transistor transistor logic device of nand gate

logic about transistor transistor logic

1. Introduction to TTL TTL stands for Transistor-Transistor Logic. It's a digital logic family that uses bipolar junction transistors (BJTs) to implement logic functions. TTL circuits offer a good balance of speed, power consumption, and noise immunity. TTL NAND gate is a universal building block as any other logic gate can be constructed from NAND gates

2. Truth table of TTL NAND gate Symbol and Truth Table of TTL NAND Gate Symbol: Triangle with two input lines and a single output line. Truth Table: A B Y 1 1 1 1 1 1 1

3. Internal Circuitry of TTL NAND Gate Internal Circuitry of TTL NAND Gate Two NPN bipolar junction transistors (Q1 & Q2) in a totem-pole configuration. Resistors (R1 & R2) provide current limiting for the base terminals. Resistor (R3) acts as a pull-up resistor for the output. Diode (D1) protects Q1 from base-emitter breakdown .

4. Operation of TTL NAND Gate When both inputs A and B are HIGH (Logic 1): Transistors Q1 is ON (active) and Q2 is OFF. Current flows through Q1, saturating it. The collector voltage of Q1 (output Y) becomes LOW (Logic 0) due to saturation. When at least one input (A or B) is LOW (Logic 0): Transistor Q1 is OFF. Transistor Q2 becomes forward biased and conducts current. The collector voltage of Q2 (output Y) is pulled high (Logic 1) by the pull-up resistor R3.

5. Key Characteristics of TTL NAND Gate Noise Immunity:  TTL gates offer good noise immunity due to the high voltage difference between logic levels (typically around 2V). Fan-out:  It represents the number of standard loads a gate can drive without degrading its output voltage. TTL gates typically have a fan-out of 10. Propagation Delay:  The time it takes for the output to change after a change in the input. TTL gates have propagation delays in the nanosecond range.

6. TTL with totem pole output

7. TTL with Totem pole output working A totem-pole output stage is a circuit configuration that provides a strong, well-defined output voltage level. It employs two bipolar junction transistors (BJTs) connected in series, acting like a totem pole. One transistor is responsible for driving the output HIGH (Logic 1), while the other drives it LOW (Logic 0). Only one transistor can be active at a time, ensuring a clean transition between logic levels.

8. Let's explore how the TTL NAND gate with a totem-pole output functions when the output needs to be LOW (Logic 0). In this scenario, both inputs A and B are HIGH (Logic 1). This forward-biases transistor Q1, allowing current to flow through it (active state). Resistor R1 restricts the base current of Q1 to a safe level. Meanwhile, transistor Q2 becomes reverse-biased and turns off (inactive state). As a result, the collector of Q1 (which is the output Y) is driven LOW (Logic 0) due to the current flow. The pull-up resistor R3 has minimal impact in this scenario because Q1 actively pulls the output low, overriding any weak pull-up effect from R3.

9. TTL with tristate buffer A TTL tristate buffer typically consists of three terminals: input (A), output (Y), and control (EN) or enable input. When the control input is active, the tristate buffer functions as a normal buffer, allowing the input signal to propagate to the output. When the control input is inactive, the tristate buffer enters a high impedance state, effectively disconnecting the output from the input. In the active mode, when the control input (EN) is asserted (typically held high in TTL logic), the tristate buffer behaves like a standard buffer. The input signal (A) is transferred to the output (Y) without any change in logic level. This means that the output follows the input faithfully. The tristate buffer essentially acts as a conduit, allowing the signal to pass through from input to output without any impedance or voltage level change.

10. In the high impedance state, when the control input (EN) is de-asserted (typically held low in TTL logic), the tristate buffer disconnects the output from the input. The internal circuitry of the tristate buffer effectively presents a very high impedance at the output terminal. This high impedance state prevents the tristate buffer from interfering with other devices connected to the same bus. It effectively isolates the output terminal, allowing other devices to drive the bus without any contention.

11. TTL with collector open An "open collector" output configuration is a common feature in TTL logic gates. In an open collector output, the transistor's collector terminal is left unconnected or "open." Instead of driving the output voltage high or low directly, the transistor acts as a switch that either pulls the output voltage low (by conducting) or allows it to float (by not conducting). This configuration allows for easy interfacing with other digital circuits, especially when different logic families or voltage levels are involved. So, a TTL NAND gate with an open collector output configuration would have a circuit arrangement where the output transistor's collector is not connected to any power supply voltage. Instead, it acts as a switch that pulls the output down to ground (logic low) when the NAND gate's output is active (logical 0), and lets the output float (high impedance state) when the output is inactive (logical 1).

12. CMOS CMOS stands for Complementary Metal Oxide Semiconductor. And CMOS based logic gates uses complementary pair of NMOS and PMOS transistors. When MOS transistors are used as logic gate then they are used as a switch. In both NMOS and PMOS transistor, the voltage applied between the gate and source acts as a control voltage. CMOS logic uses both NMOS and PMOS transistors. The PMOS transistors are used as pull-up network and NMOS transistors are used as pull-down network. And because of that, the static power consumption of the CMOS based logic gates and logic circuit is very low compared to the logic gates which is designed using only either NMOS or PMOS transistors. Moreover, CMOS based logic gates has higher noise margin compared to NMOS and PMOS based logic gates.

13. When input is low or logic ‘0’, then V SG  > V T  for this PMOS transistor and that’s why PMOS will be ON. On the other end, for NMOS transistor, V GS  < V T . that means when input is logic ‘0’, PMOS transistor will be ON and NMOS transistor will be OFF. And that’s why the output will be connected to 5V. Moreover, since PMOS passes strong logic ‘1’ , the output will be very close to supply voltage. That means when V in  is logic ‘0’ then output is logic ‘1’.