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 applications of microprogramming

Q. Show the Decimal equivalent of a binary number? In binary numbers we have two digits 0 and 1 in addition they can also be signified as a string of these two-digits known as

The fields specified by select-options and parameters statement cannot be grouped together in the selection screen. No, the fields specified by select-options and parameters s

Q. How to Access the Disk Data? Disks operate is semi-random mode of operation and generally referenced block wise. The data access time on a disk comprises two major component

Q. What is Block size of cache memory? Block Size: Block size means unit of data (few memory words) exchanged between main and cache memory. As block size increases from very

What is the function of in network access layer in TCP/IP protocol stack? Function of Network Access Layer: The network access layer is the lowest layer within Internet

The Communication Traffic gives a pictorial view of the communication traffic in the interconnection network with respect to the time in progress. The Communication Traffic shows t

What is the use of cache memory? The use of the cache memories solves the memory access problem. In certain, when a cache is included on the same chip as the processor, access

The larger the Grade of Service, the worse is the service given, it is true or false. Ans: It is true that larger the Grade of Service, the worse service is provided.

What is a deadlock? A deadlock is a situation that can increase when two units, A and B use a shared resource. Assume that unit B cannot complete its task unless unit A complet