Abstract:
A plurality of inverted transitor current mirror and pull down circuits are disclosed which are suitable for providing current mirroring and base pull down functions in a variety of integrated circuit applications. The use of the inverted transistor as a current mirror is also utilized in gaining and/or level shifting any differential or single-ended analog or digital signal. The use of a multiple emitter inverted transistor for providing multiple pull down reduces to a fraction the chip area otherwise required by the use of multiple transistors.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to current mirroring circuits and the use thereof in a plurality of integrated circuit applications and more particularly, to a plurality of inverted transistor current mirroring and pull down circuits for such applications. 
     The prior art is replete with different types of integrated circuits having current mirroring circuits. For example, current mirroring circuits are utilized in differential amplifier circuits to reduce input voltage offset errors at the quiescent operating state of such amplifiers. Such current mirroring circuits are also utilized in gaining and/or level shifting any differential or single-ended signal. 
     Presently, some prior art current mirroring circuits utilize a diode, more commonly, a diode-connected transistor connected in parallel with the base-emitter junction of a transistor. The diode and transistor are then connected in the collector circuitry of the differential amplifier as is understood. Equal quiescent currents are established by making the PN junctions of the diode and transistor equal in area as is known. An additional, but less accurate, method for providing current mirroring is to utilize a base pull down resistor in place of the diode. Both of these circuits are illustrated hereinafter. 
     In the design of large scale integrated (LSI) circuits chip space is often limited. Therefore, a need exists for reducing the number of components and devices required to perform standard circuit functions, e.g., current mirroring. A disadvantage of the prior art current mirroring circuits is that at least two component devices are required to provide this function, each requiring a minimum area on the chip. Thus a need exists for reducing component count required for providing the current mirroring function to subsequently reduce required chip area. 
     In some comparator circuit applications it is desirous to ensure that an output signal is generated only when the input signal is of a particular polarity and to positively inhibit any output signal if the input signal is of the opposite polarity. In these prior art circuits, current mirroring is utilized to drive multiple devices to actively pull down the output of the comparator to essentially ground reference to inhibit any output signal. Again, however, the use of multiple component devices is required to provide this function and these techniques suffer from the same disadvantages discussed above. 
     The present invention overcomes the problems of the above circuits by utilizing inverted transistor current mirroring and pull down circuits to minimize component count. The use of a single inverted transistor with multiple emitters to provide the above functions significantly reduces to a fraction the area required on the chip by multiple transistors. Inverted transistors as used in applications herein are typical vertical bipolar transistors wherein the emitter electrodes are utilized as collectors with the collector electrodes functioning as emitters as is known in the art. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide improved current mirroring circuits for use in integrated circuit applications. 
     It is another object of the invention to provide an improved current mirroring and active pull down circuit for use in an integrated circuit. 
     Still another object of the present invention is to provide a current mirroring circuit utilizing an inverted transistor to minimize component count. 
     A further object of the invention is to provide a simple current mirror for use in integrated circuit applications which provides active pull down to inhibit output signals from the integrated circuit when the input to the circuit is at a predetermined polarity. 
     The inverted transistor circuit configurations of the invention are suitable for providing current mirroring for active pull down and current limiting applications. Moreover, these inverted transistor circuits provide simple mirror circuits to reduce component count and therefore to reduce the space required to fabricate the circuit on the integrated chip. 
     In one feature of the invention a simple current mirror circuit is provided which comprises an inverted transistor coupled to an active circuit means, which for example, may be a differential amplifier included in a monolithic circuit. 
     In another feature of the invention, the monolithic circuit comprises a pull down circuit including an inverted transistor for providing current mirroring and active pull down for positively inhibiting any output signal from an active circuit means which is coupled to the pull down circuit. 
     In yet another feature of the invention a current limiter circuit is provided for limiting the output current of an operational amplifier. The current limiter circuit includes a current sensing circuit for providing a limiting signal when the output current exceeds a predetermined value and a pull down circuit including an inverted transistor which is responsive to the limiting signal for limiting the output current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a differential amplifier including a conventional current mirror and pull down circuit; 
     FIG. 2 is a differential amplifier shown in schematic form including an alternate current mirror and pull down circuit; 
     FIG. 3 is a schematic diagram of a differential amplifier including a current mirror and pull down circuit of an embodiment of the present invention; 
     FIG. 4 is a schematic diagram of a differential amplifier including a simple current mirror circuit of another embodiment of the invention; and 
     FIG. 5 is a schematic diagram illustrating a current limiter circuit of yet another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Alternative methods for providing current mirroring and active circuit pull down will first be described along with some problems associated therewith so that the advantages of the circuits of the present invention can be more fully appreciated. Referring to FIGS. 1 and 2, the same reference numbers are used for like components. Differential amplifier or comparators 10 and 40, which are suitable to be manufactured in monolithic integrated circuit form, are shown as including differentially connected transistor pairs 12 and 14. Transistors 12 and 14 are, for example, PNP transistors having emitter electrodes thereof coupled to current source 16 which provides current to the transistors and which is connected at terminal 18 to a suitable power supply bus line. The base electrodes of transistors 12 and 14 are adapted to receive differential input voltages V 1  and V 2  respectively. For discussion purposes, without being limited thereto, the voltage V 2  may be considered as being at a reference potential. The voltage V 1  will be considered as being either in an &#34;on&#34; or &#34;off&#34; state. The collector electrode of transistor 14 is connected to the input of Darlington output stage 20 consisting of transistors 22 and 24. The output of both amplifier 10 and 40 is provided at terminal 26, the output of Darlington amplifier 20. 
