Logic Families – IQ – 1

Logic Families » IQ – 1

1. Give the classification of logic families ?
Classified of logic families is as follows :

2. Classify the logic family by operation ?
The Bipolar logic family is classified into :
(1) Saturated logic
(2) Unsaturated logic
The RTL, DTL, TTL, I2L, HTL logic comes under the saturated logic family.
The Schottky TTL, and ECL logic comes under the unsaturated logic family.

3. State the classifications of FET devices ?
FET is classified as :
(1) Junction Field Effect Transistor (JFET)
(2) Metal oxide semiconductor family (MOS)

4. Mention the classification of saturated bipolar logic families ?
The bipolar logic family is classified as follows :
(1) RTL – Resistor Transistor Logic
(2) DTL – Diode Transistor logic
(3) I2L – Integrated Injection Logic
(4) TTL – Transistor Transistor Logic
(5) ECL – Emitter Coupled Logic

5. Mention the important characteristics of digital IC’s ?
Important characteristics of digital IC’s are as follows :
(1) Fan out
(2) Power dissipation
(3) Propagation Delay
(4) Noise Margin
(5) Fan In
(6) Operating temperature
(7) Power supply requirements

6. Define Fan-out ?
Fan out specifies the number of standard loads that the output of the gate can drive with out impairment of its normal operation.

7. Define power dissipation ?
Power dissipation is measure of power consumed by the gate when fully driven by all its inputs.

8. What is propagation delay ?
Propagation delay is the average transition delay time for the signal to propagate from input to output when the signals change in value. It is expressed in ns.

9. Define noise margin ?
It is the maximum noise voltage added to an input signal of a digital circuit that does not cause an undesirable change in the circuit output. It is expressed in volts.

10. Define fan in ?
Fan in is the number of inputs connected to the gate without any degradation in the voltage level.

11. What is Operating temperature ?
All the gates or semiconductor devices are temperature sensitive in nature. The temperature in which the performance of the IC is effective is called as operating temperature. Operating temperature of the IC vary from 0⁰C to 70⁰c.

12. What is High Threshold Logic ?
Some digital circuits operate in environments, which produce very high noise signals. For operation in such surroundings there is available a type of DTL gate which possesses a high threshold to noise immunity. This type of gateis called HTL logic or High Threshold Logic.

13. When does the noise margin allow digital circuits to function properly ?
When noise voltages are within the limits of VNA(High State Noise Margin) and VNK for a particular logic family.

14. What happens to output when a tristate circuit is selected for high impedance ?
Output is disconnected from rest of the circuits by internal circuitry.

15. What are the types of TTL logic ?
(1) Open collector output
(2) Totem-Pole Output
(3) Tri-state output.

16. List the different versions of TTL ?
(1) TTL (Std.TTL)
(2) LTTL (Low Power TTL)
(3) HTTL (High Speed TTL)
(4) STTL (Schottky TTL)
(5) LSTTL (Low power Schottky TTL)

17. How schottky transistors are formed and state its use ?
A schottky diode is formed by the combination of metal and semiconductor. The presence of schottky diode between the base and the collector prevents the transistor from going into saturation. The resulting transistor is called as schottky transistor. The use of schottky transistor in TTL decreases the propagation delay without a sacrifice of power dissipation.

18. Why totem pole outputs cannot be connected together.
Totem pole outputs cannot be connected together because such a connection might produce excessive current and may result in damage to the devices.

19. State advantages and disadvantages of TTL ?
Advantages :
(1) Easily compatible with other ICs.
(2) Low output impedance.
Disadvantages :
(1) Wired output capability is possible only with tristate and open collector types.
(2) Special circuits in Circuit layout and system design are required.

20. What is depletion mode operation MOS ?
If  the  channel  is  initially  doped  lightly  with  p-type  impurity  a  conducting  channel exists at zero gate voltage and the device is said to operate in depletion mode.

21. What is enhancement mode operation of MOS ?
If the region beneath the gate is left initially uncharged the gate field must induce a channel before current can flow. Thus the gate voltage enhances the channel current and such a device is said to operate in the enhancement mode.

22. Mention the characteristics of MOS transistor ?
(1) The n- channel MOS conducts when its gate- to- source voltage is positive.
(2) The p- channel MOS conducts when its gate- to- source voltage is negative
(3) Either type of device is turned of if its gate-to- source voltage is zero.

23. What is 14000 series ?
It  is  the  oldest  and  standard  CMOS  family.  The  devices  are  not  pin  compatible  or electrically compatible with any TTL Series.

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