Difference between TTL and CMOS

Advantages of CMOS over TTL:

1. Operates on a wider supply voltage (2 to 15 volts). Exception is 74HC CMOS limited to (2 to 7 volts).

1. Uses much less power, can operate on a small battery for a year or so.

2. Output swings the full supply voltage range.

1. Input switches at half the supply voltage for better noise immunity.

2. Input impedance is much higher so it doesn't load the driving circuit.

1. 74HC CMOS can supply 25mA of output current, both source and sink.

-Bill

The two families utilize different types of transistors in their construction, Bipolar Junction transistors for TTL and Field Effect Transistors are used for the CMOS. There are various types of both transistors, but one common way to categorize them is wether they are N or P devices, NPN and PNP for bipolar and NMOS and PMOS for the CMOS, where the N and P refers to the element used to dope the silicon. The MOS stands for Metalic Oxide Semiconductor, which is a very thin high valued insulator that is used to create the the very low input current of CMOS devices. The C in CMOS stands for complementary, which means that both N and P transistors have been fabricated together to implement the logic. This allows the rail to rail 0 - 5 volt output swings achievable with CMOS.

A bipolar transistor can be considered as a current driven switch where the FET can be viewed as a voltage controlled switch. This factor comes into play in that it is responsible for the different HIGH and LOW logic voltages and influences how and when TTL and CMOS devices can be connected together.

While Bill's post indicated a number of advantages of CMOS, mostly with regards to digital logic, when it comes to analog performce, such as an amplifier or ADC circuit, bipolar (TTL) has some advantages. Most notibly, bipolar based amplifiers and transistors can often times achieve a higher gain and operate at a much lower noise level.

Hi,

Thanks for the info. thats a lots of stuff to read.. I hope i can understand the above.

Thanks again..

Nick.

If you're going to construct a mixed TTL/CMOS-circuit, this page might be of help: