What is a Digital Integrated Circuit?
In an integrated circuit, components such as diodes, transistors, resistors, inductors etc. are integral parts of the chip. And on surface of this chip, which is very small piece of semiconductor material, miniature circuits are produced by using advanced photographic technique. The finished network is so small that microscope is required to see the connections. Digital integrated circuits are those circuits, which perform logic functions with the help of binary numbers 0 and 1; such as logic gates, flip-flops, counters, shift registers etc. Digital Integrated Circuit are most popular in realization of electronic systems in the areas of instrumentation, communication, controls and computers.
In view of the low cost and excellent performance of digital ICs in silicon monolithic form, ICs based on monolithic technology are used nowadays.
Digital ICs, on the basis of technology involved in their manufacturing, can be classified into two categories namely bipolar and MOS (metal oxide semiconductor) families. The choice depends on the use of active device in the circuit realization. NPN transistor is the most important device in bipolar digital ICs and P- and N-channel MOSFETs with complementary symmetry circuitry are popular in MOS digital ICs. Thus the logic polarity of bipolar digital ICs is always positive whereas the logic polarity of MOS digital ICs is either positive or negative depending upon its configuration.
The NAND or NOR logic gates are the universal building blocks in digital systems. So it is useful to compare the characteristics of NAND or NOR gates employing different circuit configurations/ active devices to characterize the different Digital Integrated Circuit families.
Bipolar transistors are fabricated on the chip in digital ICs. Bipolar technology is preferred for SSI (small scale integration) and MSI (medium scale integration) because it is faster.
Various bipolar families are given below:
- Resistance Transistor Logic (RTL). It was the first family group of logic circuits to be developed and packaged in IC form in early 1960s. This technology has become obsolete and is of historic interest only now.
- Diode Transistor Logic (DTL). It was the next family to be introduced after RTL. It uses resistors, diodes and transistors. This technology has also become obsolete now.
- Transistor-Transistor Logic (TTL). It uses almost transistors only. This technology is most popular one and is commonly used in SSI and MSI chips. This technology was introduced in early 1970s.
- Emitter-Coupled Logic (ECL). It is the fastest technology in logic family and is used in high speed applications.
- Integrated Injection Logic (IIL) Circuits. Integrated injection logic circuits represent an attempt to attain packing densities comparable to MOS circuits while using BJT technology. Standard bipolar technology is the fastest, however, it occupies larger space when compared to MOS circuits.
MOS (Metal Oxide Semiconductor) Families:
In these ICs, MOSFETs (metal oxide semiconductor field-effect transistors) are fabricated on the chip. It is used in LSI (large scale integration) field because more MOSFETs can be packed into the same chip area. Various MOS families are given below:
- P-channel MOSFETs (PMOS). It is the oldest and slowest type of technology and has become obsolete now.
- N-channel MOSFETs (NMOS). It is widely used in LSI field such as microprocessors and memories.
- Complementary MOSFETs (CMOS). It is a push-pull arrangement of P- and N-channel MOSFETs and is extensively used where low power consumption is required.
Saturated and Non-Saturated Logic:
The logic circuits, in which transistors are driven into saturation, are known as saturated logic circuits or simply saturated logic and the logic circuits avoiding saturation of their transistors are known as non-saturated logic. TTL is the example of a saturated logic where ECL represents a non-saturated logic. The drawback of saturated logic is the time delay that occurs in bringing the transistor out of saturation. Obviously, saturated logic circuits have low switching speeds as compared to those of non-saturated logic circuits.
Characteristics of Digital ICs:
Various logic families possess different characteristics, so one family may be best suited to one situation whereas another family in some other situation. For example, in certain cases, low power consumption may be the prime requirement whereas in some other case speed.
Various characteristics of a logic family are given below in brief:
1. Operating Speed. Speed of a logic gate depends upon the time that elapses between the application of a signal to an input terminal and the resulting change in logical state at the output terminal. It takes into consideration the transition time (rise and fall duration of a pulse) and propagation delays. Both of these times depend upon the loading and increase with the increase in load. The more inputs are attached to the output of a logic gate, the more load is to be handled by that output. High operating speed is usually the main requirement of Digital Integrated Circuits.
2. Fan-In. The fan-in of a logic gate is defined as the number of inputs (coming from similar circuits) that it can handle properly.
3. Fan-Out. In general, a logic circuit is required to drive several logic inputs. The fan-out (also sometimes called the loading factor) is defined as the maximum number of standard logic inputs that an output can drive reliably. For example, a logic gate that is specified to have a fan-out of 8 can drive 8 standard logic inputs. If this number exceeds, the output logic-level voltages cannot be guaranteed.
4. Power Dissipation. This is the amount of power dissipated in an IC. It is determined by the current, ICC, that it draws from the VCC supply and equals VCC ICC where ICC is average value of ICC (0) and ICC (1). This power is specified in mW. Lower power dissipation is desirable feature for any IC.
5. Power Supply Requirements. Every IC requires a certain amount of electrical power to operate. The power is supplied by one or more power supply voltage connected to the power pin (or pins) on the chip. Usually there is only one power supply terminal on the chip and it is marked VCC for TTL or VDD for MOS devices. Obviously low power consumption is desirable feature in any Digital Integrated Circuit.
6. Noise Immunity. Stray electric and magnetic fields can induce voltages on the connecting wires between logic circuits. These unwanted, spurious signals are known as noise and can sometimes lead to false triggering of logic levels in the circuit. The noise immunity of a logic circuit refers to the circuit’s ability to tolerate noise voltages on its inputs. A quantitative measure of noise immunity is called the noise margin. Higher the noise margin, better the logic circuit.
7. Operating Temperature Range. Digital ICs should be capable of operating for temperature ranging from 0 to 70°C for consumers and from – 55°C to + 125°C for military applications.
8. Figure of Merit. The figure of merit of a digital IC is defined as the product of speed and power. The speed is specified in terms of propagation delay time expressed in nano seconds.
Figure of merit = Propagation time (ns) x power (mW) [measured in pico joules (pJ)]
A low value of speed-power product is desirable. In a Digital Integrated Circuit if high speed or low propagation delay is desired, then there will be a corresponding increase in power dissipation and vice versa.
9. Flexibilities Available. Various flexibilities are available in different IC logic families and these must be considered while selecting a logic family for a particular job. Some of the flexibilities available are :
(i) Wire-logic Capability. Connection of gate output terminals together or using them directly to perform additional logic functions without any extra hardware.
(ii) Availability of Complement Outputs. This eliminates the need for additional inverters.
(iii) Breadth of Series. Types of different logic functions available in the series.
(iv) Popularity of Series. The cost of manufacturing depends upon the number of ICs manufactured. When the ICs are manufactured in a large number the cost per function is reduced and it will he easily available because of multiple resources.
(v) Input-Output Facilities. The number of input terminals of a gate and its input/output impedances in both 0 and 1 states are important. Former governs the fan-in and the later its fan-out. For high fan-out, the gates have low output impedance for both 0 and 1.