JPH0331014B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a logic circuit, and particularly to a current switching type logic circuit having a plurality of input terminals.

In general, the switching characteristics of logic circuits are determined by the important factors such as rise time, fall time, and propagation delay time. In particular, for current switching type logic circuits, which are characterized by high-speed operation, obtaining excellent switching characteristics is a major goal. ing.

Figure 1 shows a conventional current-switching logic circuit with n input terminals, in which input transistors are connected in parallel with the base as the input terminal.
T ₁ to _{T o} and a reference transistor T _R whose emitters are connected to the common emitter of T ₁ to _{T o} and to the first voltage source via a constant current source, and whose base is connected to the reference voltage source. The collectors of each are connected to a second voltage source via a load resistor, and changes in collector voltage from T ₁ to _{T o} are taken out as an output. However, in this type of current switching logic circuit, all the collectors of the input transistors are commonly connected, so as the number of input transistors increases, the collector capacitance of the common collector part of the input transistors increases, and the load resistance value and collector capacitance increase. The time constant of the load resistance section, which is determined by the product of the values, becomes larger,
The disadvantage was that the rise time, fall time, and propagation delay time increased.

SUMMARY OF THE INVENTION An object of the present invention is to provide a circuit that alleviates the essential drawbacks of conventional circuits and has excellent switching characteristics even when the number of input terminals increases.

According to this invention, a plurality of parallel-connected input transistor groups each having a base as an input terminal, and an emitter connected to the emitter of the input transistor group and connected to a first voltage source via a constant current source. , a reference transistor _TR whose collector is connected to a second voltage source via a resistor and whose base is connected to a reference voltage source; In a circuit that has a constant voltage transistor T _V that is connected to the second voltage source and applies a constant voltage to its base, and extracts voltage changes at the collector of the _TV as an output, the collector capacitance increases as the number of input transistors increases. A logic circuit that has excellent switching characteristics even when the number of input transistors increases, by reducing the voltage change at the collector of the input transistor and extracting the voltage change at the collector of the constant voltage transistor _TV , which has a small collector capacitance, as an output. is obtained.

FIG. 2 shows a first embodiment of the present invention in the case where the number of input terminals is n. Input transistors T ₁ to _{T o} connected in parallel with the base as the input terminal
, a reference transistor TR whose emitter is connected to the emitter of the input transistor group, and which is connected to the first voltage source via a constant current source and whose base is connected to the reference voltage source, _and whose emitter is connected to the input transistor T ₁ ~ Consisting of a constant voltage transistor T _V connected to the collector of T _o and whose base is connected to a constant voltage source, it consists of a constant voltage transistor T _V and a reference transistor.
The collectors of T _R are connected to the voltage source 2 through load resistors R ₁ and R ₂ , respectively, and the constant voltage transistor
This is a circuit that extracts voltage changes on the _TV collector as output.

In this circuit, if the base potential of the constant voltage transistor is set to an appropriate potential within a range that does not saturate the transistors T ₁ to T _o and _TV , the voltage will be constant if at least one of the input transistors T ₁ to T _o is conductive. The transistor _TV is conductive, and in other cases, the _TV is turned off.If the voltage change at the collector of the _TV is taken out as an output, it is exactly the same as the conventional current switching type logic circuit shown in Figure 1. Logical operations can be realized.

In this circuit, the emitter current of the constant voltage transistor is equal to the sum of the collector currents of each of the input transistors and depends on the emitter current.The change in V _BE of the constant voltage transistor is the change in T _BE when the input transistor is disconnected and conductive. approximately equal to. Therefore, in a normal current switching logic circuit where the input and output logic amplitudes are equal, the constant voltage transistor
Changes in V _BE and changes in the collector potential of the input transistor are approximately 1/2 of the output logic amplitude, reducing the Miller effect in which changes in the collector voltage seen from the input terminal during switching are fed back through the feedback capacitance, and improving the switching characteristics. Improved.

Furthermore, when all of the input transistors T ₁ to T _o change from a OFF state to one of them to a conductive state, the transistors T ₁ to _{T O} and
Since the collector capacitance of a _TV is discharged at a rate determined by the current value of the constant current source, the switching characteristics are the sum of charging and discharging loads during switching, which is expressed as the product of the capacitance value and the change in terminal voltage. related to.

Therefore, if the collector capacitances of T ₁ to _{T o} and T _V are all equal and their value is C, and the logical amplitude of input and output is V _W , then the sum of the discharge loads of each collector capacitance during switching is If Q is Q ₁ in Fig. 1, then Q ₁ = C x n x V _W = nCVw, and similarly, Q ₂ in the circuit of Fig. 2 is Q ₂ = C x n x V _W /2 + C x V. _W = (1+n/2).C.V _W , and when this circuit is used, the total discharge load can be reduced by (n/2-1).C.V _W , and the falling and switching times can be made faster. Furthermore, when the input transistors T ₁ to _{T o} change from a conductive state to a depleted state, the capacitance of the common collector is charged through T _V and the common collector potential increases, so that the base is connected to a constant voltage source. The base-emitter voltage V _BE of _TV decreases. The change in the collector current with respect to the change in V _BE is exponential, and the collector current of the TV decreases rapidly with a slight decrease in _{V BE} . Therefore, the change in the output voltage will change as the number of input transistors increases. Increasing the collector capacity of the common collector requires only a small delay.
This is determined by the time constant, which is the product of the collector capacitance value of the _TV and the load resistance value, and a logic circuit with excellent switching characteristics can be obtained.

FIG. 3 shows a second embodiment of the invention.
The difference from the first embodiment is that the constant voltage transistor T _V
Since the base of is directly connected to voltage source 2, it can be applied when the output logic width is small, and T ₁
~ If one or more of T _o is conductive, the _TV is conductive; otherwise, the _TV is in a disconnected state.If the change in the collector voltage of the _TV is taken out as an output, the result is the same as the first embodiment. Exactly the same logical operation and effects of the invention are obtained. Various modifications can be made to the embodiments described above within the scope of the present invention, and the present invention encompasses all of the scope of the claims.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional logic circuit.
2 and 3 are circuit diagrams showing first and second embodiments of the logic circuit according to the present invention. T ₁ ~ _{T o} ...transistor, _R1 , _R2 ...resistance,
I ₁ ... Constant current source, 1, 2 ... Voltage source, 3, 4,
5, 6, IN ₁ ~ IN _o ... terminal.

Claims (1)

[Claims] 1. In a current switching logic circuit having a plurality of input transistors, a transistor to which a constant voltage is applied to the base is inserted between the commonly connected collectors of the plurality of input transistors and the load element, and A logic circuit characterized in that a voltage change at a connection between a transistor and the load element is extracted as an output.