In linear integrated circuits, why are field-effect transistors (FETs) commonly included in input stages?

Introduction / Context:
Operational amplifiers and other linear ICs often use FETs at the input because FET gate channels behave like extremely high-impedance nodes. This design choice improves signal integrity when interfacing with high-impedance sensors or small signals.

Given Data / Assumptions:

• We discuss linear IC input stages.
• FET characteristics: very high input impedance; ability to implement large on-chip resistances.
• We are not optimizing for switching speed but for analog performance.

Concept / Approach:
FET gates draw negligible DC current, which raises the input resistance into megaohm or gigaohm ranges. In IC processes, FET geometries can also realize large value “pseudo-resistors” where discrete resistors would be bulky or impractical.

Step-by-Step Solution:
Identify analog need: avoid loading sources → require high Rin.Use FET gates to realize high input resistance.Leverage device geometry to synthesize large resistive elements on die.Therefore both (a) and (b) are correct.

Verification / Alternative check:
Compare BJT vs. JFET/MOSFET input op-amps: FET inputs excel where source impedance is high or bias currents must be minimal.

Why Other Options Are Wrong:
increase device complexity: not the primary design goal; complexity may increase but is not the reason.Single choices (a) or (b) alone understate typical benefits.None: incorrect because FETs do provide both advantages.

Common Pitfalls:
Assuming FET inputs are always superior; BJTs may offer lower noise or higher transconductance in some applications.

Final Answer:
(a) and (b) above