Abstract:
A differential stage circuit is disclosed, which includes a differential circuit, a current source coupled to supply, when activated, an operating current to the differential circuit, and a control circuit coupled to control activation and deactivation of the current source. The differential stage circuit further includes a compensation circuit configured to supply a compensation pulse to the current source when the current source is activated.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a circuit system for transmitting data to a receiver, and wherein the receiver may be quickly enabled. 
     DESCRIPTION OF RELATED ART 
     Data transmission takes place, for example, in electronic systems, such as computers, between a memory controller device and a memory device. Such a memory device could be a dynamic random access memory (DRAM). 
     While standard DRAM interfaces are typically 64 bit wide, graphic systems use much wider interface widths such as 256 bit or even 512 bit to achieve the required bandwidth. If using that many receivers, the receiver&#39;s power consumption is a concern. Hence, it is desired to provide a fast enabling of the receivers which would allow to power down the receivers during even short idle states and thus decrease the average power consumption. 
     In order to provide a circuit system for data transmission, a differential or single ended data transmission could be used. However, differential receiving stages are more advantageous, since they decrease the sensitivity to signal and supply noise for high frequency data transmission. Further, in case of differential signaling, there is no need for any additional reference signals: complementary signals may act like references for each other. In case of single ended signaling, an additional reference voltage V_REF has to be delivered to the receiver or may be generated on-chip. 
     As an example, a typical data receiver structure is shown in  FIG. 1 . The differential input pair is comprised of n-channel MOS transistors N 1  and N 2  receiving the data DAT_IN_T and the corresponding complementary data DAT_IN_C, respectively. The n-channel MOS transistors N 4  and N 5  operates as a current mirror to act as a tail current source, and capacitor C 1  ensures a stable gate bias voltage for transistor N 5 . The bias current I_BIAS may be delivered from an external circuitry (not shown). N-channel MOS transistor N 3  provides the enable functionality by receiving the enabling signal RCV_EN corresponding to the original enabling signal EN_N (after having passed the inverter I 1 ). 
     A disadvantage of this known configuration is that a good common mode rejection and output signal symmetry can only be achieved with high (ideally infinite) differential impedance of the tail current source. However, in a realistic circuit, a parasitic capacitance is present at the tail node (“TAIL” in the Figs.). In this particular implementation as shown in  FIG. 1 , the enabling transistor N 3  adds significant additional parasitic capacitance (“C parasitic”) to the tail node TAIL which drastically reduces the impedance of the tail node. In other words, a relatively high tail current I_TAIL may require a quite large transistor size of N 3 , resulting in a large parasitic capacitance so that a charge sharing effect occurs. 
     Further, when transistor N 3  is enabled, it quickly pulls the drain of transistor N 3  from supply potential to the operation point voltage. This transition may result in an overshoot-like behavior over the current with a 20-30% larger tail current for a transitional time period as shown in  FIG. 3   a . In the first graph, the differential data input signals DAT_IN_T and DAT_IN_C waveforms are shown in a particular time frame. In this time frame, the waveform of the enable signal RCV_EN is shown in the second graph, with the enable signal being activated shortly before the first data input is applied. In the third graph, the tail current I_TAIL features the discussed overshoot-like behavior for a particular duration of time, resulting in the distorted data output DAT_OUT_T and DAT_OUT_C as shown respectively in the fourth and fifth graphs of  FIG. 3   a . This means that the receiver characteristic is different for the duration of this overshoot, limiting the ability to reliably receive fast signals for this transitional time period. 
     Furthermore, transient charge sharing between drain and gate of transistor N 5  may also increase the current overshoot: when the signal EN_N is pulled low, extra charge is injected into capacitor C 1 , thereby increasing the gate potential on transistor N 5 . This effect may be reduced by a large capacitor C 1 , but in practice area constraints limit the capacitance of C 1  to a few picofarads (pF). 
     One known alternative for the  FIG. 1  configuration is shown in  FIG. 2 , improving the differential input stage&#39;s performance: the enable transistor N 6  is connected to the source network of current sourcing transistor N 4 . In this case, the enable transistor could be sized as large as needed for the low voltage drop, and the parasitic capacitance does not anymore affect the output signal quality. In doing so, much higher tail node impedance compared to the circuit in  FIG. 1  may be achieved. 
