Abstract:
An improved 3.3 V to 5 V interface buffer comprising a solid state BiCMOS device implemented on a reduced voltage process designed to operate from 3.3 V and 5 V supplies and capable of receiving a 0-3.3 V input signal while providing a external swing signal from 0-5 V. Specifically, cross coupled PMOS and NMOS devices manufactured by a 3.3 V process are utilized with level shifting diodes for achieving a device which operates on higher voltages than conventional circuit design techniques allow for a given process technique, while providing a 0-5 V output at the sending device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to digital solid state integrated circuits. More particularly, the present invention relates to interfacing circuits implemented using different fabrication processes. 
     2. Description of Related Art 
     Interfacing electronic devices which operate on different supply voltages and which necessarily present different input and output characteristics is a problem that has long been known in the art. As digital integrated circuits and the devices which they operate become smaller and smaller, much as in the portable computer market, designers have focused on ways to reduce the size, and also accordingly, the power consumption of each component in a given electronic device. In one particular example, designers have begun to develop digital integrated circuits which operate on 3.3 V references, thereby realizing as much as a 40% power reduction from their predecessors utilizing 5 V references. Because of the advent of digital integrated circuits which operate at 3.3 V, the particular problem of interfacing the input and output characteristics of 3.3 V devices with the more commonly used 5 V devices has presented designers with a wide variety of problems. 
     In the prior art, designers have typically interfaced 3.3 V devices and 5 V devices by means of an external voltage level translator operated from a 5 V supply. These extemal voltage level translators receive a given 0-3.3 V input signal from a 3.3 V device and adjust the output level so as to present a 0-5 V output level signal to the receiving 5 V device. However, inherent in this extemal translator solution, designers have had to make sacrifices. Performance characteristics, particularly speed and reliability, are reduced in any implementation where the input device must rely on a second device, not able to be located on the same die, for achieving the proper interface voltages. Additionally, more components again take up more space, thereby reducing the gain realized by utilization of 3.3 V devices. 
     Heretofore, designers have not been able to implement the level transition required at a sending device due to the inherent limitations of the manufactured 3.3 V devices. In order to achieve the size, speed and power consumption goals, a typical BiCMOS fabrication process designed to operate at 3.3 V would have gate oxide thicknesses in the range from 80-150 Å with drawn gate lengths from 0.6-0.8 μm (as compared to a typical 5 V process with gate oxide thicknesses of 150-250 Å and drawn gate lengths greater than or equal to 0.9 μm). While power consumption goals have been achieved, the reduced voltage manufacturing 3.3 V process (hereinafter the 3.3 V process) provides a relatively thin gate oxide and narrow channels that prevent the devices fabricated using a 3.3 V process from being operated at voltages above 3.3 V. At voltages above 3.3 V, hot electron effects and gate oxide breakdown are two of the reliability problems that are well known in the 3.3 V process art. 
     In the competitive digital integrated circuit market, power consumption, speed, reliability, and compatibility are hallmarks for comparing products produced by different vendors. The ability of a digital integrated circuit to provide an on-chip buffered output for interfacing with more conventional 5 V devices would represent a significant improvement to digital integrated circuit interfhcin g art. 
     SUMMARY OF THE INVENTION 
     In summary, the present invention provides an improvement in interfacing 3.3 V output and 5 V input devices by means of a solid state BiCMOS device implemented on a reduced voltage process designed to operate from 3.3 V and 5 V supplies and capable of receiving a 0-3.3 V input signal while providing a extemal swing signal from 0-5 V. Specitically, cross coupled PMOS and NMOS devices manufactured by a 3.3 V process are utilized with level shifting diodes for achieving a device which operates on higher voltages than conventional circuit design techniques allow for a given process technique, while providing a 0-5 V output at the sending device. 
     To overcome the limitations of the prior art, it is an object of the present invention to implement an output buffer which provides a 0-5 V output swing signal utilizing 3.3 V BiCMOS process devices. 
     It is a further object of the present invention, to implement a digital integrated circuit providing a 3.3 V input buffer with 0-5 V output utilizing 3.3 V BiCMOS process devices operated by both 3.3 V and 5 V supplies while maintaining all transistor voltages at less than 3.3 V. 
