Abstract:
A differential input buffer shows reduced sensitivity to input conditions such as input-trace loading and upstream driver characteristics. Varying input conditions can be measured as differences in amplitude, slew rate, and common-mode offset. Wide input-voltage swings are clamped to a limited voltage range by an input clamp circuit that uses source followers to drive p-channel clamp transistors that turn off when the input voltage is too low. A voltage divider then sets the lowest voltage input to a differential stage. The differential stage receives the clamped inputs and has two tail current sinks to reduce delay sensitivity to charging and discharging of tail capacitances. A middle voltage is applied to transistors opposite the differential transistors that receive the clamped input voltages. A bias voltage for the tail current sinks is generated by mirroring currents and setting a gate voltage by injecting and removing a same bias current from a resistor.

Description:
BACKGROUND OF INVENTION 
     This invention relates to differential input buffers, and more particularly to differential buffers that are relatively insensitive to input conditions. 
     Matching of delays in electronic circuits is a long-standing design challenge. One of many applications that require matched delays is a zero-delay buffer that uses a PLL to synchronize outputs to inputs. The PLL adjusts the circuit delay to exactly match the clock period and thus the apparent delay of the zero-delay buffer is zero with respect to the clock. 
     FIG. 1 shows a zero-delay buffer that requires precise delay matching. Zero-delay buffer chip  10  may be integrated with a larger system or may be sold as a stand-alone zero-delay chip. Clock source  18  generates a reference clock CLK that is input to chip  10 . This reference clock passes through input buffer  22  and is compared in phase to a feedback clock FB that passes through a similar input buffer  24 . Phase comparator  12  compares the phases of CLK and FB and activates charge pump  14  to charge or discharge filter capacitor  20 . As capacitor  20  is charged or discharged, the voltage across capacitor  20  changes, and voltage-controlled oscillator VCO  16  senses the capacitor voltage and adjusts the output frequency of an output clock that is buffered by output buffer  26 . 
     The FB clock output by buffer  26  can drive other logic directly, or additional internal or external buffering can generate other clocks. Many PLL&#39;s can be included in parallel to separately drive and adjust parallel clock lines that are synchronized to each other and to the reference clock. 
     Input buffers  22 ,  24  must to have closely matched delays. Otherwise a phase error is introduced that can cause VCO  16  to output the wrong frequency. The physical design of input buffers  22 ,  24  can be closely matched to avoid skews. Unfortunately, the electrical characteristics of the signal traces and drivers of the reference clock and feedback clock can differ. For example, clock source  18  may have a different driver than output buffer  26 , and may drive less current or may drive signal edges more slowly. The metal traces for CLK and FB may also differ. For example the FB clock trace may be much shorter than the reference clock CLK trace. Thus the input conditions of the signals input to input buffers  22 ,  24  can differ considerably. Input buffers that can adapt for such external mis-matches are desirable. 
     There are many measurable input conditions that can mismatch as a result of such differences in external drivers and traces. For example, measurements of rise and fall times or slew rates, high and low amplitudes and even root-mean-square (RMS) voltages of signals can differ. Often differential rather than single-ended signals are used. Then additional input-signal characteristics can be mis-matched, such as differential crossing or cross-over points and common-mode voltages. 
     One commonly-used circuit for input buffers is the differential amplifier. FIG. 2 shows a prior-art differential input buffer. Inputs signals V+, V− are applied to the gates of n-channel differential transistors  36 ,  38 , respectively. Resistor loads  32 ,  34  supply matched currents to transistors  36 ,  38 , and these currents are combined at current sink  30 . Differential outputs VO−, VO+ are taken from the drains of transistors  36 ,  38 , respectively. 
     Such a differential buffer is delay-independent of common-mode level within a certain range. The differential transistors tend to linearize I/Vin, for a range of about 2 ΔV, where ΔV is the gate bias of transistors  36 ,  38 , or Vgs−Vt, where Vgs is the average gate-to-source voltage and Vt is the n-channel transistor threshold voltage. However, non-linear delays occur beyond this 2 ΔV range. 
     One source of delay variation is the parasitic tail capacitance represented by tail capacitor  28 . When inputs V+, V− differ by more than 2 ΔV, one of differential transistors  36 ,  38  may turn off completely. All of the tail current then flows through the other differential transistor. As the input voltage V+ or V− rises even more, the tail voltage also rises rather than remaining constant. The rising tail voltage charges tail capacitor  28 , requiring some of the current. 