     However, the configuration of the circuit as shown in  FIG. 2  containing the capacitance C 1  to obtain a stable gate current, leads to a slower enable as shown in  FIG. 3   b , when the signal RCV_EN is pulled high, the source potential of transistor N 4  is rapidly pulled towards ground, and the charge sharing between gate-source capacitance of transistor N 4  and capacitor C 1  reduces the effective gate-source voltage of transistor N 4  and thereby the tail current. This results in a slow current settling behavior (see third graph in  FIG. 3   b ), thereby leading to a different receiver characteristic for the duration of this transition, so that the ability to reliably receive signals is again limited for this transitional time period. Similar as already discussed above, a large capacitance of C 1  would avoid these effects, but in practice, such a capacitor would be too large on planar IC (“integrated circuit”) technology. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a circuit system providing a compensation pulse to the current source of a differential input receiver stage. The compensation pulse is generated in response to an enable signal controlling the activation of the receiver&#39;s tail current source. 
     In a first aspect of the present invention, a circuit system for data transmission comprises a differential stage for receiving data, a tail current source configured to supply a bias current to the differential stage, an enabling element coupled to the tail current source to control the tail current to the differential stage, and a circuit providing means configured to generate a compensation pulse to the tail current source in response to the receipt of an enable signal by the enabling element. 
     According to a second aspect of the present invention, there is provided a circuit system for transmitting and receiving data, which comprises a first and a second transistor coupled to a common node and functioning as a differential stage for receiving data, a capacitor coupled to a tail current source comprising a third transistor and configured to supply a bias current to the differential stage, the third and a fourth transistor coupled in series between the common node and the capacitor, and a circuit providing means configured to generate a charge compensation pulse to the tail current source and coupled in parallel or in series to the capacitor. 
     According to a third aspect of the present invention, a method for transmitting and receiving data, comprises (a) providing a circuit system comprising a first and a second transistor functioning as a differential stage, a third transistor functioning as a tail current source for the differential stage, and a fourth transistor functioning as an enabling element for the current source; (b) providing an enable signal to the fourth transistor to control the functionality of the third transistor; and (c) generating a charge compensation pulse to the third transistor in response to the activation of the third transistor. 
     Thus, the differential stage may comprise a first and a second transistor coupled by a common node, and may provide higher immunity against input and supply noise compared to a single ended stage, especially, if data transmission occurs at high frequencies (above about 1 GHz) and with low signal swing as in the present case. 
     The tail current source for supplying the differential stage may, for example, be realized by a (symmetrical) current mirror comprising a first and a second transistor coupled by a common node via their gates. Further, the tail current source may be coupled to an enabling element controlling the operation to the differential stage, for example, by activating the transistor(s) of the current mirror. 
     The enabling element may be realized by a transistor which may be rendered conductive or non-conductive, depending on an enable signal controlling the functionality of the enabling transistor. 
     The enable signal may be a control signal for switching on the circuit system in order to allow receipt of any data. For example, the enable signal may provide an “enable” function of the circuit system, so that the system is enabled to receive any data. In realistic circuits, the enable signal may pass a buffer/inverter and not the “original” enable signal, but a corresponding enable signal may be received by the enabling element and/or the compensation pulse provider. In the following, the expression “enable signal” may mean both, the original enable signal and the actually received (“realistic”) enable signal. 
     In response to the enabling element being rendered conductive or non-conductive by the enable signal, charge sharing effects may occur as discussed above, so that the tail current source supplying the differential stage receiving the data does not provide the needed tail current for the differential stage within a particular time frame. 
     The compensation pulse provider in the circuit system according to the present invention compensates or substantially compensates the charge sharing effects by providing a charge compensation pulse to the tail current source of the differential stage as soon as the enable signal is received. 
     According to preferred embodiments of the present invention, the compensation pulse provider may be realized by a resistor divider or by a capacitance divider or by an additional current source. 
     In particular, the resistor divider and the current source may be coupled between a capacitor and the enabling element, wherein the capacitor is coupled to the tail current source. The capacitance divider may be arranged so that the first capacitor is coupled between the enabling element and the tail current source, and the second capacitor is arranged in parallel to the first capacitor, between the tail current source and a source of the enable signal. 
     In view of the fact that the tail current source for the differential stage may be a current mirror or even a transistor, and the enabling element may be a transistor, the circuit system of the present invention may be realized by providing a first and second transistor as a differential stage, a third transistor as a current source, a fourth transistor as an enabling element, and a compensation pulse provider for generating a compensation pulse to the tail current source for the differential stage. 
     If a resistor divider is used as a compensation pulse provider, the resistor divider may contain two resistors connected by a common node. The resistor divider ratio may be equal to the ratio of the capacitance of the current source and the capacitance of the capacitor between the resistor divider and the current source. 
     For example, the two resistors may have characteristic values of about 10Ω to about 100 kΩ, and the values are different for the two resistors, e.g. the first resistor may have a value of about 100Ω, whereas the second resistor may have a value of about 10Ω. In a preferred embodiment, the value of the first resistor is smaller than the value of the second resistor. 
