Digital Logic Families Interview Questions and Answers:

1. What is the difference between a bipolar logic family and unipolar logic family?

Ans. Logic family using BJTs is known as bipolar logic family whereas the logic family using unipolar transistors (MOSFETs) is known as unipolar logic family.

2. Differentiate between saturated and non-saturated logic.

Ans. The logic circuits, in which transistors are driven into saturation, are known as saturated logic circuits or simply the saturated logic and the logic circuits avoiding saturation of their transistors are known as non-saturated logic.

3. What is meant by breadth of a logic family?

Ans. The number of various functions available in a logic family is known as the breadth of the logic family.

4. What is tristate logic?

Ans. Instate logic is a logic that has three states of output viz. high impedance state in addition to 0 and 1 states of output.

5. Which is the fastest of all logic families?

Ans. ECL is the fastest of all logic families.

6. Which type of TTL gates can drive CMOS gates?

Ans. TTL with open-collector can drive CMOS.

7. On what factors the switching speed of a BJT and MOSFET depends?

Ans. The switching speed of a BJT depends upon the time required to charge base-emitter capacitance and input capacitances whereas the switching speed of MOSFET depends upon internal capacitance of the device and the internal impedance of the gate drive circuit.

8. What determines the fan-out limitations of MOS logic circuits?

Ans.

1. The input capacitance restricts the number of MOS inputs that one MOS output can drive and thus determines the fan-out limitations of MOS logic circuits.
2. The large fan-out enhances the MOS capacitance at the output of the driving gate which reduces the speed of MOS gates.

9. Define fan-out. Which factor is responsible for the limit of fan out in TTL circuits?

Ans. The fan-out (also sometimes called the loading factor) is defined as the maximum number of standard logic inputs that an output can drive reliably. For example, a logic gate that is specified to have a fan-out of 8 can drive 8 standard logic inputs. If this number exceeds, the output logic-level voltages cannot be guaranteed.

The fan out in TTL circuits is limited by the amount of current that the output transistor can sink in low state or can source in high state.

10. What do you understand by fan-out of a gate?

Ans. The fan-out (also sometimes called the loading factor) is defined as the maximum number of standard logic inputs that an output can drive reliably. For example, a logic gate that is specified to have a fan-out of 8 can drive 8 standard logic inputs. If this number exceeds, the output logic-level voltages cannot be guaranteed.

11. What is the disadvantage of using back-to-back diodes in place of multi-emitter transistor in TTL totem-pole output circuit?

Ans. During turn-off multi-emitter transistor reduces storage time considerably. Thus, the drawback of using back-to-back diodes in place of multi-emitter transistor is the increase in storage time.

12. What is the function of protecting diodes connected to multi-emitter transistor in a TTL totem-pole circuit?

Ans. The function of protecting diodes in multi-emitter transistor is to protect the circuit from the negative spikes appearing at the input terminal.

13. Why the totem-pole output cannot be wire-ANDed?

Ans. The totem-pole output cannot be wire-ANDed, because, in case, if one of the output is HIGH and another is LOW, then a large current flows from supply to ground through Q₃ of the HIGH state and Q₄ of the LOW state. This large current can damage any of the transistors.

14. Why TTL totem-pole circuit is faster than open-collector logic circuit?

Ans. TTL totem-pole circuit is faster than open-collector logic circuit because the totem-pole logic circuit makes use of active pull-up circuit while the open-collector circuit uses passive pull-up resistor, and therefore, propagation delay in totem-pole logic circuit is reduced.

15. What determines the fan-out limitations of the TTL totem-pole circuit?

Ans. The fan-out in TTL totem-pole circuit is limited by the amount of current that the output transistor can sink in LOW state or can source in HIGH state.

16. “Loading an output with more than its rated fan-out has several effects” — Write at least five effects.

Ans. Loading an output with more than its rated fan-out affect the (i) rise and fall times of the gates (ii) propagation delay time of each gate (iii) power dissipation in each gate (iv) speed of operation (v) working of transistors.

Besides above the transistors may get damaged, the device may cease to operate properly and is said to be overloaded and the output logic-level voltages cannot be guaranteed.