     In FIG. 1, a current mirror and active pull down circuit is provided by diode connected transistor 28, transistor 30 (which are coupled to the collector output circuitry of respective transistors 12 and 14) and transistor 32. The amplifier 10 functions in a known manner to produce an output signal only when transistor 14 is rendered conductive. Transistor 14 is conductive when the voltage V 1  is greater in magnitude than voltage V 2 . Conversely, when the voltage V 1  is less than the voltage V 2  then transistor 12 is conductive and transistor 14 is rendered nonconductive. 
     A problem arises in amplifier circuits 10 and 40 due to the high current gain of Darlington amplifier section 20. If for some reason a small trickle current should occur at the base of transistor 22 of Darlington amplifier 20, it is entirely possible that a false output signal could occur even though transistor 14 is in an off state. To prohibit this possibility, the aforementioned current mirror and pull down circuit is provided to inhibit any output signal from occurring when transistor 12 is conducting. When transistor 12 is rendered conductive, transistor 28 is turned on and current is mirrored to transistors 30 and 32 which saturate. With transistors 30 and 32 in a saturated state the bases of transistors 22 and 24 are essentially clamped to a ground reference potential through these saturated transistors. Hence, transistors 22 and 24 are positively biased in an off state such that the Darlington amplifier cannot be rendered conductive whenever transistor 12 is conductive. 
     However, transistors 30 and 32 being in a saturated state increases the switching time decreasing the speed of operation of the circuit comprising amplifier 10. As is understood, a finite time is required to change operating states of the differential amplifier due to the time it takes to bring transistors 30 and 32 out of saturation because of the storage time delay phenomenon. Thus, differential amplifier 10 may not be able to switch between states fast enough for some applications. 
     An alternate method which may be used to provide current mirroring and active pull down is to replace diode connected transistor 28 (FIG. 1) with resistor 34. Thus in a like manner as differential amplifier 10, with transistor 12 being conductive, transistors 30 and 32 are in a saturated state and clamp the bases of transistors 22 and 24 respectively to substantially ground potential. Resistor 34, besides pulling the base electrodes of transistors 30 and 32 to less than one base-to-emitter voltage drop to prevent conduction thereof when transistor 14 is rendered conductive also provides a discharge path for discharging the storage charge in transistors 30 and 32. 
     Differential amplifier 40 suffers from the same disadvantages as differential amplifier 10 in that a finite time is required to switch operating states of transistors 12 and 14 because of the storage delay time caused by transistors 30 and 32 being in a saturated state when transistor 12 is conducting. In both differential amplifier circuits, the storage delay time is a function of the high current gain (β) of transistors 30 and 32. Because of the high current gain, a larger storage charge is developed between the base-emitter junction of transistors 30 and 32 which subsequently must be discharged through resistor 34 before the Darlington output amplifier section can be rendered conductive. 
     Referring to FIG. 3, there is shown differential amplifier 50 of a first embodiment of the invention which overcomes the problem associated with the aforedescribed amplifiers. Differential amplifier 50 includes: an active circuit portion comprising current source 16, transistors 12, 14 and Darlington amplifier section 20. Amplifier 50 operates in the same manner as amplifiers 10 and 40. To overcome the speed problems of the aforementioned amplifiers, amplifier 50 includes a vertical NPN transistor 36 which operates as an inverted transistor. The operation and construction of inverted transistor 36 is fully described for example in the magazine article by C. M. Hart, A. Slob, and H. E. Wulms, &#34;Bipolar LSI Takes a New Direction With Integrated Injection Logic,&#34; Electronics, October 3, 1974, pp 111-118. Briefly, transistor 36 is shown as including multiple emitter electrodes coupled respectively to the base electrodes of transistors 22 and 24. The normal collector electrode of inverted transistor 36 is coupled to ground reference and the base electrode thereof is coupled to the collector of transistor 12. 
     In operation, the emitter electrodes of inverted transistor 36 operate as multiple collectors and the collector electrode as an emitter. With transistor 12 being rendered conductive, current is mirrored through inverted transistor 36 and the base electrodes of transistors 22 and 24 are pulled down to essentially ground reference potential in the same manner as previously described. Thus any output signal is positively prevented from occuring during the time duration that transistor 12 is rendered conductive. 