     Finally it is an object of the present invention to implement cross coupled NMOS and PMO S transistors and level shifting diodes fabricated utilizing a BiCMOS 3.3 V process to achieve a 3.3 V compatible interface circuit for use in 3.3 V digital integrated circuit devices. 
     The apparatus comprises a BiCMOS output buffer circuit, implemented on a 3.3 V process, which is designed to switch between 0 V and 5 V and includes a first predriver circuit having cross coupled PMOS devices coupled to an N channel output driver, both of which are referenced to 3.3 V, for providing a low output signal. The buffer further includes a second predriver circuit with level shifting diodes and cross coupled PMOS devices coupled to a P-channel output driver, both of which are referenced to 5 V, for providing the 5 V high output signal. 
     Additional objects and features of the invention will be more readily apparent from the following detailed. description of the preferred embodiment and appended claims when taken in conjunction with the drawings provided herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a first driver circuit of the preferred embodiment of the present invention. 
     FIG. 2 is a schematic diagram of a first driver circuit coupled to first and second inverter and follower circuits of the preferred embodiment of the present invention. 
     FIG. 3 is a schematic diagram of a N-channel predriver circuit of the preferred embodiment of the present invention. 
     FIG. 4 is a schematic diagram of a P-channel predriver circuit of the preferred embodiment of the present invention. 
     FIG. 5 is a schematic diagram of the main driver circuit of the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a schematic of a solid state digital electronic circuit incorporating a first driver circuit 100 of the preferred embodiment of the present invention is shown. A 0-3.3 V input signal V in  in is coupled to the first driver circuit 100 comprising PMOS enhancement mode transistors 102 and 104 and NMOS enhancement mode transistors 106 and 108. In the preferred embodiment of the present invention, the PMOS transistors are fabricated of a gate width of 24 μm and a gate length of 0.6-0.7 μm nominally, while the NMOS transistors are fabricated of a of gate width of 9 μm and a gate length of 0.6-0.7 μm nominally. Methods for fabricating NMOS and PMOS transistors with these gate dimensions in a 3.3 V process are well known to those of ordinary skill in the art. 
     The transistors in the first driver circuit 100 are configured such that the drain port of PMOS transistor 102 is coupled to the drain port of NMOS transistor 106 and simultaneously to the respective gate inputs of PMOS transistor 104 and NMOS transistor 108, thereby forming node V 1 . The source ports of both the PMOS transistors 102 and 104 are coupled to a 3.3 V reference, while their counterpart source ports of the NMOS transistors 106 and 108 are coupled to a ground reference. Finally the drain ports of the PMOS transistor 104 and the NMOS transistor 108 are coupled at node V 2 , yielding a first output signal 110 from the first driver circuit 100. 
     Input signal V in  is coupled to the gate inputs of both the PMOS transistor 102 and NMOS transistor 106 such that as the input signal V in  swings from a potential of0 to 3.3 V, the PMOS transistor 102 tums off, creating an open circuit between the drain port and source port in the PMOS transistor 102 as the gate to source input voltage becomes less negative than the threshold voltage V p  associated with a PMOS device. Simultaneously, the NMOS transistor 106 tums on, creating a conductive path from the drain port to the source port in the NMOS transistor 106 as the gate to source voltage exceeds the threshold voltage V tn  associated with an NMOS device, thereby driving the common drain ports of PMOS transistor 102 and NMOS transistor 106 at node V 1  to the ground reference. In the preferred embodiment of the present invention, the threshold voltages V tp  (absolute value) and V tn  for all of the PMOS and NMOS devices selected are approximately 0.5 V-0.7 V. Those of ordinary skill in the art will recognize that the threshold voltage level may be selected from any value that will assure that a complete activation or deactivation will occur for all devices as the gate potential for the NMOS and PMOS devices varies 3.3 V between activated and deactivated states. 
     As node V 1  transitions to the ground reference, PMOS transistor 104 tums on reflecting a gate to source voltage more negative than the threshold voltage V tp  of a PMOS device. Simultaneously, the NMOS transistor 108 tums off, refl.ecting a gate to source voltage less than the threshold level V tn  for an NMOS device, thereby resulting in a 3.3 V first output signal 110 at node V 2  from the first driver circuit 100. Those of ordinary skill in the art will realize that as the input voltage V in  swings back from 3.3 V to 0 V, a directly opposite condition than described above will occur, resulting in transistors 102 and 108 tuming on, while transistors 104 and 106 tum off, yielding a 0 V first output signal 110 at node V 2 . Those of ordinary skill in the art will also recognize that the paired PMOS and NMOS configuration functions as a follower circuit, exactly matching the input V in  at the output 110 of the first driver circuit 100. 
