must have sufficient drive current capacity to drive the large base-emitter capacitances of the transistors Q 1 , Q 2 . 
     Referring to  FIG. 4 , another example of a conventional current mode output stage  14   b  includes PMOS transistors P 1 , P 2 , NMOS transistors N 1 , N 2 , and current source circuits  20   bp ,  20   bn , and a resistance R, all interconnected substantially as shown. In this circuit  14   b , the resistance R is substantially equal to the load impedance of 100 ohms. Each of the two branch currents I 1 , I 2  is equal (mutually exclusively in time) to two times the load current  15   i  (I 1 =I 2 =2*Iload). While this current is less than the current for the circuit of  FIG. 3 , additional current Ipd will still be required for the pre-driver stage  12  (not shown). 
     Referring to  FIG. 5 , another example of a conventional voltage mode output stage  14   c  is biased between two voltage potentials, Vhigh, Vlow rather than current sources, and does not require the internal resistance R. Accordingly, the two supply currents I 1 , I 2  are needed during mutually exclusive time intervals and each one is equal to the load current  15   i  (I 1 =I 2 =Iload). However, the output impedance of this output stage  14   c  is dependent upon the channel characteristics of the individual transistors P 1 , P 2 , N 1 , N 2 , and, therefore, cannot be matched well to the 50 ohm impedance of the signal transmission medium and load  16 . Accordingly, some form of calibration must be provided, e.g., in control of the device characteristics during manufacture or additional calibration circuitry. 
     Referring to  FIG. 6 , another example of a conventional voltage mode output stage  14   d  includes the transistors P 1 , P 2 , N 1 , N 2  of the circuit of  FIG. 5 , plus resistances R 1   a , R 2   a , Rib, R 2   b , all interconnected substantially as shown. Each of the resistances R 1   a , R 2   a , R 1   b , R 2   b  is 50 ohms so as to be half of the 100 ohm load impedance  16 . Similar to the circuit  14   c  of  FIG. 5 , each of the mutually exclusive currents I 1 , I 2  is equal to the load current  15   i  (I 1 =I 2 =Iload). Improved matching between the output stage impedance and the signal transmission medium is provided by the resistances R 1   a , R 2   a , R 1   b , R 2   b . However, this then requires the channel impedances of the transistors P 1 , P 2 , N 1 , N 2 , when in their turned-on states, to be as close to zero ohms as possible. As a result, the signals  13   p ,  13   n  provided by the pre-driver stage  12  (not shown) must be capable of driving the larger input capacitances at the gate electrodes of the transistors P 1 , P 2 , N 1 , N 2 . 
     Accordingly, it would be desirable to have a differential signal driver circuit topology that minimizes the amount of supply current needed, while also avoiding large input capacitances, as well as proper matching between the impedances of the output driver circuit and signal transmission medium. 
     SUMMARY 
     In accordance with the presently claimed invention, a low voltage differential signal (LVDS) driver circuit with reduced power consumption is provided. A pre-driver stage, implemented as a differential current mode amplifier, is driven by the differential input signal and provides a corresponding differential drive signal, which drives the output stage, implemented as a differential voltage mode amplifier, which, in turn, provides the differential output signal for the load. Total current consumption equals the load current, which is provided by the output stage, plus a much smaller current used by the pre-driver stage. 
     In accordance with one embodiment of the presently claimed invention, a differential signal driver includes: 
     first and second input electrodes to convey a differential input signal; 
     differential current mode amplifier circuitry coupled to the first and second input electrodes and responsive to the differential input signal by providing a corresponding differential drive signal; 
     current source circuitry to provide a load current; 
     first and second output electrodes for coupling to a predetermined load impedance; and 
     differential voltage mode amplifier circuitry coupled to the current source circuitry, the differential current mode amplifier circuitry, and the first and second output electrodes, and responsive to the differential drive signal by conducting the load current via the load impedance when the load impedance is coupled to the first and second output electrodes. 
     In accordance with another embodiment of the presently claimed invention, a differential signal driver includes: 
     differential current mode amplifier means for receiving a differential input signal and in response thereto providing a corresponding differential drive signal; 
     current source means for providing a load current; and 
     differential voltage mode amplifier means for receiving the differential drive signal and load current, and in response thereto conducting the load current via a predetermined load impedance when the load impedance is coupled to the differential voltage mode amplifier means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a conventional differential signal driver circuit. 
         FIGS. 2A and 2B  are schematics of conventional models for voltage mode and current mode signal drivers, respectively. 
         FIG. 3  is a schematic of a conventional current mode differential signal driver. 
         FIG. 4  is a schematic of another conventional current mode differential signal driver. 
         FIG. 5  is a schematic of a conventional voltage mode differential signal driver. 
         FIG. 6  is a schematic of another conventional voltage mode differential signal driver. 
         FIG. 7  is a schematic of a differential signal driver in accordance with one embodiment of the presently claimed invention. 
         FIG. 8  is a schematic of a differential signal driver in accordance with another embodiment of the presently claimed invention. 
         FIG. 9  is a schematic of a differential signal driver in accordance with another embodiment of the presently claimed invention. 
         FIG. 10  is a schematic of a differential signal driver in accordance with another embodiment of the presently claimed invention. 
         FIG. 11  is a schematic of a differential signal driver in accordance with another embodiment of the presently claimed invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
     Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed. 