     It is thus desirable to avoid operating at higher input voltages where such delay variations due to charging and discharging of the tail capacitance occur. It is desirable to avoid large differences in differential input voltages V+, V−. It is desired to limit the difference between differential inputs V+, V−. Adjusting for input conditions that exceed 2 ΔV is desirable to produce better delay matching of differential input buffers. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a zero-delay buffer that requires precise delay matching. 
     FIG. 2 shows a prior-art differential input buffer. 
     FIG. 3 is a block diagram of a clamped differential input buffer with a well-controlled bias circuit. 
     FIG. 4 is a schematic of one embodiment of the differential input clamp. 
     FIG. 5 is a schematic of an embodiment of the first differential stage. 
     FIG. 6 is a diagram highlighting principles of a current-mirroring bias-voltage generator. 
     FIG. 7 highlights substituting a resistor that sets a voltage due to current injected and extracted from the circuit at the resistor terminals. 
     FIG. 8 is a diagram of a bias current generator. 
     FIG. 9 shows an alternate embodiment of a voltage-to-current reference. 
     FIG. 10 is a plot of delay sensitivity to input characteristics. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an improvement in differential buffers. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed. 
     The inventor has realized that input buffers can better match in delay when input voltages do not have wide swings. An input clamp on the differential inputs limits the voltages of the differential inputs before they are applied to a differential amplifier. This input clamp prevents the clamped differential inputs from differing by more than the unclamped input, preventing non-linear effects caused by charging and discharging of tail capacitance. 
     FIG. 3 is a block diagram of a clamped differential input buffer with a well-controlled bias circuit. Input clamp  300  receives differential inputs V+, V−. When V+ or V− are above a limiting voltage level, input clamp  300  reduces the input voltage to the limiting voltage. Otherwise the inputs are level-shifted and output as clamped differential inputs VC+, VC−. 
     First differential stage  302  receives the clamped differential inputs VC+, VC−. The clamped differential inputs are limited in voltage. The voltage differential is allowed to be slightly more than 2 ΔV, but is clamped at less than the full input swing, allowing first differential stage  302  to operate in the linear range. This prevents large delay mis-matches caused by charging and discharging the tail capacitor. First differential stage  302  generates differential outputs VO+, VO− which may be further amplified by second differential stage  302 ′, which generates final outputs VO 2 +, VO 2 −. The second differential stage is optional but desirable. 
     A well-controlled bias voltage is required for the tail current source in the differential stages  302 ,  302 ′. This bias voltage is generated by bias current generator  306 , which mirrors a current generated by voltage-to-current reference  304 . 
     FIG. 4 is a schematic of one embodiment of the differential input clamp. Input clamp  300  receives differential inputs V+, V− which are applied to the gates of n-channel follower transistors  62 ,  64 , respectively. Follower transistors  62 ,  64  act as source-followers, shifting the input voltages V+, V− lower by at least a threshold. Resistors  70 ,  72  sink current from follower transistors  62 ,  64  respectively. 
     The source output of n-channel follower transistor  62  is applied to the gate of p-channel clamp transistor  68 , which has its drain grounded. The source of p-channel clamp transistor  68  is the clamped output VC+. Resistors  76 ,  77  form a voltage divider to clamped output VC+ that set the clamped voltage level when p-channel clamp transistor  68  is shut off. 
     Likewise, the source output of n-channel follower transistor  64  is applied to the gate of p-channel clamp transistor  66 , which has its drain grounded. The source of p-channel clamp transistor  66  is the clamped output VC−. Resistors  74 ,  75  form a voltage divider to clamped output VC− that set the clamped voltage level when p-channel clamp transistor  66  is shut off. 
     When V+ is below the lower clamped voltage input level, n-channel follower transistor  62  turns off, and resistor  70  drives the gate of p-channel clamp transistor  68  to ground, turning it on. Since the drain and gate of p-channel clamp transistor  68  are grounded, it is in the saturated region of operation and its source is clamped to one threshold above its gate, or |Vtp|. The lower clamped voltage of VC+ is about |Vtp|, plus some adjustment from voltage divider resistors  76 ,  77 . 
     When V+ is above the clamped voltage input level, n-channel follower transistor  62  is on strongly, pulling the gate of p-channel clamp transistor  68  high. When a sufficiently high voltage is applied, p-channel clamp transistor  68  turns off. Then resistors  76 ,  77  form a voltage divider that determines the upper clamped voltage output on VC+. 
     Note that the clamped voltage input levels on inputs V+, V− can be different from the clamped voltages on clamped outputs VC+, VC−. Indeed, a benefit of this embodiment of input clamp  300  is that the maximum clamped output voltages can be set by sizing the voltage-divider resistors while the minimum clamped output voltage is set by the p-channel threshold |Vtp|. 
