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The TTL gate circuit is a fundamental building block in digital electronics, with the TTL NAND gate being its basic form. The typical TTL NAND gate circuit structure is illustrated in Figure 8-16. This circuit is composed of three key stages: the input stage, the inverting stage, and the output stage.
The input stage consists of a multi-emitter transistor T1 and a resistor R1. Here, the collector junction of T1 acts like a diode, while the emitter junction resembles two back-to-back diodes. This setup ensures that the input stage behaves similarly to a logical gate.
Moving to the inverting stage, it comprises transistor T2 along with resistors R2 and R3. These components provide complementary signals through the collector and emitter of T2, ensuring proper operation of the output stage.
Finally, the output stage is a "push-pull" configuration made up of transistors T3, T4, diode D, and resistor R4. When T3 is activated, T4 and D deactivate, and vice versa. This mechanism effectively mirrors the logical NOT function, contributing to the overall functionality of the NAND gate.
In operation, if at least one of the inputs A or B is zero (with A set to 0 having a potential of around 0.3V), the rest of the inputs would be high (around 3.6V). At this point, T1 conducts due to the low potential at its emitter junction. Given the forward conduction voltage of the emitter junction is 0.7V, the base potential of T1 would be calculated accordingly. This base potential prevents T2 and T3 from turning on, leaving them in an off state. Consequently, T4 and D turn on, resulting in a high output at terminal Y, demonstrating the logical relationship of "low input results in high output."
Conversely, when both inputs A and B are high (approximately 3.6V), UCC supplies a base current to T2 via the collector junction of R1 and T1, fully saturating T2 and subsequently T3. This leads to a low output at terminal Y, showcasing the logical function of "all high inputs result in low output." At this juncture, the collector potential of T2 would be determined, ensuring T4 and D remain off.
To summarize, the output of terminal Y is 1 when any input in the T1 emitter is 0, and it becomes 0 when all inputs are 1, thus achieving the NAND function. It's crucial to address the issue of floating inputs when using TTL circuits. When the T1 emitter is completely floating, UCC can still provide the base current to T2 through the R1 and T1 collector junctions, turning on T2 and T3 while turning off T4 and D, leading to a Y terminal output of zero. However, when there is a 0 input in the T1 emitter and the rest are floating, the 0 input still dominates, keeping T2 and T3 off and T4 and D on, resulting in a Y output of 1. This demonstrates that TTL circuit inputs left floating are equivalent to a logical 1.
When discussing the main external characteristic parameters of TTL circuits, these parameters help us understand their performance and ensure correct usage. Key parameters include:
1. Output High Level (UOH): When at least one input terminal of the NAND gate is connected to a low level, the output voltage is termed UOH. The product specification specifies UOH ≥ 2.4V.
2. Output Low Level (UOL): When all inputs of the NAND gate are connected to a high level, the output voltage is referred to as UOL. The product specification states UOL ≤ 0.4V.
3. Fan-out Factor (No): The number of input terminals of the next-level gate that can be connected to the output of the gate circuit is termed the fan-out coefficient or load capacity. Typically, No ≥ 8.
4. Average Transmission Delay Time (tpd): This measures the time delay between the input and output pulses of the NAND gate. The average of the rising and falling delay times gives the overall tpd, which is an essential parameter for evaluating the switching speed of the NAND gate.
In addition to NAND gates, TTL circuits offer various other functionalities such as AND gates, OR gates, NOT gates, NOR gates, and XOR gates. Figure 8-17 introduces some commonly used TTL gate chips, highlighting their diverse applications.
This comprehensive understanding of TTL gates not only aids in grasping their inner workings but also in applying them effectively across numerous digital systems.