Bipolar-complementary metal oxide semiconductor inverter

Abstract: A bipolar-complementary metal oxide semiconductor (Bi-CMOS) inverter includes a CMOS inverter (T1, T2), an npn-type bipolar transistor (Q1), and an n-channel MOS transistor (T3). An emitter of an n-channel MOS transistor (T2) of the CMOS inverter is connected to an emitter of the bipolar transistor (Q1) and a drain of the MOS transistor (T3). A gate of the MOS transistor (T3) is connected to an input of the CMOS inverter. When an input voltage (Vin) applied to the CMOS inverter is switched to a high level, charges at an output of the CMOS inverter (T1, T2) and at the emitter of the bipolar transistor (Q1) are pulled out. During this time, the voltage at the output of the CMOS inverter (T1, T2) decreases so as to follow the decrease of the voltage at the emitter of the bipolar transistor (Q1).

Description: BACKGROUND OF THE INVENTION The present invention generally relates to a bipolar-complementary metal oxide semiconductor inverter. As is well known, a bipolar transistor has a driving capability greater than that of a complementary metal oxide semiconductor inverter (hereafter simply referred to as a CMOS inverter). Therefore, a bipolar transistor can charge up a load quickly, compared to a CMOS inverter. From this viewpoint, recently, there has been considerable activity in the development of a bipolar-CMOS inverter (hereafter simply referred to as a Bi-CMOS inverter) obtained by combining a bipolar transistor and a CMOS stage. The CMOS inverter of the Bi-CMOS inverter performs a NOT operation for an input signal, and the bipolar transistor thereof drives a load coupled therewith in response to an output voltage of the CMOS inverter. The Bi-CMOS inverter contributes to an increased operational speed of a random access memory (RAM). Such Bi-CMOS inverters are disclosed in the Japanese Laid-Open Patent Application Nos. JP-A-57141128, JP-A-57212827 and JP-A-58080929, for example. However, the conventional Bi-CMOS inverters have the following disadvantages. That is, when the input voltage is switched from a low level to a high level, the output voltage of the CMOS inverter may fall more quickly than the output voltage of the Bi-CMOS inverter which is applied to the load. Therefore, the p-n junction formed between the base and emitter of the bipolar transistor becomes biased in the reverse direction. As a result, a breakdown may be caused at the base-emitter junction. The above problem can be overcome by making the output voltage of the CMOS inverter fall more slowly than the output voltage applied to the load. However, this causes an increased through current which passes through the bipolar transistor. The increased through current may introduce noise and causes an increased power consumption of the Bi-CMOS inverter. Alternatively, instead of being connected directly to its Vss terminal (assumed to be grounded), the source of the inverter's n-channel MOS transistor may be connected to the emitter of the bipolar transistor, and thereby to a path passing through an active device, by means of which charges at the output terminal of the CMOS inverter and at the emitter of the bipolar transistors are pulled out, when the input voltage is switched from a low level to a high level (figure 3). Examples of such circuits can be found in EP-A-0 209 805 and JP-A-0 59 25 423. SUMMARY OF THE INVENTION The present invention is based on the aforementioned alternative circuit configuration, and further proposes to discharge the base of the Bi-CMOS inverter's bipolar transistor when the input voltage Vin is switched to the high level. According to the present invention, this is achieved by adding a second n-channel MOS transistor having its drain connected to the base of the bipolar transistor, its gate connected to the gates of the p-channel and n-channel transistors of the MOS inverter, and its source connected to the negative supply source (ground). Other objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a circuit diagram of a conventional Bi-CMOS inverter; FIGS.2A and 2B show signal waveforms for explaining an operation of the Bi-CMOS inverter of FIG.1; FIG. 3 is a circuit diagram of a Bi-CMOS inverter corresponding to the preamble of claim 1, to which the present invention is applicable; FIG. 4 shows signal waveforms for explaining the operation of the Bi-CMOS inverter of FIG. 3; FIG. 5 shows the connection of the second n-charnel MOS transistor to the circuit of FIG. 3, in accordance with the present invention; and FIG. 6 shows a variation of the configuration of FIG. 3 to which the invention is applicable. DETAILED DESCRIPTION To facilitate the understanding of the present invention, a description is given of a conventional Bi-CMOS inverter as shown in the aforementioned publications, with reference to FIGS.1 and 2. Referring to FIG.1, a CMOS inverter