Integrated Circuits
Operation and use of encoders and decoders; functions of encoder types; scale of integration from SSI to VLSI; the TTL and CMOS logic families and linear ICs. Examined for Category B2.
Integrated circuits — summary notes
Free- An integrated circuit builds many components on one silicon die. Digital ICs handle discrete HIGH/LOW logic levels; linear (analogue) ICs such as operational amplifiers and voltage regulators handle continuously variable signals.
- Scale of integration: SSI (up to ~12 gates), MSI (~100), LSI (thousands), VLSI (hundreds of thousands and beyond) and ULSI. Higher integration means fewer interconnections and higher reliability per function.
- TTL (transistor-transistor logic, the 74xx family) is bipolar: fast, robust against static, but draws significant current and normally runs on a tightly regulated 5 V.
- CMOS (complementary MOS, the 4000 and 74HC families) draws almost no current when static, tolerates a wide supply range and has high noise immunity — but its high-impedance gate oxide makes it acutely static-sensitive, which links directly to sub-module 5.12.
- Packages: DIL/DIP through-hole and SOIC/QFP/BGA surface-mount. A notch or dot marks pin 1.
- Op-amps are the standard linear building block, with very high open-loop gain, high input impedance and low output impedance; feedback sets the actual gain and the function (inverting, non-inverting, follower, comparator, summing, integrator, differentiator).
- ⚠ Exam trap: CMOS wins on POWER and noise immunity, TTL on raw SPEED and static robustness. CMOS is the one that needs ESD handling.
- Integration scale
- SSI → MSI → LSI → VLSI → ULSI
- Ideal op-amp
- infinite gain · infinite input impedance · zero output impedance
Two logic families are candidates for a battery-backed memory-keep-alive circuit. Which is chosen and why?
CMOS — its static power consumption is negligible, so it can be kept alive from a small battery for long periods. TTL would flatten the battery because its bipolar inputs draw current continuously.
Why is an unused CMOS input never left floating?
A floating high-impedance input picks up noise and can drift to an intermediate voltage, switching the internal transistors partly on. That causes erratic outputs and excessive supply current, so unused inputs are tied to the supply rail or to ground.
Integrated circuits concept map
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