     Referring now to FIG. 2, the first output signal 110 is coupled to a first inverter 200 and second inverter 202 and a first follower circuit 204 and second follower circuit 206. Inverter 200 is comprised of an enhancement mode PMOS transistor 210 whose gate input is coupled to the gate input of an enhancement mode NMOS transistor 212 and the first output signal 110. The source of the PMOS transistor 210 is referenced to 3.3 V, while the source port of the NMOS transistor 212 is coupled to a ground reference. The drain ports of the PMOS transistor 210 and NMOS transistor 212 are coupled at node V 3  resulting in a first inverter output 214. As the first output signal 110 varies from a potential 0 V to 3.3 V, the first inverter output signal 214 at node V 3  swings from a potential of 3.3 V to 0 V. In the preferred embodiment of the present invention, the PMOS transistors are fabricated of a gate width of 36 μm and a gate length of 0.6-0.7 μm, nominally, while the NMOS transistors are fabricated of a gate width of 9 μm and a gate length of 0.6-0.7 μm, nominally. These gate dimensions correspond to those created with a 3.3 V fabrication process. 
     A second inverter 202, identical to first inverter 200, is comprised of an enhancement mode PMOS transistor 220 and an enhancement mode NMOS transistor 222 with coupled drain ports at node V 4 . The second inverter 202 further comprises a second inverter output 224 responsive to the first output signal 110. In the preferred embodiment of the present invention the PMOS transistors were fabricated of a gate width of 8 μm and a gate length of 0.6-0.7 μm, nominally, while the NMOS transistors were fabricated of a gate width of 3 μm and a gate length of 0.6-0.7 μm, nominally. Again, fabrication of inverter 202 is accomplished using a 3.3 V process. 
     A first follower 204, identical to the first driver circuit 100, receives the first output signal 110 at the coupled gate inputs of an enhancement mode PMOS transistor 230 and an enhancement mode NMOS transistor 234. The drain ports of PMOS transistor 230 and NMOS transistor 234 are coupled to the gate inputs of PMOS transistor 232 and NMOS transistor 236. The drain ports of PMOS transistor 232 and NMOS transistor 236 are coupled at node V 5  yielding a first inverter output signal 238. As was disclosed previously, this configuration will result in the first inverter output signal 238 following the first output signal 110, which serves as the input to the first follower 204. In the preferred embodiment of the present invention, the PMOS transistors are fabricated of a gate width of 16 μm (transistor 230) and 48 μm (transistor 232) and a gate length of 0.6-0.7 μm, nominally, while the NMOS transistors are fabricated of a gate width of 6 μm (transistor 234) and 18 μm (transistor 236) and a gate length of 0.6-0.7 μm, nominally. 
     A second follower 206, identical to follower 204, comprises enhancement mode PMOS transistors 240 and 242 coupled to enhancement mode NMOS transistors 244 and 246. The drain ports of PMOS transistor 242 and NMOS transistor 246 are coupled at node V 6 , yielding a second follower output 248. Those of ordinary skill in the art will realize that the first driver circuit 100 and the follower circuits 204 and 206 serve no logic function, as the outputs to each of these devices merely tracks the inputs provided. However, those of ordinary skill in the art will realize that the devices provide drive capability, accounting for any fan-out considerations, and, as such, may or may not be required in all applications. In the preferred embodiment, first driver circuit 100 and followers 204 and 206 provide added circuit stability and drive capacity. 
     In the preferred embodiment of the present invention the PMOS transistors are fabricated of a gate width of 8 μm (transistor 240) and 24 μm (transistor 242) and a gate length of 0.6-0.7 μm, nominally, while the NMOS transistors are fabricated of a gate width of 3 μm (transistor 244) and 9 μm (transistor 246) and a gate length of 0.6-0.7 μm. nominally. 
