Explain a TTL NAND gate and its operation, Computer Engineering

Give the circuit of a TTL NAND gate and explain its operation in brief.

Ans:

Operation of TTL NAND Gate: Fig.(d) Demonstrates a TTL NAND gate with a totem pole output.   The    totem pole output implies that transistor T4 sits atop T3 in order to give low output impedance.  The low output impedance means a short time constant RC therefore the output can change rapidly from one state to the other. T1 is a multiple type emitter transistor. Such transistor can be thought of like a combination of various transistors along with a common collector and base. Multiple emitter transistors along with about 60 emitters have been developed. In this figure, T1 has 3 emitters thus there can be three inputs A, B, C. The transistor T2 functions as a phase splitter since the emitter voltage is out of phase along with the collector voltage. The transistors T3 and T4 by the totem pole output, the capacitance CL shows the stray capacitance and so on. The diode D is added to make sure that T4 is cut off while output is low. The voltage drop of diode D remains the base-emitter junction of T4 reverse biased therefore only T3 conducts while output is low. The operation can be described briefly by three conditions as specified below:

Condition 1: At least one input is low (that is, 0).  Transistor T1 saturates. Thus, the base voltage of T2 is almost zero. T2 is cut off and forces T3 to cut off.  T4 functions as an emitter follower and couples a high voltage to load. Output is high (that is Y=1).

Condition 2: Each input is high. The emitter base junctions of T1 are reverse biased. The collector base junction of T1 is forward biased. Therefore, T1 is in reverse active mode. The collector current of T1 flows in reverse direction. Because this current is flowing in the base of T2, the transistors T2 and T3 saturate and then output Y is low.

Condition 3: The circuit is operating under II while one of the inputs becomes low. The consequent emitter base junction of T1 starts conducting and T1 base voltage drops to a low value.  Thus, T1 is in forward active mode. The high collector current of T1 shifts the stored charge in T2 and T3 and hence, T2 and T3 go to cut-off and T1 saturates and then output Y returns to high.

1437_Logic Diagram of TTL NAND Gate with Totem Pole Output.png

Fig.(d) Logic Diagram of TTL NAND Gate with Totem Pole Output

Posted Date: 5/4/2013 12:46:03 AM | Location : United States







Related Discussions:- Explain a TTL NAND gate and its operation, Assignment Help, Ask Question on Explain a TTL NAND gate and its operation, Get Answer, Expert's Help, Explain a TTL NAND gate and its operation Discussions

Write discussion on Explain a TTL NAND gate and its operation
Your posts are moderated
Related Questions
what are the types of isoquants

Write the importance of operating system. Describe the working methodology of online and real-time operating system with the help of two examples of each.

Q. What do you mean by Video Memory ? As declared before video memory is also entitled framebuffer since it buffers video frames to be displayed. The quality of a video display

Explain CGI standard. The CGI Standard: Technology utilized for building dynamic web documents is termed as Common Gateway Interface (CGI). The CGI standard gives how a serv

What are conversion routines? Non-standard conversions from present format to sap internal format and vice-versa are executed with so known as conversion routines.

Conversioin of the decimal number 82.67 into Octal number ? Ans. The binary equivalent is (1010010.10101011) 2 of decimal number 82.67. After that convert each 3-bit binary in

Determine the approaches to organizing stored program control There are 2 approaches to organizing stored program control: 1.  Centralized: In this control, all control equi

You were offered bonus marks for separating the user interface code from the main logic of your program. This design choice makes it very easy to replace the user interface without

Build U Like are a building company specialising in hard landscaping. They lay patios, build walls, lay drives etc. for their customers. A customer will remain in the system even t

Function of an IP Packet Screening Router: A screening router is the most basic type of firewall and uses only the packet filtering capability to control and monitor network tr