     FIG. 5 is a schematic of an embodiment of the first differential stage. First differential stage  302  receives clamped differential inputs VC+, VC− from input clamp  300  that are limited in voltage swing. This limited input-voltage swing limits charging and discharging of tail capacitances such as tail capacitor  54 , reducing non-linearities. 
     Clamped differential input VC+ is applied to the gate of n-channel differential transistor  42 , while input VC− is applied to the gate of n-channel differential transistor  44 . Rather than have both differential transistors  42 ,  44  connect to the same current sink, separate current sinks  50 ,  52  connect to transistors  42 ,  44 , respectively. 
     Current sink  50  receives currents from differential transistor  42  and n-channel common-mode transistor  46 . Similarly, current sink  52  receives currents from differential transistor  44  and n-channel common-mode transistor  48 . Common-mode transistors  46 ,  48  are driven by a common-mode or mid-point voltage VMID, which is between VC+ and VC−. VMID can be generated dynamically by a 50%—50% voltage divider between VC+ and VC−, or it can be generated statically at a pre-determined middle voltage such as 2.0 volts or 1.2 volts. In one embodiment, VMID is generated from the midpoint of two equal-valued resistors in series between the differential inputs. VMID can be isolated further by inserting a capacitor between the resistor midpoints and the use of VMID in the circuit. VMID follows the common-mode input. The center of the clamped differential input is filtered and responds slowly to changes in input common-mode level. 
     Load current through resistor  59  is split into parallel branches by resistors  56 ,  58 , which connect to the drains of common-mode transistors  46 ,  48 , respectively. A fast rising input induces slew current in Ctail by I=dv/dt. When the slew current exceeds the normal DC bias current Itail, most or all of the extra current dumps into Vdd through transistor  42  or  44 , whose drain connects to the Vdd supply. Thus normal operation is relatively undisturbed. An n-type source follower increases Vgs on rising edges to drive a capacitive load. On falling edges, Vgs can only be reduced somewhat, and then the slew is limited by the pulldown bias current, Itail. On rising edges, the current caused by Ctail is as large as the input slew rate demands. If the rising transistor were directly connected to the load, the load would experience an increase in current and a reduction in delay. This is avoided by having transistors  46 ,  48  with a fixed gate bias (VMID) connected to the output load. Since transistors  46 ,  48  have a fixed input voltage VMID, they cannot experience delay variations due to gate input slew. 
     When VC+ rises above VMID, the differential input changes cause the differential outputs of first differential stage  302  to swing. First differential stage  302  is non-inverting. It can be made to invert by swapping VO+ and VO−. 
     Having clamped input voltages helps to eliminate currents for charging and discharging tail capacitances, such as represented by tail capacitor  54 , which is not a real capacitor but instead represents the sum of all parasitic and other capacitances on the tail node. 
     A second differential stage can be constructed in a similar manner, or a different circuit may be used. A differential-to-single-ended converter may also be included. 
     Having well-controlled current sinks  50 ,  52  improves linearity and delay matching. Current sinks  50 ,  52  can each be implemented as an n-channel transistor with its source grounded and having a gate driven by a bias voltage. The bias voltage can be generated by a simple bias generator such as a voltage-divider network, or a more complex bias-voltage generator can be used such as described below. Since Vgs varies with temperature and according to other variables, the current source must compensate. The purpose of the bias current is to cause the input differential pair to have a fixed and controlled linear input range 2 ΔV. In this case the 2 ΔV is derived from the most convenient source, Vdd. Any reference, internal or external, could be used instead. Once this is achieved all other results may be obtained by referring to this controlled 2 ΔV. 
     FIG. 6 is a diagram highlighting principles of a current-mirroring bias-voltage generator. The bias voltage to the tail transistors implementing current sinks  50 ,  52  can be taken from the gate voltage of transistor  86 . Vgs 2 . The gate and drain of p-channel mirror transistor  84  are connected together and to the gate of p-channel mirror transistor  82 . The current I 2  through p-channel mirror transistor  84  is four times the current I 1  through p-channel mirror transistor  82 , since transistor  86  has 4 times the W/L ratio of transistor  84 . Of course, other current-scaling ratios can be substituted. 
     Current I 2  from mirror transistor  84  is sunk through n-channel transistor  86 , which has its gate driven by the drain of mirror transistor  82  as voltage Vgs 2 . 
     Current I 1  passes through a voltage source that drops the voltage by V before passing through n-channel transistor  88  to ground. The gate and drain of transistor  88  are connected together as Vgs 1 . Thus Vgs 2 =Vgs 1 +V. 
     Since current through mirror transistors  82 ,  84  is saturated and follows the saturated current equation 1=0.5*k(W/L(Vgs−Vt) 2 , where k is a constant, the equation: 
     
       
         I 2 =4*I 1   
       
     
     can be substituted as: 
     
       
         ( Vgs   2 − Vt )=4( Vgs   1 − Vt ) 
       
     
     and taking the square root of each side, 
     
       
         ( Vgs   2 − Vt )=2( Vgs   1 − Vt ). 
       
     
     Then removing Vt: 
     
       
           Vgs   2 =2 *Vgs   1 − Vt   
       
     
     and substituting Vgs 2 =Vgs 1 +V yields: 
     
       
           Vgs   1 + V =2 *Vgs   1 − Vt   
       
     
     
       
         V=Vgs 1 −Vt. 
       
     
     By setting voltage V, a desired Vgs−VT or ΔV may be generated using the circuit of FIG.  6 . Voltage V can be produced by substituting a resistor and driving a same current into and out of the resistor. FIG. 7 highlights substituting a resistor that sets a voltage due to current injected and extracted from the circuit at the resistor terminals. The voltage source is removed and resistor  94  is inserted between the drains of transistors  82 ,  88  (node N 1 ) and the gate of transistor  86 . Resistor  94  creates a voltage to set ΔV. This voltage is a floating voltage source, one that is not referred to the power supply or to ground. The floating voltage source derives a voltage-to-current current from a reference voltage across a reference resistor (not shown). This current forms a precise voltage across biasing resistor  94  by current and resistance ratios from the reference resistor. The circuit works with any other form of floating voltage source. This particular one is well suited to low-voltage application. 
     A bias current IB 1  is injected into one terminal of resistor  94  by current bias source  90 . This same current IB 1  is removed from the other terminal of resistor  94  by bias current sink  92 . Since the same current IB 1  is injected and removed from the circuit, no net current is added to node N 1  by resistor  94 . 
     This bias current IB 1  can be set as IB 1 =V/R to produce the desired voltage drop across resistor  94  to set the gate voltage Vgs 2  for transistor  86 . This gate voltage Vgs 2  is also the bias voltage for the current sinks  50 ,  52  of the first differential stage  302 . Setting bias current IB 1  through resistor  94  thus sets the tail current sinks. 
     FIG. 8 is a diagram of a bias current generator. Bias current generator  306  generates the tail voltage bias VTAIL to current sinks  50 ,  52 . VTAIL is set by the gate voltage of n-channel transistor  86 , which sinks current I 2  set by mirror transistor  84 . Mirror transistor  82  and n-channel transistor  88  are connected to one terminal of bias-setting resistor  94  while the gate voltage VTAIL of transistor  86  is the other terminal of resistor  94 . 
     Current injector  120  generates and removes bias current IB 1 . Bias current IB 1  is injected into resistor  94  by p-channel transistor  114 . The same bias current IB 1  is removed from the other terminal of resistor  94  by n-channel transistor  112 . 
     Currents through transistors  114 ,  112  match because the gate of p-channel transistor  114  is the same node as the gate of p-channel transistor  108 , and the gate of n-channel transistor  112  is the same node as n-channel transistor  110 . Since transistors  108 ,  110  are connected together and must pass the same amount of current, the p-channel and n-channel currents must match. If the W/L sizes of the p-channel transistors  108 ,  114  are the same, and the W/L sizes of n-channel transistors  110 ,  112  are the same, then the IB 1  current through transistors  108 ,  110  is mirrored to injected current IB 1  from p-channel transistor  114  and removed current IB 1  from n-channel transistor  112 . The gate and drain of n-channel transistor  110  are connected together and to the gate of n-channel transistor  112 . 
     The upper gate voltage to p-channel transistors  108 ,  114  is set by p-channel transistor  102 , which has its gate and drain connected together. Current through p-channel transistor  102  is set by voltage-to-current reference  304 . A bias voltage VB is applied to one input of comparator  100 , while the other input is feedback NFB from the source of n-channel transistor  104 . Comparator  100  drives the gate of n-channel transistor  104  to adjust current flow so that the source of n-channel transistor  104 , NFB, is the same voltage as VB. Current from n-channel transistor  104  flowing through resistor  106  produces a voltage drop NFB that is matched to VB. 
     If the resistance value of resistor  94  is R 1  and the resistance value of resistor  106  is R 2 , then V=IB 1 *R 1  and IB 1 =VB/R 2 , when currents in p-channel transistors  102 ,  108 ,  114  are the same (same transistor sizes). Of course, scaling factors could be added when the transistor sizes are ratioed. 
     Combining equations V=IB 1 *R 1  and IB 1 =VB/R 2  produces: 
     
       
           V=VB*R   1 / R   2 . 
       