     Referring now to FIG. 3, an N-channel predriver 300 is shown comprising cross coupled enhancement mode PMOS transistors 302 and 304 and enhancement mode NMOS transistors 306 and 308. The source ports of the PMOS transistors 302 and 304 are coupled to a 3.3 V reference voltage, while their counterpart source ports in the NMOS transistors 306 and 308 are coupled to a ground reference. The gate input of the PMOS transistor 304 is coupled to the drain port of the PMOS transistor 302 and to the drain port of NMOS transistor 306 at node V 7 . The gate input of the PMOS transistor 302 is coupled to the drain port of PMOS transistor 304 and to the drain port of NMOS transistor 308 at node V 8 , yielding N-channel predriver output 310. The gate input of NMOS transistor 308 is coupled to the second follower output 248 at node V 6  illustrated at connector B in FIG. 2, while the gate input of the opposing NMOS transistor 306 is coupled to the second inverter output 224 at node V 4  illustrated at connector E in FIG. 2. 
     In the preferred embodiment of the present invention, the PMOS transistors are fabricated of a gate width of 16 μm (transistor 302) and 32 μm (transistor 304) and a gate length of 0.6-0.7 μm, nominally, while the NMOS transistors are fabricated of a gate width of 12 μm (transistor 306) and 24 μm (transistor 308) and a gate length of 0.6-0.7 μm, nominally. 
     According to the preferred embodiment of the present invention, as V in , illustrated in FIG. 1, is varied from a potential of 0 V to 3.3 V, the first driver circuit 100 couples a 0 V to 3.3 V potential signal at the first driver output 110 to the second inverter 202 and second follower 206 illustrated in FIG. 2. The second follower 206 responds to the 0 V-3.3 V potential transition by providing a similar 0 V to 3.3 V potential transition via the second follower output 248 to the gate input of NMOS transistor 308 illustrated in FIG. 3. Simultaneously, the second inverter 202 provides a 3.3 V-0 V potential signal via the second inverter output 224, illustrated in FIG. 2 to the gate input of the NMOS transistor 306 of the N-channel predriver 300 illustrated in FIG. 3. 
     As the gate to source potential at NMOS transistor 308 exceeds the threshold Vm for the NMOS device, the NMOS transistor 308 turns on creating a conductive path between the drain and source ports of the NMOS transistor 308, thereby coupling the ground reference to the N-channel predriver output 310 at node V 8 . The ground reference at node V 8  results in PMOS transistor 302 turning on as the potential difference (absolute value) between the gate and source exceeds the threshold voltage V tp  for the PMOS device, thereby resulting in a conductive path forming from the source to the drain port in the PMOS transistor 302. 
     Simultaneously, the complementary NMOS transistor 306 tums off as the input to the gate of the NMOS transistor 306 received from the second inverter output signal 224 transitions the gate to source potential to less than the threshold potential V tn  of the NMOS device. An open circuit is formed between the drain and source ports of the NMOS transistor 306, which when coupled with the conductive path realized by the on state of the PMOS transistor 302, yields a 3.3 V potential at node V 7 . Finally, the 3.3 V potential coupled at node V 7  turns the PMOS transistor 304 off, a result of the gate to source potential. hlling below the (absolute value) threshold voltage V tp  for the given PMOS device. Those of ordinary skill in the art will realize that as V in  transitions back from a potential of 3.3 V-0 V, the exact opposite condition as described earlier will occur, resulting in transistors 302 and 308 turning off, while transistors 306 and 304 tum on in the N-channel predriver circuit 300 illustrated in FIG. 3, yielding a  3.3 V potential on the N-channel predriver output 310 at node V 8 . 
     Referring now to FIG. 4, a P-channel predriver 400 is shown comprising cross coupled enhancement mode PMOS transistors 402 and 404, enhancement mode NMOS transistors 406, 408, 410, and 412 and diodes 414, 416, 418 and 420. The source ports of the PMOS transistors 402 and 404 are coupled to a 5.0 V reference voltage, while the source ports in the NMOS transistors 406 and 408 are coupled to a ground reference. NMOS transistor 412 is cascaded with NMOS transistor 408, whereby the source port of the NMOS transistor 412 is coupled to the drain port of the NMOS transistor 408. Similarly, the NMOS transistor 410 is cascaded with the NMOS transistor 406. 
