Patent Publication Number: US-7902883-B2

Title: Preemphasis driver with replica bias

Description:
RELATED APPLICATION 
     This application claims the benefit, under 35 U.S.C. §119(e), of U.S. Provisional Patent Application No. 61/074,432, entitled Preemphasis Driver With Replica Bias Circuit, filed 20 Jun. 2008. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to electrical communication. 
     BACKGROUND 
     For high-speed digital-communication transmitter-driver design, precise output amplitude level control is often necessary, especially when utilizing finite impulse response (FIR) filter functionality. Because of the relatively large voltage-swing requirement for various standards such as IEEE 802.3ap 10GBASE-KR, and IEEE 802.3ak 10GBASE-CX4, previous methods for current mirroring between control circuits and output drivers tend to introduce large amplitude variation at the output of a transmitter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example mirrored driver current source. 
         FIG. 2  illustrates an example cascode topology for a mirrored driver current source. 
         FIG. 3  illustrates an example common-gate (CG) driver with an example replica driver. 
         FIG. 4  illustrates an example multi-finger CG driver with an example replica driver. 
         FIG. 5  illustrates an example multi-finger CG driver with an example switch-controlled replica driver 
         FIG. 6  illustrates example ideal driver output peak and valley voltages and ideal low-frequency-period low and high voltages. 
         FIG. 7  illustrates an example multi-finger CG driver with multiple replica drivers and a switch before the operational amplifier (op-amp) in each circuit path. 
         FIG. 8  illustrates an example method for a preemphasis driver with replica bias. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
       FIG. 1  illustrates an example mirrored driver current source. The driver current source Mdrv does not substantially match the bias current source Mbia, due at least in part to significant variations at the drain node of the Mbia transistor and the Mdrv transistor at process, temperature, and voltage (PVT) corners when power-supply voltage is 1.2V or less in complementary metal-oxide-semiconductor (CMOS) technology. One previous method for addressing this problem is to improve the bias current source with a cascode topology, as  FIG. 2  illustrates. This implementation usually reduces mirrored driver current source variation, also reducing driver output voltage amplitude variation at PVT corners. However, this implementation tends to require larger device size and higher capacitance in the signal path, which tends to degrade driver slew rate (as is the case with CG drivers) or to degrade return loss (as is the cause with low-voltage differential signaling (LVDS) drivers). Another previous method for addressing the problem of mismatched driver and bias current sources is to create a ratioed replica bias circuit corresponding to driver output that does not include any preemphasis function. 
       FIG. 3  illustrates an example CG driver with an example replica driver. The replica driver provides a low-output voltage (VOL) reference. In particular embodiments, VOL is an accurate copy of the drain voltage of the CG driver n-type metal-oxide-semiconductor (NMOS) current sources M 1  and M 2 . The op-amp in the bias circuit adjusts the gate voltage of the CG driver current sources M 1  and M 2  so that the low voltage at the driver output node TXOP or TXON substantially matches VOL. In the replica driver, the resistors (or replica resistors) may be scaled copies of driver output resistors, which are two 50Ω driver-termination resistors and two 50Ω receiver-input resistors. The resistance of the replica resistors may be m times (m&gt;1) the resistance of the driver-termination resistors. An ideal replica n-type digital-to-analog converter (NDAC) current source in the replica driver may be 1/m times the actual CG driver current, either through M 1  and M 1 A or through M 2  and M 2 A. 
     Particular embodiments implement a preemphasis driver with a replica bias circuit. Particular embodiments do so while reducing variation in peak amplitude. In particular embodiments, one difficulty in the implementation a preemphasis driver with a replica bias circuit is the problem of significant variation in the preemphasis ratio, which results from the preemphasis ratio being a ratio of two amplitudes—peak amplitude and low-frequency-period amplitude (see, for example, FIG.  6 )—whereas the replica bias circuit represents only one type of amplitude. The replica bias circuit may control one amplitude, but another amplitude will be uncontrolled. 
       FIG. 4  illustrates an example multi-finger CG driver with an example replica driver. In  FIG. 4 , the replica driver has three n-type digital-to-analog converter (NDAC) current sources (which may be NMOS) connected to VOL. Each of the NDAC current sources is a digital-to-analog converter (DAC) for current, with its current being mirrored to a corresponding p-type DAC (PDAC) in the replica bias circuits (or op-amp blocks). The resulting NDAC current determines VOL. In particular embodiments, there are three NDACs and three PDACs. The present disclosure contemplates any suitable number of NDACs and any suitable number of PDACs. 
       FIG. 5  illustrates an example multi-finger CG driver with an example switch-controlled replica driver. In  FIG. 5 , the switches in the replica driver facilitate control of CG-driver output amplitude by a single NDAC current source in the replica driver. Each of the NDAC current sources connects either to a high-output voltage (VOH) reference or to VOL through a switch. Particular embodiments digitally control the switches in the replica driver. In particular embodiments, there are three circuit paths combined at the output of the multi-finger CG driver. In particular embodiments, there are three NDAC current sources. 
     To understand how the replica driver in  FIG. 5  affects multi-finger CG driver output and preemphasis ratio, assume that the left NDAC, middle NDAC, and right NDAC current sources are NI 0 , NI 1 , and NI 2 , respectively.  FIG. 6  defines ideal driver output valley and peak voltages to be V 1  and V 2 , respectively, and defines ideal low-frequency-period low and high voltages to be V 3  and V 4 , respectively. Preemphasis ratio may be expressed in terms of V 1 , V 2 , V 3 , and V 4 . In reality, the driver-output waveforms are nonideal and are therefore denoted as V 1 ′, V 2 ′, V 3 ′ and V 4 ′. The driver tail currents are I 0 , I 1 , and I 2 . In general, I 1  is greater than I 0  and I 2 . Depending on the selections of the switches in the replica driver in  FIG. 5 , the real driver-output waveform and preemphasis ratio may be calculated as follows. 
     Let i 0 , i 1 , and i 2  be errors corresponding to I 0 , I 1 , and I 2 , respectively, due to voltage difference between VOL and the drain nodes of M 1  and M 2 . If the NDAC current sources all have the same current flow and are all connected to VOL by their switches, the real driver-output voltage and preemphasis ratio will be:
 