     
     Rather than use the previously described voltage-to-current converter, another embodiment may be substituted such as the one shown in FIG.  9 . FIG. 9 shows an alternate embodiment of an amplifier-based voltage-to-current reference. 
     Voltage VBIAS is generated by voltage source  154  and raises the source voltage of n-channel transistor  152 . The gate and drain of n-channel transistor  152  are connected together and to the gate of n-channel transistor  162  in the other leg. 
     Current is sourced by p-channel transistor  150  to the drain of n-channel transistors  52 , and by p-channel transistor  160  to the drain of n-channel transistor  162 . The gate and drain of p-channel transistor  160  are connected together to set the gate voltage of p-channel transistors  150 ,  160 . Resistor  156  receives current from the source of n-channel transistor  162  and sets VR. 
     Voltage-to-current reference  304  can generate the p-channel gate voltage to p-channel transistors  108 ,  114  of FIG.  8 . P-channel transistor  102  in FIG. 8 is the same as p-channel transistor  160  of FIG.  9 . 
     FIG. 10 is a plot of delay sensitivity to input characteristics. The delay through a prior-art differential buffer such as shown in FIG. 2 is plotted as lines  202 . The vertical axis is delay while the horizontal axis is the common-mode offset. The four lines in lines  202  are for different input amplitudes, or the maximum difference of V+ and V−. The amplitude difference is stepped from 0.6 to 1.8 volts in four steps. 
     The same conditions of input common-mode offset and amplitude difference are plotted as lines  200  for the clamped differential buffer of FIGS. 3-8. While the overall delay is higher due to the added delay of the input clamp, the variation in delay is greatly reduced. 
     For example, at an offset of −200 MilliVolt delay for the prior-art buffer varies from 820 pico-seconds (ps) to 920 ps, a difference in delay of 100 ps. In contrast, at the same −200 millivolt offset, the clamped buffer has a minimum delay of 865 ps and a maximum delay of 880 ps, for a difference in delay of only 15 ps. At the midrange of zero offset, the delay range is reduced from 40 ps (840-800 ps) for the standard buffer to 3 ps (860-857 ps) for the new clamped buffer. Thus delay range and sensitivity to amplitude is reduced by more than a factor of 10. 
     Alternate Embodiments 
     Several other embodiments are contemplated by the inventor. For example additional components such as transistors, capacitors, resistors and inductances can be added. Transistor polarities can be reversed with straightforward design changes. Any voltage-to-current converter and controlled bias can be used with the amplifier. 
     The need for matched input buffers can occur on many other kinds of circuits besides zero-delay buffers. High-speed data busses may benefit from better-matches input buffers, address and control signals that match more closely may allow a system to be run at a higher rate, and communication lines may benefit from better signal matching. Output buffers or general drivers could benefit from the invention, because their delays could match. Data and other communication receivers would benefit also, because different inputs would have equal delays. 
     Other bias-voltage generators, clamps, and differential stages can be substituted. For example amplifiers with active loads and clamps could be used instead of resistors. A fixed current could be used for the bias generator. A differential clamp of the passive or active variety could be used instead of one referred to the supplies. Resistor loads could be added above the differential transistors in the first differential stage. Rather than use resistor loads, transistor loads can be used, and controlled bias voltages can drive gates of these load transistors. 
     The abstract of the disclosure is provided to comply with the rules requiring an abstract, which will allow a searcher to quickly ascertain the subject matter of the technical disclosure of any patent issued from this disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 C.F.R. § 1.72(b). Any advantages and benefits described may not apply to all embodiments of the invention. When the word “means” is recited in a claim element, Applicant intends for the claim element to fall under 35 USC § 112, paragraph 6. Often a label of one or more words precedes the word “means”. The word or words preceding the word “means” is a label intended to ease referencing of claims elements and is not intended to convey a structural limitation. Such means-plus-function claims are intended to cover not only the structures described herein for performing the function and their structural equivalents, but also equivalent structures. For example, although a nail and a screw have different structures, they are equivalent structures since they both perform the function of fastening. Claims that do not use the word means are not intended to fall under 35 USC § 112, paragraph 6. Signals are typically electronic signals, but may be optical signals such as can be carried over a fiber optic line. 
     The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.