     The drain port of NMOS transistor 412 is connected to a series pair of diodes 418 and 420 which are connected anode to cathode. In the preferred embodiment of the present invention, the diodes 418,420 are formed from pairs of NPN transistors which have their base and collector nodes coupled, utilizing the devices&#39; inherent base to emitter diode. The drain port of NMOS transistor 412 is coupled to the cathode of diode 418, whose anode is in tum coupled to the cathode of diode 420. In this configuration, the diodes realize approximately a 0.75 V potential difference from anode to cathode as a potential is applied across each device. Those of ordinary skill in the art will recognize that the selection of diodes assures that the gate to source voltages in the PMOS transistors 402 and 404 does not exceed 3.6 V maximum potential, while allowing each device to be referenced to a 5 V potential source. Similarly, those of ordinary skill in the an will recognize that the diode pairs are selected to optimize skew characteristics and help to offset the large capacitive load presented by the larger gate oxide PMOS transistors in the P-Channel drive circuit, which will be discussed in greater detail below. 
     NMOS transistor 410 is coupled at the drain port in a similar fashion to a second pair of series diodes 414 and 416. In the preferred embodiment of the present invention, single NPN transistors form diodes 414 and 416 utilizing the devices&#39; inherent base to emitter diode. 
     The gate input of the PMOS transistor 404 is coupled to the drain port of the PMOS transistor 402 and to the anode of diode 416 at node V 10 . The gate input of the PMOS transistor 402 is coupled to the drain port of PMOS transistor 404 and to the anode of diode 420 at node V 9 , yielding P-channel predriver output 422. The gate inputs of NMOS transistors 410 and 412 are coupled to a 3.3 V reference potential. The gate input to NMOS transistor 408 is coupled to the first follower output 238 at node V 5  illustrated at connector G in FIG. 2, while the gate input of the NMOS transistor 406 is coupled to the first inverter output 214 at node V 3  illustrated at connector I in FIG. 2. 
     Referring still to FIG. 4, a bias circuit 424 comprising enhancement mode PMOS transistors 426, 428 and 430 and diodes 432 and 434 provides a small DC current to bias the diode pairs (418 and 420, 414 and 416) so as to create a steady state voltage during operation. In the preferred embodiment, the bias current is approximately 450 μA for the diode pairs. The source ports to the PMOS transistors 426, 428 and 430 are coupled to the 5 V reference potential. The drain port of PMOS transistor 430 is coupled to the anode of diode 432 and the gate inputs of PMOS transistors 426, 428 and 430 at node V 1 . The cathode of diode 432 is coupled to the anode of diode 434, while the cathode of diode 434 is coupled to ground reference. The drain port of PMOS transistor 426 is coupled to the anode of diode 416 at node V 10 , while the drain port of the PMOS transistor 428 is coupled to the anode of diode 420 at node V 9 . 
     In the preferred embodiment of the present invention, the diodes are formed from single NPN transistors utilizing the devices&#39; inherent base to emitter diode. Those of ordinary skill in the art will recognize that the PMOS transistors 426, 428 and 430 will always be tumed on as the gate to source voltage for each device is more negative than the threshold voltage V tp  for the PMOS devices. In addition, those of ordinary skill in the art will also recognize that the PMOS transistor 430 and series diode pair 432 and 434 prevent the drain to source voltages of PMOS transistors 426 and 428 from exceeding the desired maximum of 3.6 V potential across the device by providing a 1.5 V potential input at node V 11  while the PMOS transistor 430 is tumed on. 
     In the preferred embodiment of the present invention, the PMOS transistors are fabricated of a gate width of 3 μm (transistgrs 426,428 and 430), 24 μm (transistor 402), and 48 μm (transistor 404) and a gate length of 0.6-0.7 μm, nominally; the NMOS transistors are hbricated of a gate width of 64 μm (transistors 406 and 410) and 72 μm (transistors 408 and 412) and a gate length of 0.6-0.7 μm, nominally; and the NPN transistors are fabricated of an emitter width of 2.4 μm. 