     If a capacitance divider is used as a compensation pulse provider, the capacitor values may be between about 50 fF and about 20 pF. Preferably, the values are different for the two capacitors, e.g. the capacitor coupled between the current source and the enabling element may have a capacitance of about 50 fF, whereas the other capacitor may have a larger capacitance than the first one, for example, a capacitance of about 10 pF. 
     If a current source is used as a compensation pulse provider, the current source may be a current mirror. For example, the current mirror is a current operated device to control the supply current for the compensation pulse provider. In a preferred embodiment, the current mirror may comprise two transistors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the invention will be apparent from and exemplified with reference to the preferred embodiments taken in conjunction with the accompanied drawings, in which 
         FIG. 1  schematically shows a known data receiver structure; 
         FIG. 2  schematically shows another known data receiver structure; 
         FIGS. 3   a  and  3   b  schematically show the enable behavior of the differential receiver configurations as shown in  FIG. 1  and  FIG. 2 , respectively; 
         FIG. 4  schematically shows a system of a transmitter and a receiver receiving data from the transmitter according to a first embodiment of the present invention; 
         FIG. 5  schematically shows a receiver structure according to a second embodiment of the present invention; 
         FIG. 6   a  schematically shows an ideal enable behavior of a differential receiver; 
         FIG. 6   b  schematically shows the effect of the compensation pulse according to the present inventions in order to improve the differential receiver&#39;s enable behavior; 
         FIG. 7  schematically shows a receiver structure according to a third embodiment of the present invention; and 
         FIG. 8  schematically shows a receiver structure according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to  FIG. 4 , there is shown a system according to a first embodiment of the present invention, in which a transmitter and a receiver are formed respectively as, for example, an individual semiconductor chip, and coupled to data lines to each other. The transmitter transmits a data signal DAT_IN_T and, for example, a complementary data signal DAT_IN_C to the receiver through the data lines. The receiver comprises an input circuit (or buffer) receiving the true and complementary data signals DAT_IN_T and DAT_IN_C. The receiver further includes therein a bias generator for providing a bias current I_BIAS and an enable generator providing the enable signal EN_N, which are then supplied to an input circuit to control an operation thereof. 
     The input circuit of  FIG. 4  may be constituted as shown in  FIG. 5  as a second embodiment of the present invention, in which similar components as discussed with reference to  FIG. 2  above are indicated by the same reference numerals to omit further description thereof. In the circuit of  FIG. 5 , a resistor divider circuit including resistors R 3  and R 4  and an inverter I 2  are further provided, and the capacitor C 1  is connected between the node of the resistor divider (i.e., the connection node of the resistors R 3  and R 4 ) and the bias voltage supply node I_BIAS. The bias voltage I_BIAS is produced by the bias generator as shown in  FIG. 4 . The inverter I 2  inverts the enable signal EN_N, which is produced by the enable generator as shown in  FIG. 4 , and supplies the inverted enable signal to the resistor divider. While, the inverters I 1  and I 2  are provided to indicate a realistic circuit implementation for electrically decoupling signal RCV_EN and the resistor divider R 3  and R 4  from the enable signal EN_N, one of the inverters I 1  and I 2  may be omitted to supply the inverted enable signal RCV_N in common to the resistor divider and the transistor N 6 . The resistor divider including resistors R 3  and R 4  creates a charge compensation pulse to the gate of transistor N 4  in order to lower or even cancel the charge sharing effect resulting from the parasitic capacitances at the transistor N 4 . In a particular embodiment, it is possible to further reduce the parasitic capacitance of the parasitic capacitor at transistor N 4  by providing a small gate length of the transistor N 4 . According to the shown embodiment, the resistance divider ratio R 3 /R 4  is preferably equal to the ratio of the gate-source capacitance of transistor N 4  and capacitance C 1 . 
     As an example, the capacitance of capacitor C 1  is about 1 pF, and the values for resistors R 3  and R 4  are about 10 kΩ and about 100Ω, respectively. In order to keep the bias current source of I_BIAS stable within mV range, C 1  may be designed to be very large. However, due to the fact that—in practice—such a capacitor would be too large on planar integrated circuits technology, a capacitance of not more than about 3.5 pF or at most up to about 5 pF may be used. As long as the resistance of resistor R 4  is relatively small, this resistor does not negatively affect the quality of gate decoupling. In such a configuration, the current I_TAIL is stable within an extremely short period of time, maybe in about 1 ns, after the signal EN_N is pulled low. 
     With the circuit construction described above, the input buffer circuit of  FIG. 5  can operate with ideal signal waveforms as shown in  FIG. 6   a . In the first two graphs, the data input signals and the received enable signal are shown. 
