TTL Circuit in Which Transient Current Is Prevented From Flowing Therethrough

1. Field of the Invention

The present invention relates to a transistor-transistor logic (TTL) circuit. More particularly, it relates to a TTL inverter having a transient state of a high impedance output for reducing the transient current.

Transistor-transistor logic inverters are generally used as basic elements in semiconductor integrated circuits. Inverters constructed by PNP transistors or NPN transistors are known. In the following description, TTL inverters constructed with NPN transistors are explained. However, the present invention is also applicable to TTL inverters constructed with PNP transistors. 2. Description of the Prior Art

A conventional TTL inverter comprises a single phase-splitter transistor for splitting the phase of an input signal, which is applied to the base of the phase splitter transistor, into a non-inverted phase at its emitter and an inverted phase at its collector; an inverted signal output transistor for inverting the signal of the noninverted phase; and an off buffer circuit connected in series with the inverting transistor for receiving the inverted phase signal from the collector of the phase splitter transistor, whereby the input signal applied to the input end is inverted at the output end. In principle, the off buffer circuit and the inverted signal output transistor are alternately turned on and off in response to the outputs of the phase splitter transistor.

In the above-mentioned conventional TTL inverter, however, there are the following disadvantages. That is, in a transient period during which the output signal changes from an "H" (high) level to an "L" (low) level or vice versa, there is a period when both the off buffer circuit and the inverted signal output transistor are simultaneously conductive so that a large transient current flows therethrough. This transient current brings about not only an increase in power consumption but also a fluctuation of the power supply voltage, the fluctuation causing error operations of the external or internal circuits. In a large-scale integrated logic circuit, the transient currents in the inverters accumulate, resulting in a more serious problem in the above-mentioned error operations.

When the off buffer circuit comprises a single transistor for obtaining an "H" level of the output voltage higher than the "H" level given by the off buffer circuit comprises two transistors in a Darlington connection, a conventional TTL inverter requires a noninverting gate connected between the collector of the phase splitter transistor and the off buffer circuit. The noninverting gate comprises a number of elements. Because of the number of elements, the conventional TTL inverter of this type has a low integration degree and a low operating speed.

It is, therefore, a primary object of the present invention to prevent the transient current from flowing through a TTL circuit.

A second object of the present invention is to prevent error operations in various circuits having a common power supply line with a TTL circuit, the error operations being caused by the fluctuation of the power supply voltage due to the transient current flowing through the TTL circuit.

A third object of the present invention is to reduce the power consumption in the TTL circuit.

A fourth object of the present invention is to increase the degree of integration in the TTL circuit.

A fifth object of the present invention is to increase the operating speed of the TTL circuit.

The above objects are attained by a TTL circuit comprising an input end for receiving an input signal, an output end for providing a logical output in response to the input signal, an inverted signal output circuit including a first transistor having a first control electrode connected to the input end and a second transistor connected between the first transistor and the output end, the inverted signal output circuit having a first threshold voltage and being turned on when the input signal is at a high logic level to make the output end a low logic level. An off-buffer circuit is connected to the output end, being turned on to make the output end a high logical level when the input signals are at a low logical level. An off-buffer driving circuit has a third transistor connected between the input end and the off-buffer circuit and a second control electrode connected to the input end. The off-buffer driving circuit also has a second threshold voltage lower than the first threshold voltage of the inverted signal output circuit, whereby the off-buffer circuit is turned off before the turning on of the inverted signal output circuit and is turned on after the inverted signal output circuit is turned off.

The above objects and other objects, as well as the features of the present invention will be better understood from the following description of the embodiments in comparison with the prior art, with reference to the accompanying drawings, in which:

FIG. 1 is a circuit diagram of a conventional TTL inverter;

FIG. 2 is a waveform diagram at various nodes in the circuit of FIG. 1;

FIG. 3 is a waveform diagram for explaining a transient state when the circuit of FIG. 1 is switching;

FIG. 4 is a circuit diagram of a TTL inverter according to an embodiment of the present invention;

FIG. 5 is a waveform diagram for explaining a transient state when the circuit of FIG. 4 is switching;

FIGS. 6 and 7 are circuit diagrams of circuits connected to the input end of the circuit of FIG. 4;

FIG. 8 is a circuit diagram of a TTL inverter according to a second embodiment of the present invention; and

FIG. 9 is a circuit diagram of a conventional TTL inverter.

Before describing the embodiments of the present invention, an example of a conventional TTL inverter and the disadvantages therein will be described with reference to FIGS. 1 through 3. In FIG. 1, Tr.sub.1 is a phase splitter transistor, the base of which operatively receives an input signal applied to an input end A connected to the base, the collector of which provides an output having a phase inverted from that of the input signal, and the emitter of which provides another output having the same phase as the input signal. Transistors Tr.sub.2 and Tr.sub.3, in a Darlington connection, comprises an off buffer circuit. The "off buffer" circuit will be described later. A transistor Tr.sub.4 connected in series with the transistor Tr.sub.3 is an inverted signal output transistor, the base of which is connected to the emitter of the phase splitter transistor Tr.sub.1. Between the emitter of the transistor Tr.sub.1 and ground, a diode D.sub.1 and a resistor R.sub.1 are connected in series. The diode D.sub.1 and the resistor R.sub.1 function to increase the switching speed of the inverted signal output transistor Tr.sub.4 when it turns off by drawing charges on its base to ground. An output end D is connected to a node commonly connected to the emitter of the transistor Tr.sub.3 and the collector of the transistor Tr.sub.4. A capacitor C connected to the output end D is a load capacitor of the output line. To the output end D, a load resistor L is connected.

In the operation of the circuit of FIG. 1, as illustrated in FIG. 2, when the input signal applied to the input end A is at the "L" level, the transistor Tr.sub.1 is in the off state so that, on the one hand, the base B of the transistor Tr.sub.4 is connected to the emitter of the transistor Tr.sub.1 and is at the "L" level, and, on the other hand, the base of the transistor Tr.sub.2 is connected to the collector C of the transistor Tr.sub.1 and is at the "H" level. Thus, in this state, the inverted signal output transistor Tr.sub.4 is in the off state, and the off buffer circuit, consisting of the transistors Tr.sub.2 and Tr.sub.3, is in the on state so as to maintain the off state of the inverted signal output transistor Tr.sub.4. In this state, therefore, the output end D is maintained at the "H" level. The name "off buffer" is derived from this function of keeping the inverted signal output transistor Tr.sub.4 off and from the fact that this off buffer circuit includes the buffering transistor Tr.sub.2.

When the input signal applied to the input end A is at the "H" level, the phase splitter transistor Tr.sub.1 is in the on state so that its collector C is at the "L" level and its emitter connected to the base B of the inverted signal output transistor Tr.sub.4 is at the "H" level. Accordingly, the output end D is at the "L" level.

In the transient states as illustrated in FIG. 2 by the broken circles, however, there is a problem of a large transient current flowing through the transistors Tr.sub.3 and Tr.sub.4 in the conventional circuit of FIG. 1. The problem of the transient current, when the input A changes from the "L" level to the "H" level, will be explained with reference to FIG. 3. In the following description, V.sub.BE (about 0.8 V) is the base-emitter voltage when the transistor Tr.sub.1, Tr.sub.2, Tr.sub.3, or Tr.sub.4 is conductive, V.sub.CE is the collector-emitter voltage of the transistor when it is saturated, and V.sub.f (about 0.5 V), which is lower than V.sub.BE, is the anode-cathode voltage of the diode D.sub.1 when it is conductive. In FIG. 3, when the input voltage applied to the input end A is lower than the sum of the base-emitter voltage V.sub.BE of the transistor Tr.sub.1 and the anode-cathode voltage V.sub.f of the diode D.sub.1, the transistor Tr.sub.1 is in the off state so that its collector potential is equal to the power supply potential V.sub.CC and its emitter potential is equal to ground potential. Therefore, the transistors Tr.sub.2 and Tr.sub.3 are conductive and the transistor Tr.sub.4 is not conductive, with the result that the output potential at the output end D is at a high logic level V.sub.OH, which is nearly equal to V.sub.CC -2 V.sub.BE, i.e., the base potential of the transistor Tr.sub.2 minus the base-emitter voltage of the two stages of the transistors Tr.sub.2 and Tr.sub.3.

