Digital Integrated Circuit | What is an ICs?

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What is Integrated Circuit

Digital integrated circuit (ICs) were designed based on the characteristics of various semiconductor devices. Many complex digital functions evolved in different forms, and each form was called a logic family. There are basically two types of semiconductor devices. 1-Bipolar and 2-Unipolar. Digital ICs are fabricated on Bipolar or Unipolar technique. On the same basis, they are called bipolar logic family and unipolar logic family or MOS family. Click here to read this article in Hindi

Bipolar Logic Family

The main components of bipolar ICs are resistor, diode, capacitor and transistor. There are two types of operation in bipolar integrated circuit-

(A) saturated
(B) non-saturated

In saturated logic the transistors built in ICs are driven till saturation while in non-saturated logic the transistors are not driven till saturation. The basic families in the bipolar category are as follows

(A) saturated

Following are the members of Bipolar saturated logic family-

(I) Resistor transistor logic (RTL)
(II) Diode Transistor Logic (DTL)
(III) Direct coupled Transistor Logic (DCTL)
(IV) Integrated Injection Logic (I2L)
(V) Transistor Transistor Logic (TTL)

(B) non-saturated

Following are the members of Bipolar Unsaturated logic family-

(I) Schottky TTL
(II) Emitter Coupled Logic (ECL)

Diodes and transistors are used in DTL family but this design is now obsolete. Transistors are used in TTL family and it is used very widely. ECL is a high speed logic family and is used for high speed applications.

Unipolar Logic or MOS families

MOS are unipolar devices and only MOSFETs are used in MOS logic circuits. MOS family are as follows –

(I) p-channel MOSFETs (PMOS)
(III) n-channel MOSFETs (NMOS)
(III) Complimentary MOSFETs (CMOS)

SSI, MSI, LSI, VLSI

Due to the continuous development of ICs in the digital system and the technical development of ICs fabrication, it is very important to know the characteristics and their properties and defects of the ICs Logic Family. Digital ICs are classified based on the complexity of the circuit. For example, ICs is classified based on the number of GATEs required to complete a logic function. This is called the gate-equivalency classification of integrated circuit.

S.R.	Circuit Classification	Individual basic logic gates required
1	Small Scale Integration (SSI)	Less than 12
2	Medium Scale Integration (MSI)	12 or more but less than 100
3	Large Scale Integration (LSI)	100 or more but less than 1000
4	Very Large Scale Integration (VLSI)	1000 or more

In this method, the circuits are classified into SSI, MSI, LSI and VLSI classes. The basis of classification is the number of individual logic gates used in the circuit.

Characteristics of Digital integrated circuit

To compare the working of digital ICs the following characteristics are taken into consideration.

(1) Speed of operation (Propagation Delay)
(2) Power Dissipation
(3) FAN-IN
(4) FAN-OUT
(5) Noise Immunity
(6) Noise Margin
(7) Power Supply Requirements

Speed of Operation(Propagation Delay)

The speed of any digital circuit is expressed in terms of propagation delay. The output and input waveforms of a logic gate are shown in the photo below.

Delay time is measured by the time between the 50% levels of the input and output waveforms. There are 2 delay times. (i) t_pHL – It is the time in which the output goes from HIGH state to LOW state. (ii) t_pLH – It is the time in which the output goes from LOW state to HIGH state.

The propagation delay time of a logic gate is the average of these two times (t_pHL and t_pLH). In other words, propagation delay time is the time in which the output of the gate changes when the input changes. The delay time of a TTL gate is about 10ns (10×10^-9 sec).

Power Dissipation in integrated circuit

This is the amount in milliwatts (mW) of power dissipated in the ICs. The lower the power consumption in ICs, the better because it reduces the cooling and power supply cost of ICs. But due to less expenditure power, there is a possibility of increasing the propagation delay time. Power dissipation in a standard TTL gate is about 10mW.

FAN-IN

The maximum number of inputs that can be given to a logic gate is called FAN-IN. By this definition the FAN-IN for a 3-input NAND gate is three.

FAN-OUT

The number of gates that a single logic gate can drive is called the FAN-OUT. If the value of FAN-OUT for a NOR gate is 6, it means that this NOR gate can drive 6 other similar NOR gates.

Noise Immunity in integrated circuit

It is the input voltage given to a logic gate that changes the state of the output at a given supply voltage.

Noise Margin in integrated circuit

It is the ability of a logic circuit to tolerate an unwanted noise signal at its input and it is the maximum value of the noise signal that the system can reject without affecting the circuit’s performance.

Power Supply Requirements of integrated circuit

The supply voltage and amount of power required for operation of ICs are the main characteristics of integrated circuit and by these the necessary and appropriate power supply is selected for operation of integrated circuit.

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