A HIGH-SPEED/HIGH-SLEW-RATE TRI-MODAL ALL BIPOLAR BUFFER/SWITCH AND METHOD THEREOF

Abstract: A high-speed/high-slew-rate tri-modal all bipolar buffer/switch includes a unity-gain amplifier (20), a voltage source (70), and a maximum level detector (30) and a minimum level detector (40) adjusting a current source (60) to sink or source current as required to quickly make the output voltage of the switch equal to the input voltage of the switch. The maximum level detector (30) and the minimum level detector (40) compare the output voltage to the input voltage. If the output voltage does not equal the input voltage, the current source acts (60) as either a sink or source to make the output voltage equal the input voltage. In addition, the voltage switch holds a constant d.c. voltage at the output of the switch when the switch is powered down.

Description: A high-speed/high-slew-rate tri-modal all bipolar buffer/switch and method thereof. BACKGROUND OF THE INVENTION Field of the Invention The present invention is related to switches, and specifically, to tri-modal switches. Description of the Related Art In an ideal switch, when the switch is closed, the same signal appearing at the input of the switch instantaneously appears at the output of the switch. Related art methods for implementing switches are shown in Figures 1(A), 1(B), and 1(C), explained herein below. Figure 1(A) shows a CMOS dual device switch 1. The CMOS dual device switch 1 is the closest switch to an ideal switch of the switches shown in Figures 1(A) - 1(C). The CMOS dual device switch 1 acts like a passive switch, with the output drive capability thereof limited by stages which precede the switch 1. A second method of implementing a switch is a diode bridge switch 2, shown in Figure 1(B). The diode bridge switch 2 is opened when currents I j and I 2 are both turned off. A problem with the diode bridge switch 2, however, is that for proper distortion free operation, the sink or source current for a load should be much less than the current values of either Ii or I 2 . Therefore, during continuous operation of the diode bridge switch 2, current is wasted. A third method of implementing a switch is a unity gain connected operational amplifier (op-amp) 3 with power down capability, shown in Figure 1(C). A drawback, though, of the unity gain connected op-amp 3 is that trade-offs must be made between having low standby current, high current drive capability, and capacitive compensation requirements. In switches of the related art, when an a.c. load is provided at the output of the switch and the switch is closed, the output voltage does not instantaneously equal the input voltage. If the output voltage is less than the input voltage, in the related art, a high amount of current is needed to be provided to the load to increase the output voltage to make the output voltage equal the input voltage. Tri-modal buffer/switches implemented in CMOS or bi-CMOS are easy to realize because MOS devices act as very good switches. A typical MOS device provides a small resistance when the MOS device is turned on. Further, when the MOS device is turned off, a very small amount of leakage current is output by the MOS device. If MOS switches are used, the MOS switches should be preceded by an MOS or a bipolar buffer, for current drive capability. However, a bi-CMOS switch is more expensive to implement than is a bipolar switch. Further, a bipolar switch has a faster switching speed than does a CMOS switch. What is needed is a high-speed/high-slew-rate tri-modal buffer/switch implemented using all bipolar devices. SUMMARY OF THE INVENTION An object of the present invention is to implement a high-speed/high-slew-rate tri-modal all bipolar buffer/switch which can source or sink several milli-amperes of current on demand when the switch is closed. Another object of the present invention is to provide a low voltage drop across the switch when the switch is closed. A further object of the present invention is to provide a switch which, when opened, provides high impedance out the output, and draws almost no current from a load on the switch. An additional object of the present invention is to provide a switch which has a power-down mode in which the switch draws little current and provides a reference voltage at the output of the switch to prevent capacitive loads on the switch from discharging. Also an object of the present invention is to approach an ideal switch, which maintains a zero voltage drop across the switch when the switch is closed, and instantaneously brings the output voltage equal to the input voltage when the switch is closed. The above objects can be attained by the tri-model switch as defined in the Claims. The present invention is a tri-modal buffer/switch which can source or sink several milli-amperes of current on demand when the switch is closed. In addition, the bipolar buffer/switch of the present invention is implemented using bipolar semiconductor devices. Further, the bipolar buffer/switch of the present invention provides relatively high impedance between the input node and the output node of the switch when the switch is open, and provides a very small voltage drop across the switch when the switch is closed. In addition, the bipolar buffer/switch of the present invention draws almost no current away from a load when the switch is opened. Further, in the bipolar buffer/switch of the present invention, when the bipolar buffer/switch is powered-down, a constant d.c. voltage is provided at the output of the switch to keep an a.c. capacitive load charged up, so