     The structure of differential amplifier 50 offers several significant advantages over the alternative arrangements shown in FIGS. 1 and 2. Firstly, the number of devices required to provide current mirroring and active pull down is reduced from three devices to a single multiple emitter device. This advantage becomes very significant in the production of large scale integrated (LSI) chips wherein chip area becomes a critical factor. By using multiple emitter inverted transistors, the chip area required for amplifier 50 is a fraction of that for either amplifier 10 or 40. For example, the area required for multiple emitter inverted transistor 36 is approximately only 20 percent greater than that for a single device such as transistor 28. Another advantage of amplifier 50 over the aforementioned amplifiers is the improved switching speed between operating states of the differential amplifier. Since the time required for inverted transistor 36 to come out of saturation is much less than for a normally constructed transistor, switching time between operating states is greatly improved. Also, there is no need for a pull down resistor such as resistor 34 (FIG. 2) since the inverted current gain (β) of transistor 36 is typically one as opposed to typically one hundred for the normally connected transistor. 
     Referring to FIG. 4, there is shown differential amplifier 60 of another embodiment of the invention which includes inverted transistor 38. Inverted transistor 38 is utilized as both a current mirror and for single-ended differential amplifier 60. Inverted transistor 38 may also be utilized for level shifting such that the output signal developed at single ended output terminal 42 may be developed about a different voltage level than the differential input signal V 1 , V 2  that is applied to the base electrodes of transistors 12 and 14. Moreover, because the inverted gain of transistor 38 is typically unity, input voltage offset error can be controlled thereby. The use of the single inverted transistor 38 eliminates the requirement for the current mirror circuitry of FIGS. 1 and 2 including either transistors 28 and 30 or resistor 34 and transistor 30. Again, at least one device has been eliminated by the use of an inverted transistor in the differential amplifier application. 
     Referring now to FIG. 5 there is shown yet another embodiment of the invention wherein the application of an inverted transistor is utilized to provide current mirroring (n a current limiting circuit. As illustrated, current limiting circuit 70 is coupled to an active circuit 44, for example an operational amplifier, and is utilized to limit the maximum output current of the active circuit. The MC-1458S, an operational amplifier sold by Motorola, Inc. shows a typical connection of a current limiting circuit similar to circuit 70 to an active circuit such as circuit 44. Current limiting circuit 70 comprises a current sensing circuit including PNP transistors 46, 48 and resistor or circuit means 51, bias transistor 52 and inverted transistor 54. 
     In normal operation, bias transistor 52, which is responsive to a bias potential supplied thereto at the base electrode thereof from active circuit 44, biases transistor 46 in an on condition and current flows therethrough from active circuit 44 and resistor 51 (which is coupled to the active circuit 44 at output 56). Transistor 48 is biased off as the voltage drop across resistor 51 is not sufficient to bias the transistor to an on condition. Thus inverted transistor 54 is also nonconductive. In response to the output current from active circuit 44 exceedng a predetermined maximum limit, transistor 46 is caused to conduct more such that the voltage drop across resistor 51 biases transistor 48 to a conductive state. Because the β of inverted transistor 54 is typically one, the collector current developed by transistor 48 is mirrored through the inverted transistor which then steals the equivalent amount of current from the base drive of transistor 52. This in turn limits the base drive of transistor 46 and therefore limits the current therethrough. Thus if current limiting circuit 70 is in the output circuitry of active circuit 44 the output current therefrom is limited to a maximum value. The advantage of current limiting circuit 70 over other similar methods for providing current limiting is that the circuit is not responsive to any leakage current which might occur through transistor 48, since the β of transistor 54 is very low. For instance, whatever leakage current is conducted to the base of inverted transistor 54 is conducted therethrough which has little effect upon the operation of the circuit. Therefore inverted transistor 54 can be used alone for a current mirror and pull down circuit. However, in most typical current limit circuits a current mirror circuit comprising a normally connected transistor and either a diode or resistor is required in place of inverted transistor 54 to eliminate any leakage current problems. If these conventional circuits used only a single transistor, because the β of the transistor is very high, any leakage current from transistor 48 could cause transistor 46 to be rendered nonconductive to shut down the active circuit when not desired. Thus to prevent such occurrence a current mirror circuit is required in the conventional circuits. However, current limiting circuit 70 overcomes these leakage problems utilizing a single inverted device with its inherent advantage over conventional circuits. 
     In summary, the foredescribed circuits utilizing inverted transistors have several significant advantages over conventional and alternate circuits providing the same function. The number of devices required is minimized, the use of multiple emitters inverted transistors also reduces the area required for fabrication of The circuits in an integrated circuit. The speed of switching circuits utilizing the current mirrors of the present invention are greatly improved over conventional circuits since the time inverted an inerted transistor to come out of saturation is much less than a normally connected transistor. Finally, protection to leakage current problems is provided by the inverted transistor circuits of the present invention.