Abstract:
A level shift circuit and method for level shifting the common-mode voltage of a power-supply-referenced circuit to the common-mode voltage of a ground-referenced circuit. The level shift circuit and method entails performing a common-mode level shifting of input complementary signals derived from the power-supply-referenced circuit to produce output complementary signals for the ground-referenced circuit. Because the level shifting is performed in a common manner (i.e. the level shift or voltage drop is common to both complementary signals), pulse-width distortion is substantially reduced if not eliminated. The level shift circuit includes a voltage drop device common to both sides of a differential pair to produce the desired level shift of the output complementary signals. Yet another embodiment relates to a multi-stage level shift circuit and method for level shifting in steps an input common-mode voltage to an output common-mode voltage.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to level-shift circuits, and in particular, to level-shift circuits and related methods that perform common-mode level shifting of the common-mode voltage of a power-supply-referenced circuit to the common-mode voltage of a ground-referenced circuit, and for distributing the overall voltage shift among a plurality of level-shifting stages. 
     BACKGROUND OF THE INVENTION 
     In many applications, there is a need to level shift the complementary signals of a power-supply-referenced circuit where the signals are referenced from the power supply voltage Vcc to the complementary signals of a ground-referenced circuit where the signals are referenced from ground potential. For example, the power-supply-referenced circuit may be a data latch or multiplexer where the complementary output signals swing between 3.3 Volts and 3.1 Volts and the ground-referenced circuit may be a laser driver or low voltage differential signaling (LVDS) circuit where the complementary input signals swing between 1.5 Volts and 1.3 Volts. Generally, the level shifting is referred to with reference to the common-mode voltage, i.e. the average of the complementary signal levels. In the above example, the level shifting is from a common-mode voltage of 3.2 Volts (average of 3.3 Volts and 3.1 Volts) of the power-supply-referenced circuit to the common-mode voltage of 1.4 Volts (average of 1.5 Volts and 1.3 Volts) of the ground-referenced circuit. Typically, a level-shift circuit is used to perform the level shifting of the common-mode voltage of the power-supply-referenced circuit to the common-mode voltage of the ground-referenced circuit. 
     FIG. 1A illustrates a schematic diagram of an exemplary representation of a level shift circuit  100  interfacing a power-supply-referenced circuit  102  and a ground-referenced circuit  104 . In this example, the level shift circuit  100  is represented as two variable batteries  106  and  108  respectively coupling the respective outputs of the power-supply-referenced circuit  102  to the inputs of the ground-referenced circuit  102 . The ground-referenced circuit  104  may include an input differential pair of bipolar transistors Q 15  and Q 16  having bases respectively coupled to the variable batteries  108  and  106  of the level shift circuit  100  and emitters connected in common to a current source represented as bipolar transistor Q 17 . 
     The power-supply-referenced circuit  102  may include a pair of differential bipolar transistors Q 11  and Q 12  having bases configured to receive complementary signals, emitters electrically connected in common to a tail current source I 11 , and collectors respectively connected to collector resistors R 11  and R 12 . The power-supply-referenced circuit  102  further consists of emitter-follower output bipolar transistors Q 13  and Q 14  having bases respectively connected to the collectors of transistors Q 12  and Q 11 , emitters respectively connected to current sources I 12  and I 13 , and collectors connected to the power supply rail, which is connected to the source resistors R 11  and R 12  as well. The emitters of the emitter-follower output transistors Q 13  and Q 14  serve to produce the complementary output signals of the power-supply-referenced circuit  102 , and are respectively coupled to the variable batteries  106  and  108  of the level shift circuit  100 . 
     The common-mode voltage of the power-supply-referenced circuit  102  may be at a different voltage level than the common-mode voltage of the ground-referenced circuit  104 . In addition, the common-mode voltage of the power-supply-referenced circuit  104  varies with changes in the power supply voltage Vcc, with changes in temperature, and with changes in the production process. The common-mode voltage of the ground-referenced circuit  104  should be substantially independent to variations of the power supply voltage Vcc, temperature and process. Thus, the level shift circuit  100  performs two functions: (1) to provide the necessary voltage shift of the common-mode voltage of the power-supply-referenced circuit  102  to the common-mode voltage of the ground-referenced circuit  104  and (2) to isolate variations in the power supply voltage Vcc, temperature, and process from the common-mode voltage of the ground-referenced circuit  104 . Thus, the level shift circuit is represented as variable batteries  106  and  108  to perform such functions. 