When the input voltage reaches V.sub.BE +V.sub.f or more, the transistor Tr.sub.1 begins to be turned on so that the potential at its collector C begins to be lowered, and, accordingly, the transistors Tr.sub.2 and Tr.sub.3 become unsaturated, with the result that the potential at the output end D is gradually lowered. Then, when the input voltage further increases to reach 2 V.sub.BE, the transistors Tr.sub.1 and Tr.sub.4 are saturated so that they are completely turned on, with the result that the potential at the collector C is lowered to the sum of the collector-emitter voltage V.sub.CE of the transistor Tr.sub.1 and the base-emitter voltage of the transistor Tr.sub.4 and the potential at the output end D is lowered to the collector-emitter voltage V.sub.CE of the transistor Tr.sub.4. Since the potential difference between the base of the transistor Tr.sub.2 and the emitter of the transistor Tr.sub.3 is now (V.sub.CE +V.sub.BE)-V.sub.CE =V.sub.BE, the transistor Tr.sub.2 is turned off. However, there are remaining charges on the base of the transistor Tr.sub.3, so that, for an instant, the transistor Tr.sub.3 is still in the on state even after the transistor Tr.sub.2 is turned off. Therefore, there is an instantaneous current flowing from the power supply line V.sub.CC through the transistors Tr.sub.3 and Tr.sub.4 to ground. This instantaneous current causes the above-mentioned problem.

Although FIG. 3 illustrates an input voltage at the input end A higher than 2 V.sub.BE, it cannot be increased to higher than 2 V.sub.BE because it is clamped to 2 V.sub.BE by the on-state transistors Tr.sub.1 and Tr.sub.4.

When the input voltage at the input end A is decreased from 2 V.sub.BE to the "L" level, a similar instantaneous current also flows through the transistors Tr.sub.3 and Tr.sub.4 because of the remaining charges on the base of the transistor Tr.sub.4.

The primary object of the present invention is to prevent the above-mentioned instantaneous current from flowing through the transistors Tr.sub.3 and Tr.sub.4 during a transient period.

An embodiment of the present invention will now be described with reference to FIGS. 4 and 5. In FIG. 4, the main difference between the conventional circuit of FIG. 1 and the circuit of FIG. 4 is that, in FIG. 4, instead of the single-phase splitter transistor Tr.sub.1 in FIG. 1, two transistors Tr.sub.5 and Tr.sub.6 are employed. The transistor Tr.sub.5 is used for driving only the inverted signal output transistor Tr.sub.4. The transistor Tr.sub.6 is used for driving only the off buffer circuit, including the transistors Tr.sub.2 and Tr.sub.3. The base of the transistor Tr.sub.5 is connected to the input end A, its collector is connected through a resistor R.sub.4 to the power supply line V.sub.CC, and its emitter is connected to the base of the inverted signal output transistor Tr.sub.4 and, through a diode D.sub.2 and a resistor R.sub.6 connected in series for drawing the charges on the base of the transistor Tr.sub.4 when it turns on, to ground. In place of the diode D.sub.2 and the resistor R.sub.6, a single resistor may alternatively be used. The base of the transistor T.sub.r6 is connected through a resistor R.sub.5 to the input end A, its collector is connected to the base of the transistor Tr.sub.2 in the off buffer circuit and, through a load resistor R.sub.2, to the power supply line V.sub.CC, and its emitter is connected through a diode D.sub.3 to the ground. The resistor R.sub.5 is provided for preventing the base potential of the transistor Tr.sub.6 from being clamped to V.sub.BE +V.sub.f. That is, by providing the resistor R.sub.5, the potential at the input end A can be increased to 2 V.sub.BE. The transistors TR.sub.2 and Tr.sub.3 comprising the off buffer circuit, the inverted signal output transistor Tr.sub.4, the load capacitance C, and the load resistance L are the same as those in the circuit of FIG. 1.

The operation of the circuit of FIG. 4 will be described with reference to FIG. 5. When the potential of the input signal at the input end A is lower than V.sub.BE +V.sub.f, the transistors Tr.sub.5 and Tr.sub.6 are in the off state so that the transistors Tr.sub.2 and Tr.sub.3 are on and the transistor Tr.sub.4 is off, resulting in a high potential level V.sub.OH =V.sub.CC -2 V.sub.BE at the output end D. When the input potential at the input end A increases to more than V.sub.BE +V.sub.f, the transistor Tr.sub.6 begins to be turned on so that the potential at its collector C begins to be lowered to the sum of its collector-emitter voltage and the anode-cathode voltage of the diode D.sub.3 until the transistor Tr.sub.6 is completely turned on and saturated. The transistors Tr.sub.2 and Tr.sub.3 are in the on state as long as the transistor Tr.sub.6 is not saturated. Therefore, the potential at the emitter of the transistor Tr.sub.3 connected to the output end D is lowered, as is the potential at the base of the transistor Tr.sub. 2 connected to the collector C of the transistor Tr.sub.6. When the transistor Tr.sub.6 is saturated, the transistor Tr.sub.2 is not supplied with the base current so that the transistor Tr.sub.2 and then the transistor Tr.sub.3 are turned off. After the transistor Tr.sub.3 is turned off, and when the input potential further increases to reach 2 V.sub.BE, the transistor Tr.sub.4 is turned on so that the output potential at the output end D is clamped to the low logic level V.sub.OL equal to V.sub.CE of the transistor Tr.sub.4. Since the transistor Tr.sub.3 is in the off state after the transistor Tr.sub.6 is saturated, no transient current flows through the transistors Tr.sub.3 and Tr.sub.4.

