ICT (Computer Studies): Logic Gate 3

The simplest form of electronic logic is diode logic (DL).

This allows AND and OR gates to be build, but not inverters, and so is an incomplete form of logic. To build a complete logic system, valves or transistors can be used. The simplest family of logic gates using bipolar transistors is called resistor-transistor logic, or RTL. Unlike diode logic gates, RTL gates can be cascaded indefinitely to produce more complex logic functions. These gates were used in early integrated circuits.

For higher speed, the resistors used in RTL were replaced by diodes, leading to diode-transistor logic, or DTL. It was then discovered that one transistor could do the job of two diodes in the space of one diode, so transistor-transistor logic, or TTL, was created.   In some types of chip, to reduce size and power consumption still further, the bipolar transistors were replaced with complementary field-effect transistors (MOSFETs), resulting in complementary metal-oxide-semiconductor (CMOS) logic. The switching action of a transistor makes it especially suitable for use in digital logic circuits where the output is either 0 or 1 depending on the input.

Applications of such circuits are to: ·       

• Switch on a water pump on a hot sunny day ·         
• Sound an alarm if the pilot light of a boiler went off ·         
• Sound an alarm if a burglar stepped on a pressure pad or shone his torch ·         
• Switch a light on if it was a cloudy day ·         
• Add two simple binary numbers together ·         
• Switch on a fan if a darkroom door was shut and it was warm inside. 

All these things and indeed many more can be done with ELECTRONIC LOGIC CIRCUITS. These circuits are ones that can make decisions.

Different decisions need different circuits. Because of its wide use in modern digital electronics the NAND gate will be considered as a basic building block for a variety of logic circuits. In fact a number of other logic gates can be constructed from NAND gates as is shown below.

Think about just two of the examples: The hot sunny day To switch on the pump it must be hot AND sunny. This needs an AND circuit. The burglar alarm To sound the alarm the burglar has to step on the pressure pad OR switch on his torch. This needs an OR circuit.

Exercises:

1.      What is the simplest form of electronic logic?

2.      What can be used to build a complete logic system?

3.      What can the simplest family of logic gates using bipolar transistors also be known as?

4.      What are the differences between diode logic gate and resistor-transistor logic?

5.      What led to the creation of transistor-transistor logic, or TTL?

6.      What is the advantage of the switching action of a transistor?

7.      What are five uses of transistor based digital logic circuits?

8.      What are electronic logic circuits?