     FIG. 1B illustrates a schematic diagram of a prior art level shift circuit  150  for shifting the common-mode voltage of the power-supply-referenced circuit  102  (as previously described) to the common-mode voltage of the ground-referenced circuit  104  (as previously described). The prior art level shift circuit  150  consists of resistors R 13  and R 14  respectively coupling the outputs of the power-supply-referenced circuit  102  to the inputs of the ground-referenced circuit  104 , and variable current sources I 14  and I 15  coupled respectively to the resistors R 13  and R 14  and to ground potential. The respective voltage drops across the resistors R 13  and R 14  are formed by the currents induced through them by the respective current sources I 14  and I 15 . The voltage drops across the resistors R 13  and R 14  provide the appropriate level shift between the power-supply-referenced circuit  102  and the ground-referenced circuit  104 . In addition, the current sources I 14  and I 15  are made variable to absorb any variations in the quiescent voltage difference between the two circuits  102  and  104 . In addition, capacitors C 11  and C 12  may be coupled across respective resistors R 13  and R 14  to reduce the adverse effects of the resistors R 13  and R 14  on the frequency response of the circuit. 
     A drawback of the prior art level shift circuit  150  stems from the level shifting of the complementary signals being performed independently of each other. In this case, the resistor R 13  and current source I 14  perform the level shift of one of the complementary signals independently of the level shift performed by resistor R 14  and current source I 15  on the other complementary signal. In order to avoid the formation of pulse-width distortion of the signals at the ground-referenced section, it is desirable for the level shift on each of the complementary signals to be substantially the same. However, it is difficult to provide accurately-matched equal resistors R 13  and R 14  and current sources I 14  and I 15  in order to achieve substantially the same level shift for each signal. Another drawback is the adverse effects of the resistors R 13  and R 14  on the frequency response of the circuit, even though capacitors C 11  and C 12  are employed to reduce these adverse effects; desirably R 13  and R 14  should be small. Yet another drawback of R 13  and R 14  and of I 14  and I 15  is that they consume a relatively large amount of power, which is undesirable; the is especially the case when R 13  and R 14  are small, as is desirable. 
     Thus, there is a need for level shift circuits and related methods that overcome the above-mentioned drawbacks of the prior art level shift circuit. Such a need and others are met with the level shift circuits and related methods in accordance with the invention. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention relates to a method of level shifting the common-mode output voltage of a power-supply-referenced circuit to the common-mode input voltage of a ground-referenced circuit. The method entails performing a common (as distinct from independent and separate) level shifting of complementary signals derived from the power-supply-referenced circuit to produce the complementary signals for the ground-referenced circuit. Because the level shifting is performed in a common manner (i.e. the level shift or voltage drop is common to both complementary signals), pulse-width distortion is substantially reduced if not eliminated. A specific example of common-mode level shifting entails applying input complementary signals to bases of a differential pair of bipolar transistors, wherein the output complementary signals are derived from the respective collectors of the differential transistor pair, and using a voltage drop device common to both sides of the differential transistor pair to level shift the output complementary signals. 
     The common-mode level shifting method may also entail providing feedback control for controlling the amount of level shifting. Such feedback control may entail generating a feedback voltage related to the common-mode voltage of the shifted output complementary signals and adjusting the voltage drop device until the feedback voltage is substantially the same as an external reference voltage. Adjusting the voltage drop device may entail varying a tail current of the differential pair of bipolar transistors inversely with the difference between the reference voltage and the feedback voltage. In addition, generating the feedback voltage may entail applying the shifted output complementary signals to a differential pair of bipolar transistors of a following level shifting stage and generating the feedback voltage from the tail voltage of the differential pair of bipolar transistors of the following stage. 
