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Which one of the following statements is correct about an ac-coupled common-emitter amplifier operating in the mid-band region?

1. The device parasitic capacitances behave like open circuits, whereas coupling and bypass capacitances behave like short circuits.
2. The device parasitic capacitances, coupling capacitances and bypass capacitances behave like open circuits.
3. The device parasitic capacitances, coupling capacitances and bypass capacitances behave like short circuits.
4. The device parasitic capacitances behave like short circuits, whereas coupling and bypass capacitances behave like open circuits.

Correct answer: The device parasitic capacitances behave like open circuits, whereas coupling and bypass capacitances behave like short circuits.

Solution

In the mid-band region, the frequency is high enough that the coupling and bypass capacitances effectively short-circuit AC signals, allowing them to pass through, while the parasitic capacitances of the device do not significantly affect the circuit and can be treated as open circuits.

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