     Referring to  FIG. 7 , a differential signal driver  110   a  in accordance with one embodiment of the presently claimed invention includes a pre-driver stage  112   a  and an output stage  114   a . The pre-driver stage  112   a  includes NPN transistors Qp 1 , Qp 2 , resistances Rp 1 , Rp 2 , and a current source  120   p , interconnected substantially as shown. The signal phases  11   p ,  11   n  of the outgoing data signal drive the base electrodes of the transistors Qp 1 , Qp 2 , which then provide the pre-driver signal phases  13   p ,  13   n  via their collector electrodes. 
     The output stage  114   a  includes NPN transistors Qd 1 , Qd 2 , NMOS transistors Nd 1 , Nd 2 , resistances Rd 1 , Rd 2 , and a current source  120   d , all interconnected substantially as shown. The pre-driver signals  13   p ,  13   n  drive the base electrodes of the NPN transistors Qd 1 , Qd 2 , the emitter electrodes of which drive the gate electrodes of the NMOS transistors Nd 1 , Nd 2  and resistances Rd 1 , Rd 2 . The drain electrodes of the NMOS transistors Nd 1 , Nd 2 , with load current provided via the resistances Rd 1 , Rd 2 , provide the output signal phases  15   p ,  15   n.    
     The current required by the output stage  114   a , as supplied by the current source  120   d , is equal to the required load current Iload. As for matching the output impedance of the output stage  114   a  to the load  16  (as well as the signal transmission medium), it should be readily understood that the 100 ohms of the load impedance  16  is matched by the sum of the impedances of the resistances Rd 1 , Rd 2 . As should be readily understood by one of ordinary skill in the art, an AC signal analysis will show that the upper ends of the resistances Rd 1 , Rd 2 , are effectively terminated at signal ground potential due to the extremely low impedances of the emitter follower outputs at the emitters of the NPN transistors Qd 1 , Qd 2 , while at the lower ends of the resistances Rd 1 , Rd 2 , the NMOS transistors Nd 1 , Nd 2  do not affect the signal due to the high impedances of their channels while operating in saturation. Additionally, the supply current Ip required by the pre-driver stage  112   a  can be minimized by increasing the resistance values of the pre-driver resistances Rp 1 , Rp 2 , since the pre-driver output signals  13   p ,  13   n  drive the large input impedances of the output stage emitter followers (transistors Qd 1 , Qd 2 ). Accordingly, this will reduce the pre-driver stage current Ip, thereby reducing power consumption. 
     Referring to  FIG. 8 , in accordance with an alternative embodiment  110   b  of the presently claimed invention, the output common mode voltage can be sensed by including two serially connected resistances Rcm 1 , Rcm 2  between the output electrodes. The common mode voltage Vcm appearing between these resistances Rcm 1 , Rcm 2  is fed back to a voltage comparator  122   a  for comparison with a reference voltage Vref to provide a controlled supply voltage  123   a  for the pre-driver stage  112   a.    
     Referring to  FIG. 9 , in another alternative embodiment  110   c  of the presently claimed invention, the common mode voltage Vcm can be controlled by feeding it back for comparison with the reference voltage Vref in a voltage comparator  122   b  used to control shunt current sources  124   a ,  124   b . The voltage comparator  122   b , in accordance with the relative values of the reference voltage Vref and common mode voltage Vcm, provides a control voltage  123   b  for the shunt current sources  124   a ,  124   b , which shunt respective currents  125   a ,  125   b  from the circuit branches of the pre-drive circuit  112   c , thereby controlling the common mode voltage of the pre-driver output signals  13   p ,  13   n . For example, if the output common mode voltage Vcm were to increase, the control voltage  123   b  would cause the shunt currents  125   a ,  125   b  to increase, thereby causing the common mode voltage of the pre-driver output signals  13   p ,  13   n  to decrease and, in turn, cause the output common mode voltage Vcm to decrease as well. 
     Referring to  FIG. 10 , in accordance with another alternative embodiment  110   d  of the presently claimed invention, signal pre-emphasis can be provided with the addition of emitter resistances Re 1 , Re 2 , emitter capacitances C 1 , C 2 , and inductances L 1 , L 2 , all interconnected substantially as shown. As should be readily understood, signal gain will increase with frequency in accordance with the relative values of the collector resistances Rp 1 , Rp 2 , emitter resistances Re 1 , Re 2 , emitter capacitances C 1 , C 2 , and inductance values L 1 , L 2 . Alternatively, the inductances L 1 , L 2  can be omitted in which case, the relative values of the resistances Rp 1 , Rp 2 , Re 1 , Re 2  and capacitances C 1 , C 2  will determine the signal gain with frequency, in accordance with well known principles. 
     Referring to  FIG. 11 , in accordance with another alternative embodiment  110   e  of the presently claimed invention, the output stage  114   c  can include a “keep alive” current circuit with NMOS transistors N 3 , N 4  and a tail current source  122 , connected substantially as shown. Such additional circuitry can be useful for increasing the switching speed of the output stage  114   c  by maintaining the flow of a small current I 3  or I 4  depending upon which devices are currently conducting signal current. Accordingly, those devices not currently conducting signal current will nonetheless maintain a small amount of current flow so as to be turned on more fully and more quickly when it is their turn to conduct signal current. 
     As should be readily understood by someone of ordinary skill in the art, the circuit topologies discussed herein for the example embodiments of the presently claimed invention can also be implemented by switching the bipolar and MOS transistors. For example, the bipolar transistors Qp 1 , Qp 2 , Qd 1 , Qd 2  can be replaced with MOS field effect transistors, while NMOS transistors Nd 1 , Nd 2  are replaced with bipolar transistors. Additionally, depending upon a desired speed of operation or signal gain, degeneration resistances can be included or not included in the tail transistors Nd 1 , Nd 2 . 
     Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.