     Another aspect of the invention relates to a level shift circuit that implements the common-mode level shifting. The level shift circuit comprises a differential pair of bipolar transistors having bases to respectively receive input complementary signals, emitters coupled in common to a first (tail) current source, and collectors respectively coupled to first and second collector resistors. In addition, the level shift circuit comprises a pair of emitter-follower output bipolar transistors having bases respectively coupled to the collectors of the differential pair of bipolar transistors, emitters respectively coupled to second and third current sources, and collectors coupled to the power supply rail, wherein the output complementary signals are respectively produced at the emitters of the emitter-follower output bipolar transistors. To generate the common-mode level shift, the level shift circuit includes a voltage drop device connected between the power supply rail and the collector resistive elements, whereby the voltage drop device produces a voltage drop related to the desired level shift of the output complementary signals. 
     In a more specific embodiment of the level shift circuit, the voltage drop device comprises a current source, and the tail current source comprises an open-collector-output operational amplifier. The operational amplifier varies the tail current inversely with the difference between an external reference voltage applied to the positive terminal of the operational amplifier and a feedback voltage applied to the negative terminal of the operational amplifier. The feedback voltage is related to the common-mode voltage of the shifted complementary output signals. 
     Yet another aspect of the invention relates to a multi-stage level shift circuit and method of level shifting in steps an input common-mode voltage of input complementary signals to the output common-mode voltage of output complementary signals. The level shift circuit method comprises performing a plurality of common-mode level shifts in cascade to produce the output complementary signals from input complementary signals. Each level shift is performed by a common-mode level shifting circuit as described above. In addition, the multi-stage level shift circuit may include a reference voltage circuit for generating a plurality of reference voltages for controlling the amount of respective level shifts for the level shifting stages. 
     The level shift circuit may be implemented with the n-MOS, p-MOS or CMOS technologies as well as bipolar technology. Other aspects, features and techniques of the invention will become apparent to one skilled in the relevant art in view of the following detailed description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates a schematic diagram of an exemplary representation of a level shift circuit interfacing a power-supply-referenced circuit to a ground-referenced circuit; 
     FIG. 1B illustrates a schematic diagram of a prior art level shift circuit for shifting the common-mode voltage of a power-supply-referenced circuit to the common-mode voltage of a ground-referenced circuit; 
     FIG. 2 illustrates a schematic diagram of an exemplary level shift circuit in accordance with the invention; 
     FIG. 3 illustrates a schematic/block diagram of an exemplary multi-stage level shift circuit in accordance with the invention; 
     FIG. 4 illustrates a schematic diagram of another exemplary level shift circuit that can be used as an individual stage of a multi-stage level shift circuit in accordance with the invention; 
     FIG. 5 illustrates a schematic/block diagram of another exemplary multi-stage level shift circuit in accordance with the invention; and 
     FIG. 6 illustrates a schematic diagram of yet another exemplary level shift circuit in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates a schematic diagram of an exemplary level shift circuit  200  in accordance with the invention. The level shift circuit  200  comprises an input differential pair of bipolar transistors Q 21  and Q 22  having bases to respectively receive complementary signals to be level shifted, emitters connected in common to a tail current source I 11 , and collectors respectively connected to collector resistors R 21  and R 22 . The level shift circuit  200  further comprises emitter-follower output bipolar transistors Q 23  and Q 24  having bases respectively connected to the collectors of transistors Q 22  and Q 21 , emitters respectively connected to current sources I 22  and I 23 , and collectors coupled to the power supply rail. The shifted output complementary signals of the level shift circuit  200  are taken off the respective emitters of transistors Q 23  and Q 24 . 
