Digital Logic Gates

Digital Logic Gates

Standard commercially available Digital Logic Gates are available in two basic forms, TTL which stands for Transistor-Transistor Logic such as the 7400 series, and CMOS which stands for Complementary Metal-Oxide-Silicon which is the 4000 series of chips. Generally speaking, this refers to the logic technology used to manufacture the Integrated Circuit, (IC) or "chip" as it is commonly called. Generally speaking, TTL IC's use NPN type Bipolar Junction Transistors while CMOS IC's use Field Effect Transistors or FET's for both their input and output circuitry. As well as TTL and CMOS technology, simple digital logic gates can also be made by connecting together diodes and resistors to produce RTL, Resistor-Transistor Logic circuits but these are now less common.

Integrated Circuits or IC's as they are more commonly called, can be grouped together into families according to the number of transistors or "gates" that they contain. For example, a simple AND gate my contain only a few individual transistors, where as a more complex microprocessor may contain many thousands of individual transistor gates. A general classification for the number of gates within a single chip is given as:

Classification of Integrated Circuits

• Small Scale Integration or SSI - up to 10 transistors or a few gates
•  
• Medium Scale Integration or MSI - between 10 and 100 transistors or tens of gates
•  
• Large Scale Integration or LSI - between 100 and 1,000 transistors or hundreds of gates
•  
• Very-Large Scale Integration or VLSI - between 1,000 and 10,000 transistors or thousands of gates
•  
• Super-Large Scale Integration or SLSI - between 10,000 and 100,000 transistors
•  
• Ultra-Large Scale Integration or ULSI - more than 1 million transistors

While the ultra large scale ULSI classification is less well used, another level of integration which represents the complexity of the Integrated Circuit is known as the System-on-Chip or (SOC). Here individual components such as the microcontroller, memory, peripherals, I/O logic etc, are all produced on a single piece of silicon and which represents a whole electronic system within one single chip. These chips are generally used in Mobile Phones, Digital Cameras, Microcontrollers and robotic applications etc, and can contain up to 100 million individual Silicon-CMOS transistor gates within a single chip.

Moore's Law

In 1965, Gordon Moore co-founder of the Intel corporation predicted that "The number of transistors and resistors on a single chip will double every 18 months" regarding the development of semiconductor gate technology. When he made his famous comment there where approximately only 60 individual transistors on a single silicon chip. Today, Intel have placed around 1.7 Billion individual transistor gates on its new Dual-Core Itanium Processor chip.

Digital Logic States

All digital electronic circuits and microprocessor based systems contain hardware elements called Digital Logic Gates that perform the logical operations of AND, OR and NOT on binary numbers. In digital logic only two voltage levels or states are allowed and these states are generally referred to as Logic "1" or Logic "0", High or Low, True or False and which are represented in Boolean Algebra and Truth Tables by the numbers "1" and "0" respectively. A good example of a digital logic level is a simple light as it is "ON" or "OFF".

Most logic systems use "Positive logic", in which a logic "0" or "LOW" is represented by a zero voltage, 0v or ground and a logic "1" or "HIGH" is represented by a higher voltage such as +5 volts, with the switching from one voltage level to the other, from either a "0" to "1" or "1" to "0" being made as quickly as possible to prevent any faulty operation of the logic circuit. There is also a complementary "Negative Logic" system in which the values and the rules of a logic "0" and a logic "1" are reversed but in this tutorial we shall only refer to the Positive Logic convention as it is the most common.

In standard TTL (transistor-transistor logic) IC's there is a pre-defined voltage range for the input and output voltage levels which define exactly what is a logic "1" level and what is a logic "0" level and these are shown below.

TTL Input & Output Voltage Levels

There are a large variety of logic gate types in both the Bipolar and CMOS families of digital logic gates such as 74L, 74LS, 74ALS, 74HC, 74HCT, 74ACT etc, with each one having its own distinct advantages and disadvantages and the exact voltages required to produce a logic "0" or logic "1" depends upon the specific logic group or family. However, when using a standard +5 volt supply any TTL voltage input between 2.0v and 5v is considered to be a logic "1" or "HIGH" while any voltage input below 0.8v is recognised as a logic "0" or "LOW". The voltage region between these two voltage levels either as an input or as an output is called the Indeterminate Region. CMOS logic uses a different level of voltages with a logic "1" level operating at between 3 and 15 volts.

