Unit V Integrated CircuitsAn Integrated Circuit (IC) is an electronic circuit in which devices like transistor, diodes, resistors and capacitors (i.e. active and passive devices) are fabricated on a tiny single chip of silicon.

Advantages of ICs over Discrete Components: Extremely small physical size Low power consumption Reduced cost Increased system reliability Increased operating speed Increase equipment density Improved function performance High yield

Limitations of ICs: Coils or inductors can not be fabricated ICs function at fairly low voltages They handle limited amount of power They are quite delicate and can not withstand rough handling or excessive heat

Scale Of Integration:

Small Scale Integration (SSI): In this number of circuit per package is less than 12 Medium Scale Integration (MSI): In this number of circuit per package is between 13 and 99 Large Scale Integration (LSI): In this number of circuit per package is between 100 and 9,999 Very Large Scale Integration (VLSI): In this number of circuit per package is between 10,000 and 99,999 Ultra Large Scale Integration (ULSI): In this number of circuit per package is between 1,00,000 and 9,99,999 Giga Scale Integration (GSI): In this number of circuit per package is between 10,00,000 or more 12 12 - 99 100 - 9,999 10,000 - 99,999 1,00,000 - 9,99,999 10,00,000

SSI < MSI LSI VLSI ULSI GSI >

Classification of ICs by Structure:

Monolithic ICs: In these ICs all circuit components (i.e. active and passive) are fabricated inseparable within a single continuous piece of silicon crystalline material called WAFER. In Monolithic ICs all components are formed simultaneously by a diffusion process. Then a metallization process is used in interconnecting these components to form the desired circuit. Hybrid ICs: In Hybrid ICs passive components (such as resistors and capacitors) and the interconnection between them are formed on an insulating substrate, the substrate is used as a chassis for the integrated components .Active components such as transistors and diodes, as well as Monolithic ICs are then connected to form a complete circuit. Hybrid ICs are further classified as Thin Film and Thick Film, depending on the method used to form the resistor, capacitor and related interconnections on the substrate.1. Thin Film: When a suitable material is evaporated on a

substrate informing resistors, capacitors and interconnections, a Thin Film Hybrid IC is obtained

2. Thick Film: When the resistors, capacitors and

interconnections are etched on the substrate by silk screening, a Thick Film Hybrid IC is obtained.

Classification of ICs by Function:

Linear ICs: They perform amplification and other essential linear operation on signals. Non Linear ICs: They require only ON-OFF operation of the transistor, thus the design requirements for these circuits are less stringent than those of linear ICs.

Comparison among different ICs:Monolithic ICs have an advantage of lowest cost and high reliability, but they have some limitations. Isolations among components is poor Inductors can not be fabricated It is difficult to change the circuit design after it is finalized Range of values of resistors and capacitors, which are produced in ICs, is comparatively small. The Thick and Thin Film ICs have an advantage of producing resistors and capacitors with greater range of values and smaller tolerance than those of Monolithic ICs. Isolation among components in these circuits is also better. They have greater flexibility in circuit design. The performance of film circuit at high frequencies is also much better. There limitations are: Active components an not be fabricated Fabrication cost is higher than Monolithic Physical size is also larger

IC TerminologySome common terms used in fabricating ICs are: Bonding: Attaching the die on ceramic substrate and then connecting the leads to the package. Chip: An extremely small part of silicon wafer on which IC is fabricated. Circuit Probing: Testing the electrical performance of each IC chip with the help of microscope. Diffusion: A process that consist of the introduction of impurities into selected regions of a wafer to form junctions. Encapsulation: putting a cap over the IC and sealing it in an inert atmosphere. Epitaxy: A process of controlled growth of a crystalline doped layer of silicon on a single crystal substrate. Mask: A glass plate with desired pattern of diffusion or metallization. Metallization: A process for providing ohmic contacts and interconnections by evaporating aluminum over the chip. Photolithography: A process to transfer geometrical pattern from the mask to the surface of the wafer. Photoresist: A light-sensitive material that hardens when exposed to ultraviolet light. Wafer: A thin disk of semiconductor in which number of ICs are fabricated simultaneously.