     In order to provide a level shift of the common-mode voltage of the input signals to the common-mode voltage of the output signals, the exemplary level shift circuit  200  includes a variable voltage drop device V adj  represented as a variable battery. The variable voltage drop device V adj  reduces the supply voltage applied to the differential transistor pair Q 21  and Q 22  by an amount of V adj . This shifts the input signals by an amount which is related to V adj  to produce the shifted output signals. Since the variable voltage drop device V adj  is common to both sides of the differential transistor pair Q 21  and Q 22 , the level shifting is common to both complementary signals. Thus, the level shifting of the complementary signals is performed in a common-mode manner, and consequently, eliminates or reduces pulse width distortion that typically results from unequal level shifting of the complementary signals. The maximum amount of downward level shifting performed by the level shift circuit  200  is approximately one base-emitter voltage drop V be . The circuit can also produce an upwards level shift. Although the variable voltage drop device V adj  is represented as a variable battery, it shall be understood that the variable voltage drop device V adj  can be any device that causes a desired level shift to both complementary signals. 
     FIG. 3 illustrates a schematic/block diagram of an exemplary multi-stage level shift circuit  300  in accordance with the invention. The multi-stage level shift circuit  300  shifts the common-mode voltage of a power-supply-referenced circuit  302  to the common-mode voltage of a ground-referenced circuit  304 . As previously discussed, the output of the power-supply-referenced circuit  302  may include emitter-follower output transistors Q 33  and Q 34  respectively coupled to current sources I 32  and I 34 . Also, the outputs of the power-supply-referenced circuit  302  are taken off the respective emitters of transistors Q 33  and Q 34 . The ground-referenced circuit  304  may include an input circuit having differential transistor pair Q 35  and Q 36  with bases to receive the level-shifted complementary signals and emitters connected in common to a current source I 37 . 
     In this case, the exemplary level shifting circuit  300  comprises a plurality of level shifting stages  306 - 1  through  306 -N that perform the desired total level shift of the common-mode voltage of the power-supply-referenced circuit  302  to the appropriate common-mode voltage of the ground-referenced circuit  304 . One or more of the level shifting stages  306 - 1  through  306 -N may be of the type that perform common-mode level shifting, such as the level shift circuit  200  shown in FIG.  2 . The level shifting stages  306 - 1  through  306 -N respectively shift the common-mode voltage in discrete steps. The total level shift may be distributed equally or unequally between the level shifting stages  306 - 1  through  306 -N. 
     The exemplary level shift circuit  300  may further comprise a reference voltage circuit  308  that generates reference voltages V R1  through V RN  that set the respective common-mode voltages at the output of respective level shifting stages  306 - 1  through  306 -N. In other words, the progression of the level shifting performed by the level shifting stages  306 - 1  through  306 -N correlates with the progression of the reference voltages from V R1  to V RN . As an example, if the common-mode output voltage of the power-supply referenced circuit is (V cc −0.9V) and if the common-mode input voltage of the ground-referenced circuit  304  is required to be 1.5V, and if the total shift is to be accomplished in three stages, then the common-mode output voltages of shift cells 1-3 might be (2/3V cc −0.1V), (1/3V cc +0.7V) and 1.5V. For situation in which V cc =3.0V, these common-mode voltages correspond to 1.9V, 1.7V and 1.5V, with a downwards shift of 0.2V per stage. However, if V cc =3.6V, they correspond to 2.3V, 1.9V and 1.5V, and a downwards shift of 0.4V per stage. In these examples, the level shift is uniformly distributed between the stages, but this is not essential to the invention, and may not always be desirable. As will be shown in a following embodiment, each level shifting stage uses the reference voltage to control the amount of level shifting. 
     In the exemplary embodiment, the voltage reference circuit  308  is configured as a voltage divider comprising a plurality of series-connected resistors R R1  through R RN  with voltage offsets  310   a-b  represented as batteries on either side of the voltage divider. The voltage reference circuit  308  may be connected to the power supply rail voltage Vcc or some other voltage source. 
     FIG. 4 illustrates a schematic diagram of another exemplary level shift circuit  400  that can be used as an individual stage of a multi-stage level shift circuit in accordance with the invention. The level shift circuit  400  comprises an input differential pair of bipolar transistors Q 41  and Q 42  having bases to respectively receive complementary signals to be level shifted and collectors respectively connected to collector resistors R 41  and R 42 . The level shift circuit  400  further comprises output emitter-follower bipolar transistors Q 43  and Q 44  having bases respectively connected to the collectors of transistors Q 42  and Q 41 , emitters respectively connected to current sources I 42  and I 43 , and collectors coupled to the power supply rail. The shifted outputs of the level shift circuit  400  are taken off the respective emitters of transistors Q 43  and Q 44 . 
     In order to provide a controlled level shift, the exemplary level shift circuit  400  further comprises an open-collector-output operational amplifier  402  having an output coupled to the emitters of the differential transistor pair Q 41  and Q 42 , a positive terminal for receiving a reference voltage from a reference voltage circuit, and a negative terminal for receiving as feedback the tail voltage of differential pair Q 41  and Q 42  of the following stage. If the exemplary level shift circuit  400  is not the first stage of a multi-stage level shift circuit, the tail voltage of differential pair Q 41  and Q 42  is coupled to the negative terminal of the open-collector-output operational amplifier of the previous stage. If the exemplary level shift circuit  400  is the first stage of a multi-stage level shift circuit, the tail voltage of differential pair Q 41  and Q 42  may be coupled to a reference voltage circuit in order to generate the respective reference voltages for the multi-stage level shift circuit, as discussed in further detail below. In addition, a current source I 41  is provided between the power supply rail and the collector resistors R 41  and R 42  to generate the necessary voltage drop to provide the desired level shift. 
     In operation, the exemplary level shift circuit  400  controls the amount of level shifting by feeding back the output of the level shift circuit  400  to the negative terminal of the open-collector-output operation amplifier  402  by way of the base-emitter drop of the input differential transistor pair of the following stage. The open-collector-output operational amplifier  402  functions as a voltage-controlled current source that causes the tail current of the input differential pair Q 41  and Q 42  to vary inversely with the difference between the reference voltage and the tail voltage of the following stage. Thus, if the tail voltage of the next stage is below the reference voltage (meaning that the output of the level shift circuit is below the appropriate level), the tail current of the input differential pair Q 41  and Q 42  is decreased, to increase the collector voltages of the differential transistor pair Q 41  and Q 42 , thereby increasing the output voltages to cause the tail voltage of the following stage to substantially equal the reference voltage. The opposite effect occurs if the tail voltage of the following stage is above the reference voltage. In this manner, the feedback loop maintains the tail voltage substantially at the reference voltage. 
     In the above manner, the reference voltage circuit can be designed to generate reference voltages for each of the level shifting stage in order to set the various level shifts for the stages. For instance, a reference voltage circuit can be designed to cause each level shifting stage in a multi-stage level shifting circuit to level shift by a variable amount V x , up to a maximum downwards shift of approximately one base-emitter voltage drop for each level shifting stage. Referring to FIG. 4 again, the tail voltage for the differential pair Q 41  and Q 42  can be set by the previous stage to be at V Ri . Thus, the common-mode voltage for the input signals of the level shift circuit  400  is at (V Ri +V be ), one base-emitter voltage V be  above the tail voltage V Ri . The reference voltage for level shift circuit  400  is set at V R(i+1) =(V Ri −V x ), which sets the tail voltage of the following stage also at (V Ri −V x ). Thus, the common-mode voltage of the output of the level shift stage  400  is (V Ri +V be −V x ), one base-emitter voltage V be  above the tail voltage (V Ri −V x ) of the following stage. Thus, the level shift circuit  400  has shifted the common-mode voltage by the desired amount V x , due to the same V x  voltage change progression of the reference voltage circuit. 
     FIG. 5 illustrates a schematic/block diagram of another exemplary multi-stage level shift circuit  500  in accordance with the invention. The multi-stage level shift circuit  500  shifts the common-mode voltage of a power-supply-referenced circuit  502  to the common-mode voltage of a ground-referenced circuit  504 . As previously discussed, the output of the power-supply-referenced circuit  502  may include emitter-follower output transistors Q 53  and Q 54  respectively coupled to current sources I 52  and I 54 . Also, the outputs of the power-supply-referenced circuit  502  are taken off the respective emitters of transistors Q 53  and Q 54 . The ground-referenced circuit  504  may include an input circuit having a differential transistor pair Q 55  and Q 56  with bases to receive the level-shifted complementary signals and emitters connected in common to a current source I 57 . 