Then from the above observations, we can define the ideal Digital Logic Gate as one that has a "LOW" level logic "0" of 0 volts (ground) and a "HIGH" level logic "1" of +5 volts and this can be demonstrated as:

Ideal Digital Logic Voltage Levels

Where the opening or closing of the switch produces either a logic level "1" or a logic level "0".

Simple Basic Digital Logic Gates

Simple digital logic gates can be made by combining transistors, diodes and resistors with a simple example of a Diode-Resistor Logic (DRL) AND gate and a Diode-Transistor Logic (DTL) NAND gate given below.

Diode-Resistor circuit	Diode-Transistor circuit
2-input AND gate	2-input NAND gate

The simple 2-input Diode-Resistor AND gate can be converted into a NAND gate by the addition of a single transistor inverting (NOT) stage. Using discrete components such as Diodes, Resistors and Transistors to make digital logic gate circuits are not used in practical commercially available logic IC's as these circuits suffer from propagation delay or gate delay due to the pull-up resistors, there is no "Fan-out" which is the ability of a single output to drive many inputs, and also they do not turn fully "OFF" as a Logic "0" produces an output voltage of 0.6v (diode voltage drop), so the following TTL and CMOS circuits are used instead.

Basic TTL Logic Gates

The simple Diode-Resistor AND gate above uses separate diodes for its inputs, one for each input. As a transistor is made up off two diode circuits connected together, these input diodes can be replaced by one single NPN transistor with multiple emitter inputs as shown below.

2-input NAND gate

As the gate contains a single stage inverting NPN transistor circuit (TR2) an output Logic level "1" at Q is only present when Both the emitters of TR1 are connected to Logic level "0" or ground allowing base current to pass through the emitter and not the collector, thus producing a NAND gate function. In standard TTL logic gates, the transistors operate either completely in the "cut off" region, or else completely in the saturated region, Transistor as a Switch type operation.

Emitter-Coupled Logic Gates

Emitter Coupled Logic or ECL is another type of digital logic gate that uses bipolar transistor logic where the transistors are not operated in the saturation region, as they are with the standard TTL digital logic gate. Instead the input and output circuits are push-pull connected transistors with the supply voltage negative with respect to ground. This has the effect of increasing the speed of operation of the ECL gates up to the Gigahertz range compared with the standard TTL types, but noise has a greater effect in ECL, because unsaturated transistors operate within their active region and amplify as well as switch signals.

With improvements in the circuit design to take account of propagation delays, current consumption, fan-in and fan-out requirements etc, this type of TTL bipolar transistor technology forms the basis of the prefixed "74" family of digital logic IC's, such as the "7400" Quad 2-input AND gate, or the "7402" Quad 2-input OR gate. Sub-families of the 74xx series ICC's are available relating to the different technologies used to fabricate the gates and are denoted by the letters between the 74 and the device number e.g. 74L00 or 74ALS00 AND gate, were the "L" stands for "Low-power TTL" and the "ALS" stands for "Advanced Low-power Schottky TTL".

Basic CMOS Logic Gates

One of the main disadvantages of the TTL logic series is that the gates are based on bipolar transistor logic technology and as transistors are current operated devices, they consume large amounts of power from a fixed +5 volt power supply. Also, TTL bipolar transistor gates have a limited operating speed when switching from an "OFF" state to an "ON" state and vice-versa called the "gate" or "propagation delay". To overcome these limitations CMOS logic gates using "Field Effect Transistors" or FET's were developed. As these gates use P and N-channel MOSFET's as the input device, at quiescent conditions their power consumption is almost zero, (1 to 2uA) with speed of operation upwards of 100MHz.

2-input NAND gate

This CMOS gate example contains 3 N-channel MOSFETS, one for each input FET1 and FET2 and one for the output FET3. When both the inputs A and B are at logic level "0", FET1 and FET2 are both switched "OFF" giving an output logic "1" from the source of FET3. When one or both of the inputs are at logic level "1" current flows through the corresponding FET giving an output state at Q equivalent to logic "0", thus producing a NAND gate function.

As with the TTL gates, improvements in the circuit design with regards to speed and low power consumption has resulted in the standard 4000 CMOS "CD" family of IC's being developed. As with the standard TTL digital logic gates, all the major digital logic gates are available in the CMOS package. E.g. CD4011, a Quad 2-input NAND gate, or the CD4001, a Quad 2-input NOR gate along with all their sub-families. However, one main disadvantage with the CMOS range of IC's compared to their equivalent TTL types is that they are easily damaged by static electricity so extra care must be taken when handling these devices.