AN INTEGRATED CIRCUIT INCLUDING A HIGH VOLTAGE SWING OPEN COLLECTOR DRIVER

Abstract: It is well known that the base width of a transistor has a direct bearing on transistor speed and the punch-through voltage. A solution to the transistor speed versus break down (punch through) voltage problem is disclosed. Output transistors (T10, T11) are serially connected between power supply rails along with associated transistor base driving circuitry. The circuit arrangement ensures that the supply voltage divides between the series connected output transistors and prevents excessive voltage from being applied to each output transistor.

Description: Background of the Invention A key factor in improving the speed of modern digital computers is to reduce the propagation delay of the circuits used to perform the necessary logic and communications operations. The speed or propagation delay of a circuit is ultimately limited by the speed of the individual transistors from which it is built. A fundamental limit to the speed of a transistor is given by the time required for the minority carrier distribution in the base region to respond to a change in base current and cause an amplified change in collector current. A measure of this time is given by the minority carrier base transit time. T=W²/2D for a linear carrier distribution where T is the base transit time, W is the transistor base width, and D is the carrier diffusivity in the base. The base width is thus seen to have a direct bearing on the transistors speed and thus affects the speed of machines, such as computers, which are made from them. In order to minimize the propagation delay of a circuit, transistors with the smallest obtainable base width are required. The logic circuits in a computer do not exist in isolation. They must be able to electrically communicate with circuits and devices made from different materials and with different operating characteristics. Examples of such interfaces would be communications with field effect transistor memory array chips and communications with sections of the machine built from industry standard 5 volt logic circuits. These other circuits have specific electrical interface specifications in regard to voltage and current levels that must be met with the same type of devices and on the same integrated circuit chip that form the high speed digital logic circuits. A critical parameter for the devices forming the interface circuits is the transistor breakdown voltage. This voltage limits the use of the transistors to those interface applications that will not overstress the device and cause device or circuit failure. A measure of this is given by the transistor collector to emitter punch-through voltage, BV= (qQ/(Ke))(W+Q/(2N)) where q is the magnitude of the electronic charge, Q is the base doping, Ke is the permittivity of the semiconductor, W is the base width, and N is the collector doping. The base width is thus seen to have a direct bearing on the transistor breakdown voltage and thus limits the range of interface applications. In order to maximize the breakdown voltage of the transistor, the largest possible base width is required. These two desirable transistor characteristics, high speed and high breakdown voltage, are seen to be in conflict. The doping concentrations and base widths that result in high speed devices severely limit the transistor breakdown voltage and thus limit the use of the transistors in interface circuit type applications. In fact, transistors presently designed for state of the art fast switching speeds are incapable of sustaining the voltage levels (three to five volts) required to interface to FET memory array chips and industry standard TTL circuits. The conventional solution to this problem is to use special interfacing integrated circuits that can receive the levels generated by the high speed logic chips and convert them to the larger standard interface signal levels. These special interfacing integrated circuits are made by a process different from that of the high speed circuits and will support the higher voltage levels, although at a correspondingly lower performance and with greater propagation delay. This approach causes problems in the design of a high performance system not only because the interfacing circuits are manufactured by a lower performance process but because the additional circuits introduce more circuit chips both in the logic path and in the machine, causing physical packaging difficulties and total logic path and transmission line delay increases. This causes the assembled digital system to run slower and cost more than it would if the interface circuits could be integrated at will on the high speed logic chips. My invention presents a circuit solution to the transistor speed vs. breakdown voltage problem. Transistors, formed by a process that results in maximum speed, are connected in series such that the total applied voltage divides between them. In this manner, no one transistor must support the entire voltage swing and each device