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DIGITAL ELECTRONICS

UNIT V

LESSON 22:

INTRODUCTION TO LOGIC FAMILIES

In this lecture you will study the basic classification of logic families and the characteristics of digital IC’s.

On completion of this section you should be able to explain the following:

 Types of Bipolar logic families. Types of saturated logic families. Types of nonsaturated logic families. Types of Unipolar logic families. Characteristics of digital IC’s such as speed of operation, power dissipation etc,.

MOS devices are unipolar devices and only MOSFETs are employed in MOS logic circuits. The MOS logic families are:

1. PMOS,

2. NMOS, and

3. CMOS

While in PMOS only p-channel MOSFETs are used and in NMOS only n-channel MOSFETs are used, in complementary MOS (CMOS), both p-channel and n-channel MOSFETs are employed and are fabricated on the same silicon chip.

We will discuss all the above logic families in the forth-coming lectures.

22.2. Characterstics of Digital ICS

With the widespread use of ICs in digital systems and with the development of various technologies for the fabrication of ICs, it has become necessary to be familiar with the characteristics of 1C logic families and their relative advantages and disadvan- tages. Digital ICs are classified either according to the complexity of the circuit, as the relative number of individual basic gates (2- input NAND gates) it would require to build the circuit to accomplish the same logic function or the number of compo- nents fabricated on the chip.

The classification of digital ICs is given in Table 22.1.

Classification of digital ICs

22.1. Introduction

Basically, there are two types of semiconductor devices: bipolar and unipolar. Based on these devices, digital integrated circuits have been made which are commercially available. Various digital functions are being fabricated in a variety of forms using bipolar and unipolar technologies. A group of compatible ICs with the same logic levels and supply voltages for performing various logic functions have been fabricated using a specific circuit configuration, which is referred to as a logic family.

22.1.1 Bipolar Logic Families

The main elements of a bipolar 1C are resistors, diodes (which are also capacitors) and transistors. Basically, there are two types of operations in bipolar ICs:

1. Saturated, and

2. Non-saturated.

In saturated logic, the transis- tors in the 1C are driven to saturation, whereas in the case of non-saturated logic, the transistors are not driven into saturation. The saturated bipolar logic families are:

1. Resistor-transistor logic (RTL)

2. Direct-coupled transistor logic (DCTL)

3. Integrated-injection logic IC Classification Equivalent individual Number of Basic gates components Small-scale integration (SSI) Less than12 Up to 99 Medium-scale integration (MSI) 12-99 100-999 Large-scale integration (LSI) 100-999 1,000-9,999 Very large-scale integration Above 1,000 Above 10,000 (VLSI)

(I 2 L)

4. Diode-transistor logic (DTL)

5. High-threshold logic (HTL)

6. Transistor-transistor logic (TTL).

The non-saturated bipolar logic families are:

1. Schottky TTL, and

2. Emitter-coupled logic (ECL).

22.1.2 Unipolar Logic Families

Table: 22.1

The various characteristics of digital ICs used to compare their performances are:

1. Speed of operation,

2. Power dissipation,

3. Figure of merit,

4. Fan-out,

5. Current and voltage parameters,

DIGITAL ELECTRONICS

6. Noise immunity,

7. Operating temperature range,

8. Power supply requirements, and

9. Flexibilities available.

22.2.1 Speed of Operation

The speed of a digital circuit is specified in terms of the propagation delay time. The input and output waveforms of a logic gate are shown in the figure 22.1. The delay times are measured between the 50 per cent voltage levels of input and output waveforms. There are two delay times: t pHL , when the output goes from the HIGH state to the LOW state and t pLH , corresponding to the output making a transition from the LOW state to the HIGH state. The propagation delay time of the logic gate is taken as the average of these two delay times. Fig: 22.1 22.2.2 Power Dissipation

This is the amount of power dissipated in an 1C. It is deter-

mined by the current, I CC , that it draws from the V cc supply, and

is given by V CC x I CC . I CC is the average value of Icc (0) and Icc (1). This power is specified in mill watts.

22.2.3 Figure of Merit

The figure of merit of a digital 1C is defined as the product of speed and power. The speed is specified in terms of propaga- tion delay time expressed in nanoseconds.

