Transistor operating modes in practice: Is it accurate to state that “transistors are controlled to operate either as a switch or as a variable resistor”?

Introduction / Context:
Engineers commonly describe transistor use cases as “switching” (digital) and “amplifying” (analog). This question probes whether calling a transistor a “variable resistor” correctly captures analog operation, particularly for BJTs versus FETs.

Given Data / Assumptions:

• Device of interest: BJT (bipolar junction transistor).
• Context spans both digital switching and linear amplification.
• Focus is on physical behavior, not a rough two-terminal metaphor.

Concept / Approach:
In digital mode, a BJT is driven between cutoff and saturation, approximating an on/off switch. In analog mode, a BJT operated in forward-active region behaves primarily as a transconductance device: small changes in base-emitter voltage (or base current) modulate collector current with current gain beta and transconductance gm. While one can bias certain transistors to approximate a two-terminal “variable resistor,” that description fits MOSFETs in the ohmic (triode) region more naturally than BJTs, which are current-controlled and exhibit exponential I-V characteristics across the base-emitter junction.

Step-by-Step Solution:

Verification / Alternative check:
Small-signal models show r_pi and ro as parasitic resistances around a dependent current source; the core transfer is current gain or transconductance, not variable resistance per se.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming any linear operation equals “variable resistor”; ignoring that BJTs are three-terminal current-controlled devices.

Final Answer:
Incorrect