is operated within its breakdown voltage rating. However, because of the multiple connection of series transistors, an arbitrarily large voltage swing may be built up, satisfying any interfacing requirement. Furthermore, the disclosed circuits are true digital logic circuits having only two possible output voltage levels near each power supply extreme, resulting in maximum circuit efficiency. The circuit voltage gain in each of the binary states is zero, insuring stable output levels. A most important aspect of operation of the disclosed circuits is that the series transistors maintain the voltage division even during the dynamic switching interval when the circuit output changes between the binary states. Thus the transistors are always protected from overvoltage, even while switching states from "on" to "off". Specific embodiments of my invention are fully disclosed hereinafter. The key feature of my invention centers around an upper series connected transistor with its emitter and base resistors and base voltage-dropping base-collector diode. As fully described hereinafter, these components act to control the voltage seen by the transistor and keep it within its safe operating region during switching. Several similar stages may be combined in series to increase the output swing beyond what is shown. Description of the Prior Art Previously, digital logic circuits did not have this problem since the transistor breakdown voltages were sufficient for circuit communication. However, analog circuits such as high voltage amplifiers had to face the large signal swing verses transistor breakdown voltage problem and to use a form of series connected output transistor. However, the requirement for the analog circuits are quite different from the digital logic circuit requirements. For example, analog circuits must be linear amplifiers across the complete signal swing in order to minimize distortion. Digital logic circuits, on the other hand, must have an amplification (or gain) of zero at the two stable output states in order to generate the discrete logic levels. Also, digital logic circuits are designed to minimize the power dissipation by having either high current with low voltage, or high voltage with low current across them in their static states. Linear analog circuits must support high current and voltage simultaneously. Numerous driver circuits including open collector driver circuits are known to the art. CIRCUIT DESIGNER'S CASEBOOK, vol. 14D C. 1975, page 1 discloses a circuit containing a plurality of high speed transistors, having a bipolar logic state which output swing is greater than the CE-breakdown voltage of its component transistors according to the preamble portion of Claim 1. Document LEISTUNGS-MOSFET PRAXIS, J.P. Stengl, pages 155-7, 1985 discloses the use of a chain of serially cascode connected FET semiconductors in order to provide the output stage of an integrated circuit chip. The following patents are directed to transistor switching circuits and driver circuits. It is to be appreciated, with reference to the subject invention, that the following art is not submitted to be the only prior art, the best prior art, or the most pertinent prior art. Background Art U.S. PATENTS : 3,769,524 entitled "Transistor Switching Circuit" granted October 30, 1973 to K. F. Mathews. 3,962,590 entitled "TTL Compatible Logic Gate Circuit" granted June 8, 1976 to J. Kane et al. 3,974,402 entitled "Logic Level Translator" granted August 10, 1976 to D. L. Fett et al 4,070,600 entitled "High Voltage Driver Circuit" granted January 24, 1978 to W. J. Butler et al. 4,251,737 entitled "Dottable Active Collector Driver Circuit" granted February 17, 1981 to G. J. Gaudenzi. Summary of the Invention A primary object of the invention is to provide digital circuitry which efficiently and reliably employs high speed transistors having narrow-base widths. A further object of this invention is to provide digital interface circuitry which obviates the punch-through voltage and other collector to base and collector to emitter breakdown voltage problems of high speed transistors having narrow base width. A further object is to provide an improved output stage for logic circuits employing high speed transistors having narrow base width. A still further object is to provide improved high speed logic circuits. The invention employs series connected output transistors connected between the power supply rails along with the associated transistor base driving circuitry that ensures that the power supply voltage divides between the series connected transistors, thus preventing excessive voltage from being applied to each transistor. These and other features and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention as illustrated in the accompanying drawings. Brief Description of the Drawing Figure 1 discloses a high voltage swing open collector driver. Figure 2 discloses an embodiment of a high voltage open collector driver in accordance with my invention. Figure 3 discloses another high voltage