Advantages of CMOS logic family over TTL

Negligible power dissipation

The main advantage of CMOS logic family is their extremely low power consumption. This is because there is no direct conducting path from Vdd to ground in either of input conditions. So there is practically zero power dissipation in STATIC conditioms.There is only dynamic power dissipation, i.e. power is dissipated only during the switching of MOSFETs.

 

High fan out

For TTL logic family the maximum fan out is found to be around 10.Whereas for CMOS logic family fan out rating rating may exceed 50. Hence a larger no of gates can be driven by the output of a single gate.

 

Compatibility with TTL

CMOS logic family is compatible with TTL for 5V supply.

 

Temperature stability

CMOS logic family is found to have wider temperature range in which they can operate satisfactorily. The operating temperature range for CMOS is -55 to 125°C   whereas for TTL it is around 25 to 70 °C.

 

Other advantages

1. Nominal supply voltage for CMOS is from 3V to 15V whereas for TTL it’s upto 5V.
2. Noise immunity is excellent.
3. Simple circuitary as MOSFETs are voltage controlled devices. Hence there is no need of biasing resistors to adjust current to desired level.

 

On the similar lines as that for TTL logic family CMOS also provides many features. These features include-
– CMOS with open DRAIN output as there is open collector output in TTL logic family.
– This is because outputs of conventional CMOS gates can’t be shorted together. Thus an external pull up resistor is connected between Vdd and output of CMOS gate (DRAIN).
– Also there is CMOS logic family with tri state inputs as well the third state being High Impedance state.

 

Disadvantages of CMOS logic family-

Slow operating speed – The main disadvantage of CMOS logic family is their slow speed of operation. Propagation delay time for CMOS family is found to around 50ns whereas it is around 4 to 12 ns for TTL logic family.

 

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