V1′=V1
 
V2′=V2
 
 V 3 ′=V 3−37.5 *i 1−12.5*( i 0 +i 2)
 
 V 4 ′=V 4−12.5 *i 1−37.5*( i 0 +i 2)
 
               PE   ⁢           ⁢     ratio   ′       =       1   -       (       V   ⁢           ⁢     4   ′       -     V   ⁢           ⁢     3   ′         )       (       V   ⁢           ⁢     2   ′       -     V   ⁢           ⁢     1   ′         )         =       (       V   ⁢           ⁢   2     -     V   ⁢           ⁢   1     -     V   ⁢           ⁢   4     +     V   ⁢           ⁢   3     -     25   *     (       i   ⁢           ⁢   1     -     i   ⁢           ⁢   0     -     i   ⁢           ⁢   2       )         )       (       V   ⁢           ⁢   2     -     V   ⁢           ⁢   1       )               
The implementation illustrated by  FIG. 4  would have substantially the same result.
 
     Let i 0 , i 0   b , i 1 , i 2 , and i 2   b  be errors corresponding to I 0 , I 1 , and I 2 , respectively, due to voltage difference between VOL and the drain nodes of M 1  and M 2 . If the NDAC current sources NI 0  and NI 2  are connected to VOH and the NDAC current source NI 1  is connected to VOL, the real driver-output voltage and preemphasis will be:
 
 V 1 ′=V 1+37.5*( i 0 +i 1 +i 2)
 
 V 2 ′=V 2+12.5*( i 0 +i 1 +i 2)
 
 V 3 ′=V 3−12.5*( i 0 b+i 2 b )
 
 V 4 ′=V 4−37.5*( i 0 b+i 2 b )
 
     
       
         
           