that when the switch is turned on, the switch is immediately returned to the state of the switch when the switch was turned off, thereby preventing the a.c. capacitive load from discharging. These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation is more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. BRIEF DESCRIPTION OF THE DRAWINGS Further details of the present invention are explained below, with the aid of the attached drawings in which: Figures 1(A), 1(B), and 1(C) show switches of the related art; Figure 2 is a block diagram of the high-speed/high-slew-rate tri-modal all bipolar buffer/switch of the present invention; and Figure 3 is a detailed circuit diagram of the switch of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Figure 2, a high-speed/high-slew-rate tri-modal all bipolar buffer/switch 10 according to the present invention is shown. An input voltage to the switch 10, swin, is received by the non-inverting (+) input of a unity gain operational amplifier 20, which may be a conventional unity gain operational amplifier, as well as by a maximum level detector 30 and a minimum level detector 40. The output voltage of switch 10, swout, is output from the unity gain operational amplifier 20, fed back to the inverting (-) input of the unity gain operational amplifier 20, and is a second input to the maximum level detector 30 and the minimum level detector 40. The output of the maximum level detector 30 is input to the non¬ inverting (+) input to an amplifier 50, which may be conventionally implemented, and the output of the minimum level detector 40 is input to the inverting (-) input of the amplifier 50. The output of amplifier 50 controls a current source 60, which may be conventionally implemented and which produces current I main . Further, the output of the unity gain operational amplifier 20 is coupled to a voltage generator 70, which may be conventionally implemented and which generates a constant d.c. voltage when the switch 10 is powered down, thereby holding the output of the switch 10 at a constant d.c. voltage level when the switch 10 is powered down. The switch 10 equalizes output voltage, swout, to input voltage, swin, by controlling I main to source or sink as much current as necessary to achieve the above¬ mentioned function. When input voltage is received by amplifier 20, which is a unity gain operational amplifier having no voltage drop thereacross when the switch 10 is closed, swin is transferred to the output of the amplifier 20 as swout, without gain or attenuation. Current source 60, which generates current I majn , controls the amplifier 20 to provide a high sink and a high source capability to unity gain operational amplifier 20. Switch 10 does not have a fixed amount of current output, as in the related art in which current is wasted, but controls the output of switch 10 by current I main through the feedback of output voltage swout from the output of amplifier 20 to the detector 30 and the detector 40. Therefore, the present invention provides either more or less current, depending on the demand thereon. As shown in Figure 2, maximum level detector 30 and minimum level detector 40 sense input voltage swin and output voltage swout. If swin is lower than swout or if swout is lower than swin, then current I main is changed by the amplifier 50 to match output voltage swout to input voltage swin. In an ideal switch, when the switch is closed, the same signal appearing at the input of the switch instantaneously appears at the output of the switch. In the present invention, current source 60 provides a sufficient amount of current, e.g., 5 mili- amperes, to almost instantaneously match the voltage level of the output of switch 10 to the voltage level of the input of switch 10, which approaches the above-mentioned characteristics of an ideal switch. Also in the present invention, when the switch 10 is opened, the current source I main is shut-down, making the op-amp 20 high impedance at the output thereof, with minimal leakage current of only a few nano-amperes. Therefore, the switch 10 of the present invention closely approximates the characteristics of an ideal switch. Unlike a normal switch, which is bi-modal (i.e., open or closed), the switch 10 of the present invention has a third state, called powerdown, in addition to being open or closed. As shown in Figure 2, voltage generator 70 provides a d.c. reference voltage at the output of unity gain operational amplifier 20 when the switch 10 of the present invention is powered down. When input signal ipwd, which is generated and transmitted by conventional logic control circuits, is received by the voltage generator 70, the voltage generator 70 generates the d.c. reference voltage. Referring now to Figure 3, npn transistors 21 and 22, and pnp transistors 23 and 24 form the unity gain operational amplifier 20. The emitter of transistor 21 is coupled to the emitter of transistor 22. Further, the emitter of transistor 23 is coupled to the emitter of transistor 24. As shown in Figure 3, feedback from the output of the amplifier 20 to the inverting terminal (-) of the unity gain operational amplifier 20 is provided between the collector of transistor 24 and the base of transistor 22. The above-mentioned feedback provides for the unity gain of the unity gain operational amplifier 20. Also as shown in Figure 3, input voltage swin is input to the amplifier 20 through the base of transistor 21. Further, the output of unity gain operational amplifier 20 is the collector of transistor 24. As shown in Figure 3, current __,-;_ is the