open collector driver. Figure 4 discloses idealized waveforms to be viewed in conjunction with the high voltage swing open collector driver of Figure 1 as an aid in understanding the explanation and operation thereof. Figure 5 discloses idealized waveforms to be viewed in conjunction with the high voltage swing open collector driver of Figure 1 as an aid in understanding the explanation and operation thereof. Figure 6 discloses a further high voltage swing open collector driver. Description of the Preferred Embodiment Transistors with vertical structures sacrifice breakdown voltage in order to get faster transistor switching speeds, resulting in devices with a BV CEO of 2.5V. (BV CEO is defined as the collector to emitter break down voltage with the base open.) The system power supplies (-2.2V, -0.6V, 0.0V, +1.4V) have a voltage differential of 3.6 volts, sufficient to generate signal swings that can interface with industry standard TTL devices, but beyond the BV CEO limit of high speed (narrow base) transistors. In accordance with the invention, driver circuits herein after disclosed in detail, can generate a logic swing across the full voltage (3.6V) without exceeding the BV CEO (2.5V) of the transistors. In reference to the schematic diagram, Figure 1; the input to the driver circuit may be a logic circuit such as known current switch or TTL logic circuits. Current switch logic circuits are well known to the art and accordingly no detailed discussion thereof is deemed necessary. In figure 1 a current switch logic circuit is shown with first and second terminals respectively connected to the bases of transistors T₁ and T₂. The collector load resistor R₁ of the current switch is arranged to give the maximum collector swing without saturating the input transistor. The current switch output collectors, T₁ and T₂, are connected to an emitter follower level shifter circuit that drops the collector voltage swing to the most negative supply voltage, -2.2V, generating the base drive current for the high voltage output stage. The emitter-follower level shifter circuit may, as shown in Figure 1, be comprised of the serial connection of transistor TE, diode connected transistor TD, Schottky diode D₂ and resistor R₃. The base of transistor TE is connected to the collectors of transistors T₁ and T₂. The output from the emitter-follower level shifter circuit is taken from the juncture of Schottky diode D₂ and resistor R₃. Reference is made to the circuitry shown in Figure 1 and enclosed within the broken line labelled "High Voltage Output Stage". Serially connected between an off-chip pad and a source of potential are transistors T₁₁ and T₁₀. The off chip pad is connected to a terminator resistor connected in turn to a source of potential (+ 1.4V). The emitter of T₁₁ is connected to the collector of T₁₀. The emitter of T₁₀ is connected to a source of potential (-2.2V). The base of T₁₁ is connected to a source of reference potential (0.0V) via a resistor R₅. Resistor R₆ connects the emitter of T₁₁ to the source of reference potential (0.0V). The base of T₁₀ is connected via resistor R₄ to the output of the emitter follower level shifter circuit. With no base current into transistor T₁₀, T₁₀ is off, V BE =OV, V CE =2.2V and transistor T₁₁ is off, V BE =OV, V CE =1.4V. Turning on transistor T₁₀ results in its collector voltage dropping 0.8V, at which point it will be clamped by the turn-on of transistor T₁₁. Transistor T₁₀ is on, V BE =0.8V, V CE =1.4V and Transistor T₁₁ is on, V BE =0.8V, V CE =2.2V. Line current due to the discharge of line capacitance and due to the developing voltage across the pull-up terminator resistor RT starts to flow through the series connected transistors T₁₁ and T₁₀. The pad voltage drops until the anti-saturation SBD clamp on the transistor T₁₁ begins to conduct. As is apparent from Figure 1 of the drawing the open collector driver and output pad are on a first chip (CHIP ONE) whereas the terminating circuit (shown in the drawing as a terminating resistor RT is on a second chip (CHIP TWO). Alternatively, both the open collector driver, the terminating circuit and the output pad could be on the same chip. Transistor T₁₀ is on, V BE =0.8V, V CE =1.4V and Transistor T₁₁ is on, V BE =0.8V, V CE =0.2V. The current through the transistor T₁₁ SBD clamp flows through the resistor R₅ in series with the base, causing a voltage drop to the base of transistor T₁₁. As the pad voltage drops, the SBD drags the base down and the emitter of transistor T₁₁ will drop. The emitter of transistor T₁₁, or the collector of transistor T₁₀, will continue to drop until the anti-saturation SBD clamp on transistor T₁₀ begins to conduct. Transistor T₁₀ is on, V BE =0.8V, V CE =O.2V and Transistor T₁₁ is on, V BE =0.8V, V CE =0.2V. The collector of transistor T₁₁ has moved 3.2V with a maximum of 2.2V appearing across the devices. The same thing happens in reverse for turn-off. Removing the base drive from transistor T₁₀, releases its collector, and allows it to charge up to 0.0V. Simultaneously, transistor