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.
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 Q15 and Q16 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 Q17.
The power-supply-referenced circuit 102 may include a pair of differential bipolar transistors Q11 and Q12 having bases configured to receive complementary signals, emitters electrically connected in common to a tail current source I11, and collectors respectively connected to collector resistors R11 and R12. The power-supply-referenced circuit 102 further consists of emitter-follower output bipolar transistors Q13 and Q14 having bases respectively connected to the collectors of transistors Q12 and Q11, emitters respectively connected to current sources I12 and I13, and collectors connected to the power supply rail, which is connected to the source resistors R11 and R12 as well. The emitters of the emitter-follower output transistors Q13 and Q14 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 R13 and R14 respectively coupling the outputs of the power-supply-referenced circuit 102 to the inputs of the ground-referenced circuit 104, and variable current sources I14 and I15 coupled respectively to the resistors R13 and R14 and to ground potential. The respective voltage drops across the resistors R13 and R14 are formed by the currents induced through them by the respective current sources I14 and I15. The voltage drops across the resistors R13 and R14 provide the appropriate level shift between the power-supply-referenced circuit 102 and the ground-referenced circuit 104. In addition, the current sources I14 and I15 are made variable to absorb any variations in the quiescent voltage difference between the two circuits 102 and 104. In addition, capacitors C11 and C12 may be coupled across respective resistors R13 and R14 to reduce the adverse effects of the resistors R13 and R14 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 R13 and current source I14 perform the level shift of one of the complementary signals independently of the level shift performed by resistor R14 and current source I15 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 R13 and R14 and current sources I14 and I15 in order to achieve substantially the same level shift for each signal. Another drawback is the adverse effects of the resistors R13 and R14 on the frequency response of the circuit, even though capacitors C11 and C12 are employed to reduce these adverse effects; desirably R13 and R14 should be small. Yet another drawback of R13 and R14 and of I14 and I15 is that they consume a relatively large amount of power, which is undesirable; the is especially the case when R13 and R14 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.
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.