During a switching period when the input potential is lowered from 2 V.sub.BE to a level lower than V.sub.BE, the transistor Tr.sub.4 is first turned off and then the transistor Tr.sub.6 is turned off. Accordingly, the transistor Tr.sub.3 is turned on only after the transistor Tr.sub.4 is turned off. Thus, in this case also, no transient current flows through the transistors Tr.sub.3 and Tr.sub.4.

As is well known, a multi-emitter transistor, as illustrated in FIG. 6, or an AND circuit comprising the PNP transistors, as illustrated in FIG. 7, may be connected to the input end A of the circuit of FIG. 4.

In the embodiment of FIG. 4, in order to obtain a high operating speed, Schottky barrier transistors are employed as the transistors Tr.sub.2, Tr.sub.4, Tr.sub.5, and Tr.sub.6. However, the scope of the present invention also covers the case when these transistors are not Schottky barrier transistors. Also, the transistors Tr.sub.2 through Tr.sub.6 may alternatively be PNP transistors. Further, as an alternative to the Schottky barrier diode D.sub.3, a conventional transistor having a base-emitter voltage lower than V.sub.BE may be used by connecting its collector to its base.

FIG. 8 shows another embodiment of the present invention. The differences between FIGS. 4 and 8 are that, in FIG. 8, the single transistor Tr.sub.3, instead of the two transistors Tr.sub.2 and Tr.sub.3 of the off buffer circuit in FIG. 4, constitutes an off buffer circuit. By eliminating the transistor Tr.sub.2 and by directly connecting the collector of the transistor Tr.sub.60 to the base of the transistor Tr.sub.3, the high logic level at the output end D of FIG. 8 is nearly equal to V.sub.CC -V.sub.BE, which is higher by V.sub.BE than the high logic level V.sub.CC -2 V.sub.BE at the output end D of FIG. 4. Conventionally, a TTL inverter having such a high "H" level output cannot be formed as simply as the circuit of FIG. 8. That is, in FIG. 8, when the output potential at the output end D is the "L" level, both the transistors Tr.sub.60 and Tr.sub.40 are in the on state so that the potential difference between the collector C of the transistor Tr.sub.60 and the output end D is (V.sub.CE +V.sub.f)-V.sub.CE =V.sub.f, which is lower than V.sub.BE. Therefore, the transistor Tr.sub.3 is surely off at this state. However, in the conventional circuit of FIG. 1, if the transistor Tr.sub.2 is eliminated and the collector of the transistor Tr.sub.1 is directly connected to the base of the transistor Tr.sub.3, the transistor Tr.sub.3 would always be conductive because when the output potential is at the "L" level, the potential difference between C and D would be V.sub.BE so as to maintain the on state of the transistor Tr.sub.3.

Conventionally, in order to obtain a TTL inverter having a relatively high "H" level output, a considerably complex circuit 90 was required between the transistor Tr.sub.50 and the transistor Tr.sub.3, as illustrated in FIG. 9. The circuit 90 acts as a noninverting gate. The noninverting gate 90 comprises a number of elements such as transistors Tr.sub.91, Tr.sub.92, and Tr.sub.93, resistors R.sub.91, R.sub.92, and R.sub.93, and a diode D.sub.91. As mentioned before, such a number of elements as the above-mentioned ones not only deteriorates the degree of integration but also lowers the operating speed. By comparing the circuit of FIG. 8 with the conventional circuit of FIG. 9, it will be apparent that the embodiment of FIG. 8 is advantageous with respect to the degree of integration and the operating speed.

From the foregoing description, it will be apparent that, according to the present invention, in a TTL inverter, by providing two separate transistors for driving an off buffer circuit and for driving an inverted signal output transistor, respectively, and by making the input threshold voltage for turning on or off the one transistor different from that of another transistor, no transient current flows through the TTL inverter so that fluctuation of the power supply voltage due to the transient current can be prevented. Consequently, error operations of various circuits having a common power supply line with the TTL inverter can be prevented.

Further, according to the present invention, a TTL inverter having a relatively high "H" level output can easily be formed without decreasing the degree of integration and the operating speed.