is made up of MOS type field effect transistors (hereafter simply referred to as FETs) T1 and T2. FETs T1 and T2 are p-channel and n-channel FETs, respectively. Gates of the FETs T1 and T2 are mutually connected and are also connected to an input terminal IN. Drains of the FETs T1 and T2 are mutually connected. An output voltage V1 of the CMOS inverter is applied to an npn-type bipolar transistor Q1, which is used for driving a load (not shown) connected to an output terminal OUT. A FET T3 is an n-channel FET, and is connected as shown. A positive power source voltage V DD is applied to a source of the FET T1, and another positive power source voltage V CC is applied to a collector of the transistor Q1. Generally, the voltages V DD and V CC are set identical to each other. Alternatively, the voltages V DD and V CC may be set different from each other. Sources of the FETs T2 and T3 are connected to the ground which acts as a negative power source. An input voltage Vin is applied through the input terminal IN to the gates of the FETs T1, T2 and T3. The CMOS inverter inverts the input voltage Vin and produces a voltage V1. The polarity of the voltage V1 is opposite to that of the input voltage Vin. The voltage V1 is applied to the base of the transistor Q1, which then supplies an output voltage Vout to the output terminal OUT through the emitter of the transistor Q1. The above Bi-CMOS inverter is designed so that the voltage V1 is decreased to the low level by turning ON the FET T2 through which the charge at the output terminal of the CMOS inverter is passed to the ground. The output voltage Vout is decreased to the low level by turning ON the FET T3 through which the charge at the output terminal OUT is passed to the ground. When the input voltage Vin is changed from the low level to the high level as shown in FIG.2A, the voltage V1 represented by a solid line may decrease faster than the output voltage Vout represented by a broken line. At this time, the p-n junction formed between the base and emitter of the transistor Q1 becomes biased in the reverse direction. In case where the transistor Q1 is constructed by a transistor having a low base-emitter breakdown voltage V BEB , the transistor Q1 may be broken down. As a result, the circuit operation of FIG.1 has a low reliability. In order to overcome the above problem, it is conceivable to apply a FET of a small capability to the FET T2. In this case, as shown in FIG.2B, the fall time of the voltage V1 is lengthened, compared to the fall time of the output voltage Vout. Thereby, it becomes possible to prevent occurrence of the breakdown at the p-n junction formed between the base and emitter of the transistor Q1. However, during a time when the voltages Vout and V1 fall, the p-n junction between the base and emitter of the transistor Q1 is forwardly biased, and therefore the transistor Q1 is kept ON. Thereby, a large quantity of the through current passes through the transistor Q1, which may introduce noise on the output signal. Additionally, an increased power is consumed. The above problems are eliminated by the circuit of FIG. 3, corresponding to the preamble of claim 1. In FIG.3, elements identical to those in FIG.1 are indicated by the same references. One of the features of this circuit is that the source of the FET T2 is connected through a connection line 10 to the emitter of the transistor Q1 and the drain of the FET T3. Other connections between the elements are the same as those in FIG.1. That is, the input terminal IN is connected to the gates of the FETs T1, T2 and T3. The drains of the FETs T1 and T2 are mutually connected, and are also connected to the base of the transistor Q1. The positive power source voltage V DD is applied to the source of the FET T1, and the positive power source voltage V CC is applied to the collector of the transistor Q1. In operation, during a time when the input voltage Vin is kept at the low level, the FETs T1 and T2 are ON and OFF, respectively, and therefore the output voltage V1 of the CMOS inverter is kept at the high level. Further, during that time, the FET T3 is OFF, because the input voltage Vin at the low level is applied to the gate thereof. Therefore, the output terminal OUT is kept at a voltage approximately equal to the voltage Vcc. On the other hand, during a time when the input voltage Vin is kept at the high level, the FETs T1 and T2 are OFF and ON, respectively, and the FET T3 is ON. Therefore, the voltage V1 at the low level is outputted from the CMOS inverter. The voltage V1 at the low level is applied to the base of the transistor Q1. As a result, the transistor Q1 is OFF, and the output voltage Vout at the low level is obtained at the output terminal OUT. The above is the operation which is performed When the Bi-CMOS inverter is in the stationary states. At a transient time of the input voltage