The 555 Timer

The 555 is a monolithic timing circuit that can produce accurate and highly stable time oscillation. The IC was designed and invented by Hans R. Camenzind. It was designed in 1970 and introduced in 1971 by Signetics (later acquired by Philips). The original name was the SE555/NE555 and was called "The IC Time Machine". The 555 gets its name from the three 5-kOhm resistors used in typical early implementations The 555 timer is one of the most popular and versatile integrated circuits ever produced. It includes 23 transistors, 2 diodes and 16 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8). The 555 has three operating modes:

Monostable mode: In this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, touch switches, Frequency Divider, Capacitance Measurement etc. Astable - Free Running mode: The 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc. Bistable mode: The 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.

The 555 as Monostable Multivibrator:A monostable multivibrator, often called a one shot multivibrator, is a pulse-generating circuit in which the duration of the pulse is determined by the RC network connection externally to the 555 timer. In a stable state the output of the circuit is approximately zero or at logic-low level. When an external trigger pulse is applied, the output is forced to go high. The time the output remains high is determined by the external RC network connection to the timer. At the end of the timing interval, the output automatically reverts back to its logic-low stable state. The output stays low until the trigger pulse is again applied. Then the cycle repeats. The monostable circuit has only one stable state hence it is named monostable.

The 555 as an Astable Multivibrator:An astable multivibrator, often called a free-running multivibrator, is a rectangular-wave-generating circuit. Unlike the monostable multivibrator, this circuit does not require an external trigger to change the state of the output; hence it is named free-running.

The 555 as an Astable Multivibrator:This circuit diagram shows how a 555 timer IC is configured to function as an astable multivibrator. An astable multivibrator is a timing circuit whose 'low' and 'high' states are both unstable. As such, the output of an astable multivibrator toggles between 'low' and 'high' continuously, in effect generating a train of pulses. This circuit is therefore also known as a 'pulse generator' circuit. Operation: In this circuit, capacitor C1 charges through R1 and R2, eventually building up enough voltage to trigger an internal comparator to toggle the output flipflop. Once toggled, the flip-flop discharges C1 through R2 into pin 7, which is the discharge pin. When C1's voltage becomes low enough, another internal comparator is triggered to toggle the output flip-flop. This once again allows C1 to charge up through R1 and R2 and the cycle starts all over again. C1's charge-up time t1 is given by: t1=0.693(R1+R2)C1. C1's discharge time t2 is given by: t2=0.693(R2)C1. Thus, the total period of one cycle is t1+t2 = 0.693 C1(R1+2R2). The frequency f of the output wave is the reciprocal of this period, and is therefore given by: f = 1.44/(C1(R1+2R2)), wherein f is in Hz if R1 and R2 are in megaohms and C1 is in microfarads.

The 555 as Monostable Multivibrator:This circuit diagram shows how a 555 timer IC is configured to function as a basic monostable multivibrator. A monostable multivibrator is a timing circuit that changes state once triggered, but returns to its original state after a certain time delay. It got its name from the fact that only one of its output states is stable. It is also known as a 'one-shot'. Operation: In this circuit, a negative pulse applied at pin 2 triggers an internal flip-flop that turns off pin 7's discharge transistor, allowing C1 to charge up through R1. At the same time, the flip-flop brings the output (pin 3) level to 'high'. When capacitor C1 as charged up to about 2/3 Vcc, the flip-flop is triggered once again, this time making the pin 3 output 'low' and turning on pin 7's discharge transistor, which discharges C1 to ground. This circuit, in effect, produces a pulse at pin 3 whose width t is just the product of R1 and C1, i.e., t=R1C1. The reset pin, which may be used to reset the timing cycle by pulling it momentarily low, should be tied to the Vcc if it will not be used.