Figure of merit = propagation delay time (ns) x power (mW)

It is specified in pico joules (ns x mW = pJ)

A low value of speed-power product is desirable. In a digital

circuit, if it is desired to have high speed, i.e. low propagation delay, then there is a corresponding increase in the power dissipation and vice-versa.

22.2.4 Fan-Out

This is the number of similar gates, which can be driven by a gate. High fan-out is advantageous because it reduces the need for additional drivers to drive more gates.

22.2.5 Current and Voltage Parameters

The following currents and voltages are specified which are very useful in the design of digital systems.

· High-level input voltage, V IH : This is the minimum input voltage which is recognized by the gate as logic 1.

· Low-level input voltage, V IL : This is the maximum input voltage, which is recognized by the gate as logic 0.

· High-level output voltage, V OH : This is the minimum voltage available at the output corresponding to logic 1.

· Low-level output voltage, V OL : This is the maximum voltage available at the output corresponding to logic 0.

· High-level input current, I IH : This is the minimum current, which must be supplied by a driving source corresponding to 1 level voltage.

· Low-level input current, I IL : This is the minimum current which must be supplied by a driving source corresponding to 0 level voltage.

· High-level output current, I OH : This is the maximum current, which the gate can sink in 1 level.

· Low-level output current, I OL : This is the maximum current which the gate can sink in 0 level.

· High-level supply current, I CC (1): This is the supply current when the output of the gate is at logic 1.

· Low-level supply current, I CC (0): This is the supply current when the output of the gate is at logic (0). The current directions are illustrated in figure 22.2. Fig: 22.2

22.2.6 Noise Immunity The input and output voltage levels defined above are shown in the figure 22.3. Stray electric and magnetic fields may induce unwanted voltages, known as noise, on the connecting wires between logic circuits. This may cause the voltage at the input to a logic circuit to drop below V IH or rise above V IL and may produce undesired operation. The circuit’s ability to tolerate noise signals is referred to as the noise immunity, a quantitative measure of which is called noise margin. Noise margins are illustrated in the figure 22.3.

The noise margins defined above are referred to as dc noise margins. Strictly speaking, the noise is generally thought of as an a.c. signal with amplitude and pulse width. For high speed ICs, a pulse width of a few microseconds is extremely long in comparison to the propagation delay time of the circuit and therefore, may be treated as d.c. as far as the response of the logic circuit is concerned. As the noise pulse width decreases and approaches the propagation delay time of the circuit, the pulse duration is too short for the circuit to respond. Under this condition, a large pulse amplitude would be required to produce a change in the circuit output. This means that a logic circuit can effectively tolerate large noise amplitude if the noise is of a very short duration. This is referred to as ac noise margin and is substantially greater than the dc noise margin. It is generally supplied by the manufacturers in the form of a curve between noise margin and noise pulse width.

DIGITAL ELECTRONICS Fig: 22.3

22.2.7 Operating Temperature The temperature range in which an 1C functions properly must be known. The accepted temperature ranges are: 0 to + 70 °C for consumer and industrial applications and -55 °C to + 125 °C for military purposes.

22.2.8 Power Supply Requirements

The supply voltage(s) and the amount of power required by an 1C are important characteristics required to choose the proper power supply.

22.2.9 Flexibilities Available

Various flexibilities are available in different 1C logic families and these must be considered while selecting a logic family for a particular job.

Some of the flexibilities available are:

 1 The breadth of the series: Type of different logic functions available in the series. 2 Popularity of the series: The cost of manufacturing depends upon the number of ICs manufactured. When a large number of ICs of one type are manufactured, the cost per function will be very small and it will be easily available because of multiple sources. 3 Wired-logic capability: The outputs can be connected together to perform additional logic without any extra hardware. 4 Availability of complement outputs: This eliminates the need for additional inverters. 5 Type of output: Passive pull-up, active pull-up, open- collector/drain, and tristate. These will be explained in subsequent sections.

In our next lecture we study RTL, DCTL and I 2 L circuits.

References

Modern Digital Electronics – R P Jain.

Notes
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