T₁₁ turns off, allowing the pad to charge up to 1.4V. In reference to the schematic diagram, Fig. 2; the operation is similar to that shown in Fig. 1. The difference is the introduction of diode connected transistor TL connected in parallel to the base-emitter junction of T₁₀. The emitter of TL is connected to the emitter of T₁₀ and the base and collector of TL are connected to the base to T₁₀. This establishes the well known current mirror configuration where the emitter current to T₁₀ is limited to some multiple of the emitter current of TL as determined by the ratio of the emitter area of T₁₀ to the emitter area of TL. This provides for current limiting and over current protection of the output transistors T₁₀ and T₁₁ in the event that the chip output pad is inadvertently shorted to a power supply voltage. In reference to the schematic diagram, Fig. 3; a third series transistor, T₁₂, has been connected into the high voltage output stage. The chip pad is connected to the collector of T₁₂. The off chip pad is connected to a terminator resistor connected in turn to a higher source of potential (+2.8V). The emitter of T₁₂ is connected to the collector of T₁₁. The emitter of T₁₁ is connected to the collector of T₁₀. The emitter of T₁₀ is connected to a source of potential (-2.2V). The base of T₁₂ is connected to a source of reference potential (+1.4V) via a resistor R7. Resistor R8 connects the emitter of T₁₂ to the source of reference potential (+1.4V). As in Fig. 1, the base of T₁₁ is connected to a source of reference potential (0.0V) via a resistor R₅. Resistor R₆ connects the emitter of T₁₁ to the source of reference potential (0.0V). The base of T₁₀ is connected via resistor R₄ to the output of the emitter follower level shifter. A total of 5V appears across the series connected output transistors. Operation is similar to that described in reference to Fig. 1, with extension to incorporate the additional transistor T₁₂. With no base current into transistor T₁₀, transistors T₁₀, T₁₁ and T₁₂ are off. V BE of T₁₀ = 0V, V CE of T₁₀ = 2.2V, V BE of T₁₁ = 0V, V CE of T₁₁ = 1.4V, V BE of T₁₂ = 0V, V CE of T₁₂ = 1.4V. Turning on transistor T₁₀ results in the activation of transistors T₁₁ and T₁₂, drawing current from the terminator resistor connected to the output pad. V BE of T₁₀ = 0.8V, V CE of T₁₀ = 1.4V, V BE of T₁₁ = 0.8V, V CE of T₁₁ = 1.4V, V BE of T₁₂ = 0.8V, V CE of T₁₂ = 2.2V, The pad voltage will drop, activating in turn the anti-saturation SBD clamps on the output transistors T₁₂, T₁₁ and T₁₀ until V BE of T₁₀ = 0.8V, V CE of T₁₀ = 0.2V V BE of T₁₁ = 0.8V, V CE of T₁₁ = 0.2V V BE of T₁₂ = 0.8V, V CE of T₁₂ = 0.2V and the output of the driver, the collector of T₁₂, has moved 4.4V with a maximum of 2.2V appearing across any device. The same thing happens in reverse for turn-off. It is apparent that the number of series connected transistors is not limited to the two or three described herein, but that the transistors may be connected in this manner in series in a sufficient number to provide for arbitrarily large output voltage swings while limiting the voltage appearing across any one of them. In reference to the idealized waveforms, Figure 4, to be viewed in conjunction with the high voltage swing open collector driver of Figure 1; the input signal applied to the base of transistor T₁ is shown switching around 0V. This causes the output, the collector of T₁₁ to switch while maintaining voltage division across the output transistors T₁₁ and T₁₀ as shown by the T₁₀ collector waveform. The power supply voltage VCC and VEE are shown for reference. In reference to the idealized waveforms, Figure 5, to be viewed in conjunction with the wave form of Figure 4; the actual base-emitter voltage and the base-collector voltage for the series output transistors T₁₀ and T₁₁ are shown illustrating the maintenance of the voltage division during the switching interval. In reference to the schematic diagram, Figure 6; it is to be recognized that the high voltage swing open collector driver need not be driven by a shifted emitter follower signal level out of a current switch logic circuit. Here is illustrated an example where the output series transistors, T₁₀ and T₁₁, are driven by a TTL type logic gate. The collector of the input TTL transistors T₁ and T₂ are connected to the base of the series output transistor T₁₀. The input logic signals, applied to the emitters of the input transistors T₁ and T₂, cause the input transistors to become active in either the normal or the inverse mode. When the input transistors are active in the normal mode, there is no base current into the series output transistor T₁₀. When the input transistor is active in the inverse mode, current flows into the base of the series output transistor T₁₀. In the same manner as described in reference to Figure 1, switching base current into transistor T₁₀ is converted into an output voltage wing at the collector of T₁₁ while maintaining the voltage split across series connected transistors T₁₀ and T₁₁.

