BJT saturation indicator: Approximately what is VCE when a silicon transistor is driven into saturation?

Introduction / Context:
Recognizing saturation is vital in switching applications. When a BJT saturates, both junctions are forward biased and the collector-emitter voltage collapses to a small value, which sets the lower bound of the output low level and affects power dissipation.

Given Data / Assumptions:

• Silicon BJT used as a switch.
• Heavy base drive sufficient to enter saturation.
• Typical room-temperature operation.

Concept / Approach:
In saturation, VBE is forward biased (around 0.7–0.9 V) and VBC is also forward biased. The collector-emitter path behaves like a low-resistance segment, and VCE(sat) is typically small, often quoted near 0.2 V for small-signal devices (exact value depends on current and device type).

Step-by-Step Solution:

Verification / Alternative check:
Consult datasheets: VCE(sat) is specified versus IC and IB, often 0.05–0.3 V for small-signal BJTs. Power devices may exhibit higher values, but 0.2 V remains a common nominal figure.

Why Other Options Are Wrong:

• VCC (option a) is the supply, not the drop in saturation.
• VB (option b) refers to base potential, not the collector-emitter drop.
• 0.7 V (option d) is characteristic of a base-emitter junction, not VCE(sat).

Common Pitfalls:
Confusing VCE(sat) with VBE or assuming a fixed 0.2 V across all currents and devices; the value varies but remains “small.”

Final Answer:
About 0.2 V.