Vin, the Bi-CMOS inverter operates as follows. As shown in FIG.4, when the input voltage Vin is changed from the low level to the high level, both the FETs T2 and T3 are turned ON. At this time, the FET T3 forms a path which connects the source of the FET T3 and the emitter of the transistor Q1 to the ground. A charge at the emitter of the transistor Q1 passes through the FET T3, and a charge at the output terminal of the CMOS inverter passes through the FETs T2 and T3. Thereby, the output voltage Vout decreases, and at the same time the voltage V1 is decreased. Since the source of the FET T2 is connected to the emitter of the transistor Q1, as shown in FIG.4, the output voltage V1 of the CMOS inverter changes so as to follow up change of the output voltage Vout obtained at the emitter of the transistor Q1. Therefore, the p-n junction between the base and emitter of the transistor Q1 is prevented from being biased in the reverse direction, and at the same time is not forwardly biased. As a result, the probability of occurrence of the breakdown at the p-n junction can be eliminated, and the occurrence of the through current can be avoided. The present invention proposes to modify the Bi-MOS inverter of FIG. 3 by adding a second n-channel MOS transistor, as shown in FIG. 5. The second n-channel FET T5, shown in FIG. 5, has a drain connected to the base of the transistor Q1, and a gate connected to the input terminal IN. A source of the FET T5 is connected to the ground. The FET T5 is used for discharging the base of the transistor Q1 when the input voltage Vin is switched to the high level. The FET T3 acts to pull the output voltage Vout down to the low voltage. The driving capability of a conventional MOSFET is sufficient for this operation. However, the second n-channel transistor of claim 1 may also be connected to the circuit shown in FIG. 6, where an npn-type bipolar transistor Q2, which generally has a higher driving capability, is used in place of the FET T3.

Claims: A bipolar-complementary metal oxide semiconductor (Bi-CMOS) inverter comprising a complementary metal oxide semiconductor (CMOS) inverter including p-channel and n-channel MOS transistors (T1, T2), an input voltage (Vin) being applied to gates of the MOS transistors; and a bipolar transistor (Q1), a base of the bipolar transistor being connected to an output terminal of the CMOS inverter, and output voltage (Vout) of the Bi-CMOS inverter being obtained at the emitter of the bipolar transistor (Q1); the source of the n-channel MOS transistor (T2) being connected to the emitter of the bipolar transistor and thereby to a path passing through an active device (T3, Q2), by means of which charges at the output terminal of the CMOS inverter (T1, T2) and at the emitter of the bipolar transistor (Q1) are pulled out, when the input voltage (Vin) is switched from a low level to a high level, characterized in that it further comprises a second n-channel MOS transistor (T5), and in that a drain of the second n-channel MOS transistor (T5) is connected to the base of the bipolar transistor (Q1), a gate of the second n-channel MOS transistor (T5) is connected to the gates of the p-channel and n-channel MOS transistors (T1, T2) of the CMOS inverter, and a source of the second n-channel transistor (T5) is connected to a negative power source (ground). A Bi-CMOS inverter as claimed in claim 1, characterized in that a source of the p-channel MOS transistor (T1) and a collector of the bipolar transistor (Q1) are connected to a positive power source (V DD , V CC ) and in that the means for pulling out charges (T3) comprises a third n-channel MOS transistor (T3), a gate of the third n-channel MOS transistor (T3) is connected to the gates of the p-channel and n-channel MOS transistors (T1, T2) of the CMOS inverter, and a drain of the third n-channel MOS transistor (T3) is connected to the emitter of the bipolar transistor (Q1), and a source of the third n-channel MOS transistor (T3) is connected to a negative power source (ground). A Bi-CMOS inverter as claimed in claim 1, characterized in that a source of the p-channel MOS transistor (T1) and a collector of the bipolar transistor (Q1) are connected to a positive power source (V DD , V CC ) and in that the means for pulling out charges (Q2) comprises a second bipolar transistor (Q2), and a base of the second bipolar transistor (Q2) is connected to the gates of the p-channel and n-channel MOS transistors (T1, T2) of the CMOS inverter, a collector of the second bipolar transistor (Q2) is connected to the emitter of the bipolar transistor (Q1), and an emitter of the second bipolar transistor (Q2) is connected to a negative power source (ground).

Inventors: KOSHIZUKA ATSUO, KOSHIZUKA, ATSUO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1988
Main classification: H03K19
IPC codes: H03K17, H01L21, H03K19, H01L27