FE Electrical & Computer Practice Question Solution included after investigation
CASE E-027 · FE

The Transistor That Refused to Amplify 100×

An NPN switch has VCC = 5 V, Rc = 100 Ω, β = 100, and Ib = 1 mA. βIb predicts 100 mA, but with VCE(sat) ≈ 0.2 V the collector circ

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Case E-027 engineering failure visual
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3

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4

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Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

An NPN switch has VCC = 5 V, Rc = 100 Ω, β = 100, and Ib = 1 mA. βIb predicts 100 mA, but with VCE(sat) ≈ 0.2 V the collector circuit can supply only about 48 mA, so the transistor saturates.

Electronics Electrical & Computer Design Verification Root Cause Analysis Safe Correction
E-027
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What was missed

Applying Ic = βIb without checking whether the collector circuit can support that current.

Corrected model

βIb predicts 100 mA, but the collector loop allows only about 48 mA; therefore the BJT is saturated and Ic is circuit-limited.

QUICK ANSWER

βIb predicts 100 mA, but the collector loop allows only about 48 mA; therefore the BJT is saturated and Ic is circuit-limited.

ENGINEERING TAKEAWAY

Check the device model and operating region first, then apply the corresponding circuit equation.

CASE QUESTIONS

Questions this case should settle.

Why can a BJT collector current be much lower than beta times base current?

βIb predicts 100 mA, but the collector loop allows only about 48 mA; therefore the BJT is saturated and Ic is circuit-limited.

NEXT FE PRACTICE QUESTION

The Op-Amp That Tried to Output -10 V on ±5 V Rails

Electronics · 5–10 min

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