     The multi-stage level shift circuit  500  comprises a plurality of level shifting stages  506 - 1  through  506 -N of the same type as level shift circuit  400  shown in FIG.  4 . The multistage level shift circuit  500  further comprises a voltage reference circuit  508  to generate a plurality of reference voltages V R1  to V RN  to set the respective amounts of level shifting of the level shifting stages  506 - 1  through  506 -N. In this case, the voltage reference circuit  508  comprises a voltage divider having a plurality of series-connected resistors R R1  to R RN , a voltage offset  510   b  in the form of a diode-connected transistor to set the tail voltage of the ground-referenced circuit  504  to approximately one base-emitter voltage drop V be  above ground, and a voltage offset  510   a  in the form of a Schottky diode and a diode-connected transistor  512  coupled to the tail voltage of the first level shifting stage  506 - 1 . The Schottky diode  510   a  provides a minimum level shift, for example 0.3 Volt, for the first level shifting stage  506 - 1  to prevent saturation of the differential transistor pair Q 41  and Q 42  and/or the current source I 41  in the case of a relatively low V CC  Voltage (See FIG.  4 ). The diode-connected transistor  512  restricts the maximum shift for the first level shifting stage  506 - 1  to one base-emitter voltage drop V be , the maximum shift permitted in one stage. 
     FIG. 6 illustrates a schematic diagram of an exemplary level shift circuit  600  in accordance with the invention. The level shift circuit  600  is similar to level shift circuit  400  but with a specific embodiment for the open-collector-output operational amplifier, and capacitors to provide stability to the feedback loop and to filter power supply noise. Specifically, the level shift circuit  600  comprises an input differential pair of bipolar transistors Q 61  and Q 62  having bases to respectively receive complementary signals to be level shifted and collectors respectively connected to collector resistors R 61  and R 62 . The level shift circuit  600  further comprises output emitter-follower bipolar transistors Q 63  and Q 64  having bases respectively connected to the collectors of transistors Q 62  and Q 61 , emitters respectively connected to current sources I 62  and I 63 , and collectors coupled to the power supply rail. The shifted outputs of the level shift circuit  600  are taken off the respective emitters of transistors Q 63  and Q 64 . 
     The exemplary level shift circuit  600  further comprises an open-collector-output operational amplifier comprising a p-n-p differential transistor pair Q 65  and Q 66  having bases to respectively receive the reference voltage and the tail voltage of the following stage, emitters respectively connected to resistors R 63  and R 64 , and collectors respectively connected to the collectors of current-mirror transistors Q 66  and Q 67 . The collector of the transistor Q 67  is coupled to the bases of transistors Q 66  and Q 67  to provide the current-mirror function of the transistors. The emitters of the transistor Q 66  and Q 67  are respectively coupled to resistors R 65  and R 66 . A current source I 64  is provided between the power supply rail and the resistors R 63  and R 64  to set the currents through the differential transistor pair Q 65  and Q 66 . The operational amplifier further includes an output transistor Q 68  having a base coupled to the collector of transistors Q 65  and Q 66 , a collector coupled to the emitters of the differential transistor pair Q 61  and Q 62 , and an emitter coupled to ground. 
     In operation, the collector current through the operational-amplifier output transistor Q 68  varies inversely with the difference between the reference voltage and the feedback voltage (i.e. the tail voltage of the following stage). Thus, if the feedback voltage is lower than the reference voltage (meaning that the output common-mode voltage is below the desired value), the current through the output transistor Q 68  decreases, which consequently increases the common-mode voltage of the level shift circuit  600  until the feedback voltage substantially equals the feedback voltage. The opposite effect occurs if the feedback voltage is above the reference voltage. The capacitor CF and Cc provide stability to the feedback loop and filter out noise. 
     It will be appreciated that the peak-to-peak signal voltages developed at the collectors of Q 61  and Q 62 , and therefore the peak-to-peak complementary signal output voltages from the circuit, are determined by current source I 61  and collector resistors R 61  and R 62 , and are therefore independent of the amount of the level shift performed by the circuit. 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.