Logic Families Interview Question Answer - 2

Important Questions of Logic Families

It includes advantages and disadvantages of totem pole, current sinking, current sourcing transistor, state of TTL, unit load, output of TTL, passive pull up, open collector TTL logic, AND operation of TTL, disadvantage of wired AND operation of TTL, Tri state TTL, tri – state configuration of the TTL and meaning of buffer and driver.

Describe the advantages and disadvantages of totem – pole output of the TTL logic IC.   

Advantages

• Low power dissipation
• Fast rise time at the output of the TTL

Disadvantages

• Current spikes due to totem – pole connection
• The outputs of the totem – pole cannot be wire ANDed.

Explain the term: Current sinking and Current sourcing transistor  

Current sinking transistor

• It is a transistor which pulls down the output voltage to a logic LOW level.

Current sourcing transistor

• It is a transistor which pulls down the output voltage to a logic HIGH level.

At which state the TTL acts as current sinking and current source?  

• The TTL acts as current sinking in LOW state whereas current source in HIGH state.

Explain the term: Unit load      

Unit load

• The current drawn or sourced back by similar gates is known as unit load.

Which type of output does the TTL consists of?

• The output of the TTL may consist of either open collector or totem – pole.

Explain the term: Passive pull up      

Passive pull up

• The output of the open collector TTL is pulled to logic high level through external resistor. 
• This is called as passive pull up.

Why the totem – pole TTL is faster than the open collector TTL logic?

                OR

Why the open collector TTL logic is not suitable for fast switching speed application?     

• The output of the open collector TTL is slower than the totem pole TTL because the load capacitance of the open collector TTL charges through with large time constant.

What is wired AND operation of the TTL? What is its purpose?

Wired AND operation

• The connection of the output wires together is called as AND operation of the TTL.
• The advantages of wired AND operation is that it requires fewer gates as compared to standard TTL arrangement.

Why the wired AND operation cannot be done in the totem – pole TTL ?  

Wired AND operation

• The wired AND operation cannot be done in the totem – pole TTL because when one output is LOW, the other at HIGH state.
• As both the output are ANDed, large current flow from supply to ground through transistors.

In which type of output, the wired AND operation is possible in the TTL?

• Open collector TTL gates

Describe the disadvantage of wired AND operation of TTL?     

• The wired AND operation of TTL reduce switching speed due to absence of pull up transistors.

Explain the term: Tri state TTL        

Tri state TTL

• The tri state TTL combines the advantage of wire AND operation of open collector TTL and high-speed operation of totem pole TTL.

What are the three output states of the tri – state TTL?    

• The tri state of the TTL output are: HIGH, LOW and HIGH impedance

Describe the advantage of the tri – state configuration of the TTL.

Advantage of tri – state configuration

• The advantage of tri – state configuration of the TTL is that the outputs of the tri state can be connected without sacrifice switching speed.

Explain the term: Buffer / Driver      

Buffer / Driver

• The logic circuit is called as buffer / driver when it has greater output voltage / current capability than that of an ordinary logic circuit.

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