Question 3-32D3

An input coupling capacitor in an amplifier serves two main purposes: to pass the AC signal from the previous stage to the amplifier's input and to block any DC voltage from the previous stage from disturbing the amplifier's precise DC biasing. If this capacitor fails open, it becomes an infinite resistance to both AC and DC. 1. **No AC signal path:** The AC input signal from the preceding stage can no longer pass through to the amplifier's input. Consequently, there will be no signal to amplify, resulting in **no amplification**. 2. **DC unchanged:** The capacitor's failure as an open circuit means it continues to block DC from the previous stage, and it does not affect the amplifier's internal DC bias network. The voltage and current levels established by the amplifier's own power supply and bias resistors remain intact. Therefore, **DC within the circuit will measure normal**. **Why other options are incorrect:** * **B) Improper biasing will occur:** This is incorrect. An open coupling capacitor prevents *external* DC from affecting the amplifier, and it doesn't alter the amplifier's *internal* DC bias points. Improper biasing would occur if the capacitor shorted, allowing DC from the previous stage to enter. * **C) Oscillation and thermal runaway may occur:** This is unlikely. An open input simply removes the signal. Oscillation typically requires feedback paths, and thermal runaway relates to excessive current and heat, which isn't a direct consequence of an open input coupling capacitor. * **D) An AC hum will appear on the circuit output:** This is incorrect. If anything, an open input coupling capacitor would prevent any hum originating from previous stages or the signal source from reaching the amplifier. Hum usually indicates power supply issues or poor grounding within the amplifier itself, not an open input.