current output from the emitter of npn transistor 28. Current I main flows to ground. Transistors 24 and 28 form a current mirror. The output current, which is the current flowing from the collector of transistor 24, is directly proportional to the current flowing from the collector of transistor 28, which current is split between transistors 21 and 22. Therefore, if the amount of current flowing from transistor 28 increase, the amount of current flowing from transistor 24 increases, as well. The current flowing from the collector of transistor 21 through the emitter of transistor 21 is transmitted to the collector of transistor 23 and is mirrored through transistor 24 through the current mirror formed by transistors 23 and 24. Therefore, changing current I main changes the output current of switch 10. Also as shown in Figure 3, maximum level detector 30 includes npn transistor 31 and npn transistor 32. Further, minimum level detector 40 includes npn transistor 44 and npn transistor 41 , which supply level-shifted versions of voltages to pnp transistor 43 and pnp transistor 45. As shown in Figure 3, the input voltage to switch 10, swin, is input to transistor 21 in unity gain operational amplifier 20 and, at the same time, through resistor Rl , for example a 30 kilo-ohm resistor, into the base of transistor 31 in maximum level detector 30, and to the base of transistor 41 in minimum level detector 40. Further, the output of switch 10, swout, is fed back to the base of transistor 44 in minimum level detector 40 and the base of transistor 32 in maximum level detector 30. Operation of the maximum level detector 30 is explained, with reference to Figure 3. With output voltage swout being fed back to transistor 32, and input voltage swin being input to transistor 31 , transistor 31 and transistor 32 form a differential pair sensing the input and the output of the switch 10. If the input voltage level of input voltage swin is greater than the output voltage level output voltage swout, the voltage level at the base of transistor 31 is greater than the voltage level at the base of transistor 32. Therefore, transistor 31 is conducting, and transistor 32 is off. As a result, a current flowing into the collector of transistor 34 is only flowing from one of transistor 31 and transistor 32. Therefore, the large voltage (of the voltages at the bases of transistors 31 and 32) will end-up on the emitter of transistor 33, hence a maximum level detector. Likewise, a similar activity happens in the minimum level detector 40. As shown in Figure 3, output voltage swout is transferred through resistor R2, for example a 30 kilo-ohm resistor, to the base of transistor 44. Resistor R2 is also coupled to ground through capacitor Cl . Further, input voltage swin is transferred through resistor Rl to the base of transistor 41. Both transistor 41 and 44 are conventional emitter-follower transistors. Therefore, input voltage swin and output voltage swout are applied, respectively, to the bases of pnp transistors 43 and 45 through emitter follower transistors 41 and 44, respectively. Transistors 43 and 45 in minimum level detector 40 act similarly to transistors 31 and 32 in maximum level detector 30. Accordingly, transistors 43 and 45 sense which of output voltage swout or input voltage swin is lower than the other. If input voltage swin is higher than output voltage swout, transistor 45 is conducting. If input voltage swin is smaller than output voltage swout, transistor 43 is conducting. As a result, the emitter voltage of either transistor 43 or 45 will equal the lower value of input voltage swin and output voltage swout and controls npn transistor 51 in amplifier 50. As shown above, the maximum level detector 30 and the minimum level detector 40 determine whether input voltage swin is higher than output voltage swout or whether output voltage swout is higher than input voltage swin. Regardless of which one is higher, the higher value ends up as a control signal on the base of npn transistor 52 and the lower value ends up as a control signal on the base of npn transistor 51 in the amplifier 50. The output of amplifier 50 eventually controls transistor 28 in the current source 60. As shown in Figure 3, transistor 51 and transistor 52 form a differential pair of transistors. Therefore, the control signal appearing at the bases of transistor 51 and transistor 52 causes a current change in the pnp transistor 53. The current change is then mirrored back to pnp transistor 25, which provides current to npn transistor 29. The current flowing into npn transistor 29 is then mirrored to npn transistor 28. Accordingly, switch 10 of the present invention determines whether input voltage swin is greater than output voltage swout or whether input voltage swin is less than output voltage swout. In both cases, the current I main increases to allow more sinking or sourcing capability to the amplifier 20. The resistor R5, transistor 26, and diode Dl combination is normally off. Only if the amplifier 20 load demands more than about 10 mili-amperes of current, npn transistor 26 turns on, taking over current from transistor 25 through diode Dl , thereby limiting the increase in current I ma ; n . If output voltage swout is equal to input voltage swin, neither maximum level detector 30 nor minimum level detector 40 demands current and transistor 28 is running at a low bias current. Also as shown in Figure 3, voltage generator 70 provides a reference d.c. voltage when switch 10 is powered down. Voltage generator 70 is implemented using bipolar devices. As shown in Figure 