     In an ideal data receiver circuit, the time dependent run of the curve of the tail current I_TAIL corresponds to the pull-up and pull-down of the enable signal EN_N. In this case, the input data DAT_IN_T and DAT_IN_C transmitted to the receiver during the time frame the receiver is enabled, can be non-distortedly received. Hence, the data output as shown in the last two graphs in  FIG. 6   a  is reliably produced. Due to the uniform receiving conditions provided by a fast enabling of the receiver, even different symbols may easily be received over the whole time frame. 
     Turning to  FIG. 6   b , there are shown internal voltage and current waveforms of the  FIG. 5 . In the first line of graphs, the enabling signal RCV_EN is shown, identical to the corresponding signal in  FIG. 6   a . Without compensation (dotted curve), the tail current I_TAIL is reduced in response to the enabling signal because of the charge sharing between the source-gate capacitance of transistor N 4  and capacitor C 1 . The shown slow current settling behavior (see the dotted curve in the third graph) leads to the disadvantages discussed with reference to  FIG. 2  and  FIG. 3   b , above. In other words, the bias current I_BIAS at the node (NODE) linking C 1  and N 4  should ideally be constant, even if an enabling signal is received, but—with the configuration as shown in FIG.  2 —the charge sharing effect between C 1  and N 4  leads to the peaks as shown by the dotted curve in the second graph in  FIG. 6   b : if the enable signal is pulled up or down, the bias current at the node I_BIAS is pulled down and up and slowly settles in response to change of the enable signal RCV_EN. 
     According to the present invention, a compensation pulse is provided in order to equalize the non-uniform bias current I_BIAS. As shown in the fourth graph of  FIG. 6   b , a charge compensation pulse through C 1  may be configured to interfere with the bias current I_BIAS at the node (NODE) so as to provide a resulting curve of an almost constant value for the bias current, i.e. the continuous curve of the second graph in  FIG. 6   b . In particular, the area between the dotted curve and the continuous curve for the bias current I_BIAS represents the charge to be compensated in order to obtain almost ideal conditions. If a compensation is provided, the almost constant bias current thus leads to a tail current I_TAIL (third graph in  FIG. 6   b ) coming close to the ideal curve as shown in  FIG. 6   a  (third graph). In doing so, signals may be received without distortion, since the conditions during the reception are uniform. 
       FIG. 7  schematically shows a circuit according to a third embodiment of the present invention based on the same general concept as discussed above. The embodiment in  FIG. 7  has similar components as the configuration shown in  FIG. 5 , but is different therefrom in that the resistor divider R 3 /R 4  is replaced by using a capacitive divider. The capacitive divider is realized by adding a further capacitance C 2  to the circuit, as shown in  FIG. 7 . 
     In an example, typical values for the capacitors C 1  and C 2  are about 1 pF and about 15 fF, respectively, so that an AC divider ratio close to about 100:1 may be achieved, thereby delivering a compensation of about 15 mV (with 1.5 V supply) to the gate of N 4 . 
     In particular, the capacitance divider C 1 /C 2  provides a charge compensation pulse in this respect that the charge sharing between C 1  and N 4  is compensated in response to the change of the enable signal RCV_EN at C 2 . As discussed above with reference to  FIG. 6   b , the charge compensation pulse leads to an almost constant tail current I_TAIL, thus providing for uniform signal receiving conditions. 
       FIG. 8  schematically shows a circuit according to a fourth embodiment of the present invention based on the same general concept as discussed above. The embodiment in  FIG. 8  has similar components as the configuration discussed with reference to  FIGS. 5 and 7 , but uses a current source S 1  in order to provide the compensation pulse to the node between C 1  and N 4 . The current supply of S 1  is triggered by the input signal EN_N via transistor P 1 . 
     Hence, a charge compensation pulse is provided to lower or even cancel the charge sharing effect between C 1  and N 4  so that a compensated tail current I_TAIL as, for example, shown in  FIG. 6   b  by the continuous curve is achieved. 
     As have been discussed so far, a simple and reliable solution is provided to improve the fast enablement of a differential stage based receiver. Preferably, the compensation pulse generated by the circuits or in the method according to the present invention results in enable times of less than 1 ns, thereby allowing systems and methods to disable the receiver much more frequently, thus achieving substantial power saving. 
     While the invention has been illustrated and described in detail in the foregoing description, such illustration and description are to be considered illustrative or exemplary and non-restrictive; the invention is thus not limited to the disclosed embodiments. Features mentioned in connection with one embodiment described herein may also be advantageous as features of another embodiment described herein without explicitly showing these features. Variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage.