What is Transistor Transistor Logic (TTL) Circuit?

Texas instruments introduced transistor transistor logic (TTL), a widely used family of digital devices in 1964. It is fast, inexpensive and easy to use. Transistor Transistor Logic family consists of variety of SSI and MSI chips, with the help of that all kinds of digital circuits and systems can be built up.

The basic Transistor Transistor Logic circuit is the NAND gate. The circuit diagram is shown in Fig. 47.3.

It can be seen that transistor Q₁ has two emitters so it has two emitter-base junctions which can be used to turn transistor Q₁ on. Up to eight emitters can be used in a multiple-emitter input transistor for an eight-input NAND gate. Other special feature of this circuit is totem-pole arrangement made by transistors Q₃ and Q₄ on the output side of the circuit.

Protecting diodes connected in multiple-emitter transistor protect the transistor from damage due to excessive input voltages.

Now let us have a look on the circuit operation. In the circuit diagram resistance R₁ and transistor Q₁ act like a 2-input AND gate. Transistor Q₂ acts like an inverter, which inverts the output of transistor Q₁ and hence the circuit operates like a NAND gate. Transistors Q₃ and Q₄ form a totem-pole connections at the output stage, either Q₃ or Q₄ is on at a time. When Q₃ is on, the output is high; when Q₄ is on, the output is low.

Now consider a case when both inputs A and B, are high. Emitter diodes of Q₁ stop conducting and collector diode gets forward biased. With this base of transistor Q₂ becomes high and it starts conducting. So a potential drop occurs across resistance R₄, and base of transistor Q₄ becomes high. So transistor Q₄ becomes saturated and gives low output.

Consider the other case also, when both inputs or either is low, then base of Q₁ becomes approximately at 0.7 V which reduces the base voltage of Q₂ at almost zero volt, therefore, transistor Q₂ is cut off. Almost no current flows through resistances R₂ and R₄ and, therefore, base voltage of Q₄ becomes zero and Q₄ is cut off. Base voltage of Q₃ becomes high and since Q₃ acts as an emitter follower, it produces high output.

The function of diode D₁ is to prevent even the slight conduction of Q₃, when the output is low. Voltage drop across the diode D₁ keeps the emitter diode of Q₃ reverse biased and so only Q₄ conducts during the low output condition.

Totem-pole transistors are used to get low output imped­ance. During high output state Q₃ conducts and provides output impedance of 70 Ω. During low output state, Q₄ gets saturated and provides output impedance of approximately 12 Ω. So in both states, output impedance is low and it ensures that output voltage changes its value from one state to another quickly be­cause stray capacitance is rapidly charged or discharged through low output impedance.

The fan-out in TTL totem-pole circuit is limited by the amount of current that the output transistor can sink in LOW state or can source in HIGH state.

Function table for the two-input Transistor Transistor Logic NAND gate is given in Table 47.1.