Predict how all transistor currents (IB, IC, and IE) and the output voltage signal will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults) :
| Column - I | Column - II |
| (A) Capacitor Cin fails becomes open circuit | (P) Transistor in cutoff mode |
| (B) Solder bridge (short) past resistor R1: | (Q) Transistor saturation mode |
| (C) Resistor R1 fails and becomes open circuit | (R) No output signal across load |
| (D) Resistor RE fails and becomes open circuit | (S) Transistor in active mode |
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In a common-emitter amplifier, transistor currents (IB, IC, IE) and output voltage are governed by biasing and signal coupling. The key relationships are IC = βIB and IE = IC + IB.
For each fault:
(A) Cin open: AC input signal is blocked, but DC bias is unchanged. Transistor remains in active mode (S). IB, IC, IE DC values are normal, but there is no output AC signal (R).
(B) Short past R1: Base voltage rises significantly, increasing IB and driving the transistor into saturation (Q). IC becomes nearly VCC/RC.
(C) R1 open: Base current IB becomes zero, forcing the transistor into cutoff (P). IC ≈ 0 and IE ≈ 0. No output signal (R).
(D) RE open: Emitter current path is broken, so IE ≈ 0. This also forces IB and IC to nearly zero, putting the transistor in cutoff (P). No output signal (R).
Final Answer: (A) matches (S) and (R); (B) matches (Q); (C) matches (P) and (R); (D) matches (P) and (R).