1. Field of the invention
This invention concerns logic circuits which are capable of high-speed operation with low power consumption.
2. Description of the Prior Art
For logic circuits of which high-speed and high load driving capacity are required, TTL type logic circuits are frequently used.
FIG. 1 is a circuit diagram of a TTL type NAND gate circuit. In the NAND gate circuit shown in FIG. 1, the base electrode of an NPN type Schottky barrier bipolar transistor (hereafter called an "S bipolar transistor") Q1 is connected to an input terminal A via a diode D1 and, at the same time, is connected to another input terminal B via a diode D2. The conductivity of the S bipolar transistor Q1 is controlled according to the input signals applied to the input terminals A and B. The base electrode of an NPN bipolar transistor Q2 is connected to the emitter electrode of the S bipolar transistor Q1. The conductivity of the S bipolar transistor Q2 is controlled by the S bipolar transistor Q1. A resistor R1 is inserted between the collector electrode of the S bipolar transistor Q2 and a voltage source terminal Vcc. The current flowing in the resistor R1 is controlled by the S bipolar transistor Q2. The base electrode of an NPN type S bipolar transistor Q3 is connected to the collector electrode of the S bipolar transistor Q2. The emitter electrode of an NPN bipolar transistor Q4, which is in a Darlington-connection with the S bipolar transistor Q3, is connected to an output terminal OUT.
The bipolar transistor Q4 and an NPN type S bipolar transistor Q5, the base electrode of which is connected to the emitter electrode of the bipolar transistor Q2, are connected in totem-pole configuration between the voltage source terminal Vcc and the ground terminal GND. The collector electrode of the S bipolar transistor Q5 is connected to the output terminal OUT. The conductivity of the bipolar transistor Q4 and the S bipolar transistor Q5 is controlled by the S bipolar transistor Q2. The base current of the S bipolar transistor Q3 is regulated by the resistor R1. The base electrode of the bipolar transistor Q4 and the output terminal OUT are respectively connected to the collector electrode of the S bipolar transistor Q2 via corresponding Schottky barrier diodes D3 and D4.
In this circuit, when the S bipolar transistor Q2 changes from the conducting state to the non-conducting state, the base potential of the S bipolar transistor Q3 rises according to a time constant determined by the resistor R1 and the parasitic capacitance which exists in the S bipolar transistor Q2 and diodes D.sub.3 and D.sub.4.
Here, when the resistance value of the resistor R1 is small, the base potential of the S bipolar transistor Q3 rises, as shown by a in FIG. 2. When the base potential exceeds Vbe (base to emitter voltage), the S bipolar transistor Q3 and the bipolar transistor Q4 changes into the conductive state and an output signal with a steep rise is outputted from the output terminal OUT, as shown by b in FIG. 2.
On the other hand, when the resistance value of the resistor R1 is large, since the current flowing in the resistor R1 is less than that of when the resistance value is small, the rise of the base potential of the S bipolar transistor Q3 is more gentle than in the case of a, as shown by c in FIG. 2. For this reason, the rise of the output signal becomes more gentle than in the case of b as shown by d in FIG. 2, and thus the rise of the output signal is delayed.
As explained above, the rising rate of the base potential of the S bipolar transistor Q3 depends on the resistance value of the resistor R1. Thus the rising of the output signal also depends on the resistance value of the resistor R1.
Therefore, in order to speed up the rising rate of the output signal, it is desirable to make the resistance value of the resistor R1 small. However, if the resistance value of the resistor R1 is made smaller, the current flowing in the resistor R1 increases. Thus, a problem of increasing the power consumption of the entire circuit occurs.
On the other hand, if the resistance value of the resistor R1 is made larger, the power consumption of the entire circuit becomes smaller than when the resistance value of the resistor R1 is small. However, on the contrary, the rising rate of the output signal is slower, and thus the speed of operation becomes slower.