Linear integrated circuits — why “resistor ratio” design is preferred: In monolithic linear ICs, designers rely on resistor <em>ratios</em> rather than precise absolute values primarily because:

Introduction / Context:
Monolithic linear integrated circuits (op-amps, references, instrumentation amplifiers) often implement functions using on-chip resistors. Semiconductor processes can produce resistors with good matching on the same die but relatively poor absolute tolerance. Consequently, designers specify performance using resistor ratios, which remain stable across process variations and temperature tracking.

Given Data / Assumptions:

• On-chip passive components exhibit process-dependent absolute values.
• Layout techniques (common centroid, identical geometry) improve matching between resistors.
• The question seeks the primary reason for ratio-based design.

Concept / Approach:
Absolute resistance (e.g., target 10 kΩ) may deviate by ±20% or more due to sheet-resistance and dimensional variations. However, two resistors fabricated adjacently with identical shapes track tightly (e.g., mismatch in the few hundred ppm to several thousand ppm). If a circuit’s transfer function depends on R2/R1, the ratio remains accurate even when both R1 and R2 shift together. This enables precise gains, filters, and references without laser-trim or external components.

Step-by-Step Solution:
Identify process limitation: poor absolute tolerance, good matching.Exploit matching by designing functions dependent on ratios (e.g., gain = 1 + R2/R1).Conclude that the core reason is the impracticality of precise absolute values on chip.

Verification / Alternative check:
Datasheets often specify gain accuracy as a ratio-dependent spec and may use laser trimming only when absolute accuracy is required (e.g., precision references).

Why Other Options Are Wrong:
“Ratios increase input resistance” is not generally true; input resistance is largely set by topology/transistor devices. “Ratios increase amplifier gain” is misleading; ratios set gain accurately but do not universally “increase” it.

Common Pitfalls:
Assuming external resistor tolerances always fix accuracy; board-level tolerances and temperature coefficients must also be considered.

Final Answer:
Precise resistor values are not possible with IC processes.