     According to the preferred embodiment of the present invention, as V in  is varied from a potential of 0 V to 3.3 V, the first driver circuit 100 couples a 0 V to 3.3 V potential signal at the first driver output 110 to the first inverter 200 and first follower 204. The first follower 200 responds to the 0 V-3.3 V potential transition by providing a similar 0 V to 3.3 V potential transition via the first follower output 238 illustrated in FIG. 2 to the gate input of NMOS transistor 408 illustrated in FIG. 4. Simultaneously, the first inverter 200 provides a 3.3 V-0 V potential signal via the first inverter output 214 illustrated in FIG. 2 to the gate input of the NMOS transistor 406 of the P-channel predriver 400 illustrated in FIG. 4. 
     As the gate to source potential at NMOS transistor 408 exceeds the threshold V tn  for the NMOS device, the NMOS transistor 408 turns on creating a conductive path between the drain and source ports of the NMOS transistor 408, thereby coupling the ground reference to the source port of the NMOS transistor 412. The NMOS transistor 412 will tum on as the gate to source potential reaches 3.3 V, thereby creating a conductive path from the drain to the source of the NMOS transistor 412 and resulting in the coupling of the cathode of diode 418 to the ground reference. 
     Bias circuit PMOS transistor 430 will remain on during steady state operations due to the -3.5 V gate to source potential realized whenever the 5 V supply reference is powered on. This will result in a 1.5 V potential at node V 1 , thereby turning on PMOS transistors 426 and 428 as each of their respective gate to source voltages become more negative than the V tp  for the PMOS devices. In the preferred embodiment of the present invention, the PMOS transistors 426 and 428 act as variable resistors, providing drive current for the diode pairs 414 and 416,418 and 420 As the PMOS transistor 428 conducts, a 1.5 V potential is realized at node V 9  due to the two diode drops across diodes 420 and 418 to ground, resulting in the P-channel predriver output 422 referenced at a potential of 1.5 V. The 1.5 V potential at node V 9  also results in PMOS transistor 402 tuming on as the potential difference (absolute value) between the gate and source exceeds the threshold voltage V tp  for the PMOS device, thereby forming a conductive path from the source to the drain port in the PMOS transistor 402. 
     Simultaneously, the complementary NMOS transistor 406 tums off as the input to the gate of the NMOS transistor 406 received from the first inverter output signal 214 illustrated in FIG. 2, transitions the gate to source potential to less than the threshold potential V tn  of the NMOS device. An open circuit is formed between the drain and source ports of the NMOS transistor 406, which when coupled with the conductive path realized by the on state of the PMOS transistor 402, yields a 5.0 V potential at node V 10 . Finally, the 5.0 V potential coupled at node V 10  results in the PMOS transistor 404 tuming off, a result of the gate to source potential falling below the (absolute value) threshold voltage V tp  for the given PMOS device. Those of ordinary skill in the art will realize that as V in  transitions back from a potential of 3.3 V-0 V, the exact opposite condition as described above will occur, resulting in transistors 402, 408 and 412 tuming off, while transistors 404, 406 and 410 turn on in the P-channel predriver circuit 400, yielding a 5.0 V potential on the P-channel predriver output 422 at node V 9 . 
     Referring now to FIG. 5, a main driver 500 is shown comprising enhancement mode PMOS transistors 502 and 504, and enhancement mode NMOS transistors 506 and 508. The source port of the PMOS transistor 502 is coupled to a 5.0 V reference voltage, while the source port in the NMOS transistor 508 is coupled to a ground reference. NMOS transistor 506 is cascaded with NMOS transistor 508, whereby the source port of the NMOS transistor 506 is coupled to the drain port of the NMOS transistor 508. Similarly, the PMOS transistor 502 is cascaded with the PMOS transistor 504, whereby the drain of the PMOS transistor 502 is coupled to the source of PMOS transistor 504. Finally, the drain port of PMOS transistor 504 is coupled the drain port of the NMOS transistor 506, resulting in the main driver output 510. 
     The gate input of the PMOS transistor 502 is coupled to the P-channel predriver 400 by the P-channel predriver output 422 at node V 9  illustrated at connector K in FIG. 4. The gate input of the NMOS transistor 508 is coupled to the N-channel predriver 300 by the N-channel predriver output 310 at node V 8  illustrated at connector C in FIG. 3. The gate input of PMOS transistor 504 is connected to a 1.7 V reference at node PBIAS illustrated in FIG. 5, while the gate input of the NMOS transistor 506 is coupled to a 3.3 V reference at node NBIAS. In the preferred embodiment of the present invention, a voltage source referenced from 5 V and divided to the correct levels (1.7 V and 3.3 V respectfully) can be implemented. However, those of ordinary skill in the art will recognize that while the selection of voltage levels provides protection for the 504 and 506 transistors, the values may be varied as required as long as a 3.6 V maximum device voltage is not exceeded. 
     In the preferred embodiment of the present invention, the PMOS transistors are fabricated of a gate width of 570 μm and a gate length of 0.9 μm ,nominally, while the NMOS transistors are fabricated of a gate width of 270 μm and a gate length of 0.9 μm, nominally. 
     According to the preferred embodiment of the present invention, as V in  is varied from a potential of 0 V to 3.3 V, the first driver circuit 100 couples a 0 V to 3.3 V potential signal at the first driver output 110 to the first inverter 200 and second inverter 202 and first follower 204 and second follower 206. The followers 204, 206 respond to the 0 V-3.3 V potential transition by providing a similar 0 V to 3.3 V potential transition via the first follower output 238 and second follower output 248 to the N-channel predriver 300 illustrated in FIG. 3 and to the P-channel predriver 400 illustrated in FIG. 4. Simultaneously, the first inverter 200 and second inverter 202 provide a 3.3 V-0 V potential signal via the first inverter output 214 and second inverter output 224 to the N-channel predriver 300 and to the P-channel predriver 400. 
     P-channel predriver 400 outputs a 1.5 V reference signal via the P-channel predriver output 422 at node V 9  illustrated in FIG. 4, thereby causing the PMOS transistor 502 illustrated in FIG. 5 to turn on as the gate to source voltage level exceeds the threshold level (absolute value) for the particular device. A conductive path forms from the source to the drain ports in the PMOS transistor 502, resulting in a 5 V potential at the source port of PMOS transistor 504. With a source port referenced to 5 V and PBIAS referenced to 1.7 V, the gate to source voltage will exceed the transition voltage V tp  (absolute value) for the PMOS transistor 504, thereby tuming on the PMOS transistor 504. In the onstate, PMOS transistor 504 forms a conductive path from the source to the drain and provides a 5 V reference output signal at the main driver output 510. 
     N-channel predriver 300 outputs a 0 V reference signal via the N-channel predriver output 310 at node V 8  illustrated in FIG. 3 thereby causing the NMOS transistor 508 illustrated in FIG. 5 to turn off as the gate to source voltage level falls below the threshold level V tn  (absolute value) for the particular device. An open circuit forms from the source to the drain ports in the NMOS transistor 508, resulting in a 2.5 V potential at the source port of NMOS transistor 506 as the open input source port floats to a V tn  (absolute value) below the NBIAS voltage. With the drain port of NMOS 506 referenced to 5 V and NBIAS referenced to 3.3 V, the source port of NMOS 506 will not exceed 2.5 V. This has the effect of dropping the 5 V at node OUT equally across NMOS transistors 506 and 508 and ensures the drain to source voltages on NMOS transistors 506 and 508 individually remain below 3.3 V. Those of ordinary skill in the art will realize, that as V in  transitions back from 3.3 V to 0 V potential, the exact opposite condition as described will occur, whereby the PMOS transistors 502 and 504 will remain turned off, while the NMOS transistors 506 and 508 are turned on providing a 0 V potential output at the main driver output 510. 
     In a second embodiment of the present invention, the voltage sources connected at PBIAS and NBIAS are varied so as to provide a variable output impedance from 50Ω-10MΩ, depending on the voltage level selected. The variable impedance is achieved by driving the PMOS transistor 504 (or the NMOS transistor 506) into the saturation region by varying the gate to source voltage. When either PMOS transistor 504 or NMOS transistor 506 are in full conduction, that is the when the gate to source voltage is much greater than the transition threshold voltage, the output impedance of the main driver output 510 is approximately 50Ω-80 Ω. As the gate to source voltage is brought nearer the threshold level, the PMOS (NMOS) devices act as variable resistors, whereby the output impedance (impedance as seen looking back into the device) varies with voltage for devices in saturation. 
     While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.