3, switch enable signal swen opens or closes the switch 10. Switch enable signal swen is input to switch logic unit 80, which is conventionally implemented. In the switch logic unit 80, in addition to conventional logic controls, the bias currents for the switch 10 as a whole are generated. When the foregoing outputs are activated, switch 10 is opened because transistor 28, which generates current I majn , is powered down. Therefore, no current will be available for biasing switch 10, hence making the amplifier 20 high impedance, like an open switch. Voltage generator 70 provides a reference d.c. voltage to output voltage swout when switch 10 is powered down. The reference d.c. voltage equals 1.4 volts. Signal Ipwd is a current generated elsewhere in the circuit when the switch is powered down. Signal Ipwd is input to the collector and base of npn transistor 73, then to the collector and base of npn transistor 72, which are both configured as diodes. As the current from signal Ipwd flows through transistor 73 and transistor 72 to ground, a voltage drop of two base-emitter junction voltages, which is approximately equal to 1.4 volts, develops at the base of transistor 73. The current mirror made of npn transistors 72 and 76 is activated, providing bias current for the differential amplifier made up of npn transistors 71 and 75. The collector of transistor 71 is connected to the collector of transistor 23. The collector and base of npn transistor 75 are shorted together and connected to output voltage swout. Although the unity gain amplifier 20 is powered down, transistors 71, 75, 23 and 24 are active and produce a new unity gain connected op-amp which drives the switch 10 output with a reference voltage of approximately 1.4 volts. The present invention is not limited to the embodiments, described above, but also encompasses variations thereof. The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims: CLAIMS: 1. A tri-modal switch for receiving an input voltage and outputting an output voltage, said tri-modal switch characterized by comprising: a unity-gain operational amplifier (20), having an input and an output for connecting a load and receiving the input voltage, outputting the output voltage substantially equal to the input voltage, and outputting an output current; and current means (60) for providing an output current to the unity-gain operational amplifier (20), an amount of the output current of the unity-gain operational amplifier being sourced to the load or sunk from the load by the unity-gain operational amplifier (20) being dependent upon the output current of the current means (60). 2. A tri-modal switch according to Claim 1 , further comprising a voltage generator (70), coupled to the unity-gain operational amplifier (20), generating a d.c. voltage if the unity-gain operational amplifier (20) is powered down. 3. A tri-modal switch according to Claim 1 or 2, wherein the current means (60) comprises: detector means (30, 40), coupled to the unity-gain operational amplifier, for receiving the input voltage and the output voltage, for determining whether the input voltage is greater than the output voltage and whether the input voltage is less than or equal to the output voltage, and for outputting a control signal based on the determination, and a current source (60), coupled to the detector means and to the unity-gain operational amplifier, and generating a current into the unity-gain operational amplifier based on the control signal. 4. A tri-modal switch as claimed in Claim 1 or 2 wherein the current means comprises: a maximum level detector (30), coupled to the unity-gain operational amplifier (20), receiving the output voltage of the unity-gain operational amplifier (20) and the input voltage, determining which one of the input voltage and the output voltage is greater, and generating and outputting a maximum level detector control signal based on the ouφut voltage and the input voltage; a minimum level detector (40), coupled to the unity-gain operational amplifier, receiving the output voltage of the unity-gain operational amplifier (20) and the input voltage, determining which one of the input voltage and the output voltage is lesser, and generating and outputting a minimum level detector control signal based on the output voltage and the input voltage; an amplifier (50), coupled to the maximum level detector and to the minimum level detector, having a non-inverting input and an inverting input and receiving the greater of the input voltage and the output voltage at the non-inverting input and the lesser of the input voltage and the output voltage at the inverting input, and outputting a current generator control signal based thereon; a current generator (60), coupled to the unity-gain operational amplifier and to the amplifier, one of sinking current from the unity-gain operational amplifier if the output voltage is greater than the input voltage and for sourcing current to the unity-gain operational amplifier if the output voltage is less than the input voltage, based on the current generator control signal. 5. A tri-modal switch as claimed in Claim 1 or 2, wherein: the current means sinks current from the unity-gain operational amplifier if the output voltage is greater than the input voltage and sources current to the unity-gain operational amplifier if the output voltage is less than the input voltage.

Inventors: FOTOWAT-AHMADY ALI, NAVID NASROLLAH S, TABATABEI ALI, FOTOWAT-AHMADY, ALI, NAVID, NASROLLAH, S., TABATABEI, ALI
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: H03K17
IPC codes: H03K17