Claims: A circuit comprising an at least a first and a second integrated circuit, said first integrated circuit containing a plurality of high speed transistors, each of said transistors having a narrow base-width whereby each of said high speed transistors has a low BVceo parameter, said first integrated circuit including at least one driving circuit having a plurality of high speed transistors, said at least one driving circuit including: input means for receiving an input signal having a voltage swing of k volts, output means coupled to and responsive to said input means for providing an output signal having a voltage swing of 1 volts where the magnitude of said output voltage swing 1 is materially greater than the voltage magnitude of the BVceo parameter of the plurality of high speed transistors employed in the said at least one driving circuit of said first integrated circuit, comprising a first and second serially connected high-speed transistor (T11, T10) each including an emitter, base and collector, said emitter of said first transistor (T11) connected to the collector of said second transistor (T10), said emitter of said second transistor (T10) connected to a first source of potential (-2.2V); characterized in that: said input means includes a logic circuit having at least first and second inputs and emitter follower output means connected to the input of said output means of said driving circuit, and said output means further includes: a first resistor (R4) connected between said base of said second transistor (T10) and said emitter-follower output means of said logic circuit; a second resistor (R5) connected between a source of reference potential (0.0V) and the base of said first transistor (T11); and a third resistor (R6) connected between said source of reference potential (0.0V) and the emitter of said first transistor (T11); and a third transistor (TL) having an emitter, base and collector, said emitter of said third transistor (TL) connected to said emitter of said second transistor (T10), and a common connection of said collector of said third transistor (TL) with said base of said third transistor (TL) to said base of said second transistor (T10), and a first Schottky diode connected across the base-collector junction of said first transistor (T11); and a second Schottky diode connected across the base-collector junction of said second transistor (T10); and said second integrated circuit contains load circuit means (RT), said load circuit means (RT) connected between the collector of said first transistor (T11) of the said first integrated circuit and a second source of potential (1.4V). Circuit according to claim 1 characterized in that said output means of the said first integrated circuit further includes a fourth transistor (T12) having an emitter, base and collector, said emitter of said fourth transistor (T12) connected to the collector of said first transistor (T11); a fourth resistor (R7) connected between said base of said third transistor (T12) and a second source of reference potential (1.4V); a fifth resistor (R8) connected between said emitter of said fourth transistor (T12) and said second source of reference potential (1.4V); and said load circuit means (RT) of the said second integrated circuit being connected between the collector of said fourth transistor (T12) and a third source of potential (2.8V). Circuit according to claim 2 characterized in that said logic circuit is a current switch circuit. Circuit according to claim 2 characterized in that said logic circuit of the said first integrated circuit is a TTL circuit. Circuit according to claim 1 characterized in that said emitter-follower circuit means of the said first integrated circuit includes fourth and fifth transistors (TE, TD), each of said fourth and fifth transistors (TE, TD) having a collector base and emitter, said collector of said fourth transistor (TE) connected to a second source of potential (1.4V), said base of said fourth (TE) transistor connected to said output of said logic circuit, and a common connection between said emitter of said fourth transistor (TE), said collector of said fifth transistor (TD) and said base of said fifth transistor (TD); a third Schottky diode (D2) connected between said emitter of said fifth transistor (TD) and said output of said emitter follower circuit means; and, a fourth resistor (R3) connected between said emitter of said output terminal of said emitter follower circuit means and said first source of potential (-2.2V).

Inventors: FREEMAN, LEO BOYES
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: H03K17
IPC codes: H03K17, H03K19