             
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       FIG. 7  illustrates an example multi-finger CG driver with multiple replica drivers and a switch before the op-amp in each circuit path. The left top NDAC current source outputs the voltage VOL Peak , which is the same as the ideal voltage V 1 . The left bottom NDAC current source outputs the voltage VOL LF , which is the same as the ideal voltage V 3 . In particular embodiments, the circuits in  FIG. 7  provide more flexibility in selecting the gate voltage for each CG driver, since each op-amp that controls its CG driver can be configured to VOL Peak  or VOL LF . Particular embodiments may select VOL LF  for the main tap. Particular embodiments may select VOL Peak  for the circuit paths providing preemphasis. 
     In particular embodiments, configurations like the one illustrated by  FIG. 7  may be more accurate in controlling preemphasis ratio than configurations like the one illustrated by  FIG. 5 . In particular embodiments, configurations like the one illustrated by  FIG. 7  produce less error at the output of the transmitter. Let symbols i 0 , i 1 , and i 2  be errors corresponding to I 0 , I 1 , and I 2 , respectively, due to voltage difference between VOL and the voltages at the drain node of M 1  and M 2 . With VOL Peak  for preemphasis finger and VOL LF  for main finger, the real driver-output voltage and the preemphasis ratio will be:
 
 V 1 ′=V 1+37.5 *i 1
 
 V 2 ′=V 2+12.5 *i 1
 
 V 3 ′=V 3−12.5*( i 0 +i 2)
 
 V 4 ′=V 4−37.5*( i 0 +i 2)
 
               PE   ⁢           ⁢     ratio   ′       =       1   -       (       V   ⁢           ⁢     4   ′       -     V   ⁢           ⁢     3   ′         )       (       V   ⁢           ⁢     2   ′       -     V   ⁢           ⁢     1   ′         )         =       (       V   ⁢           ⁢   2     -     V   ⁢           ⁢   1     -     V   ⁢           ⁢   4     +     V   ⁢           ⁢   3     -     25   *     (       i   ⁢           ⁢   1     -     i   ⁢           ⁢   0     -     i   ⁢           ⁢   2       )         )       (       V   ⁢           ⁢   2     -     V   ⁢           ⁢   1     -     25   *   i   ⁢           ⁢   1       )               
This result minimizes the variation in preemphasis ratio.
 
     In particular embodiments, the replica architecture is not only applicable to CG drivers, but is applicable to LVDS drivers and current-mode logic (CML) drivers as well. 
     With respect to previous methods for addressing the problem of mismatched driver and bias current sources by improving the bias current source with a cascode topology, particular embodiments may facilitate the use of fewer and smaller devices and the use of less chip area in improving the bias current source. Particular embodiments, when applied to CG drivers, may provide lower signal-path capacitance. Particular embodiments, when applied to CG drivers, may provide faster slew rate. Particular embodiments, when applied to LVDS drivers may provide better return loss. 
     With respect to previous methods for addressing the problem of mismatched driver and bias current sources by creating a ratioed replica bias circuit corresponding to driver output that does not include any preemphasis function, particular embodiments may facilitate realization of preemphasis functionality. Particular embodiments may provide further reduction of variation in preemphasis ratio than a naïve extension of such previous methods. 
       FIG. 8  illustrates an example method for a preemphasis driver with replica bias. The method starts at step  800 , where a replica driver that includes multiple NDAC current sources produces a reference voltage based on current supplied by the NDAC current sources. At step  802 , an op-amp in a driver bias circuit in each of multiple driver fingers that are coupled to the replica driver adjusts current-source gate voltage in the output driver to produce voltages at output nodes of the driver fingers that approximately match the reference voltage produced by the replica driver, at which point the method ends. Particular embodiments may continuously repeat the steps of the method of  FIG. 8 , according to particular needs. Although the present disclosure describes and illustrates particular steps of the method of  FIG. 8  as occurring in a particular order, the present disclosure contemplates any suitable steps of the method of  FIG. 8  occurring in any suitable order. Although the present disclosure describes and illustrates particular components carrying out particular steps of the method of  FIG. 8 , the present disclosure contemplates any suitable components carrying out any suitable steps of the method of  FIG. 8 . 
     The present disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend.