The purpose of coupling capacitor in a transistor amplifier is to ________

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Examoogle · Accepted Answer

Pass AC and block DC

Explanation: A coupling capacitor is used in multi-stage transistor amplifiers to transfer the AC signal from one stage to the next while blocking the DC bias voltages. This isolation prevents the DC operating point of one stage from affecting the biasing of the next stage.

Working Principle: The impedance of a capacitor is given by $X_C = \frac{1}{2\pi fC}$. For AC signals, the capacitor acts as a low-impedance path (assuming proper sizing for the frequency range), effectively 'coupling' the signal. For DC (frequency $f = 0$), the capacitor acts as an open circuit ($X_C = \infty$), effectively 'blocking' the DC voltage.

Key Formulas: $X_C = \frac{1}{2\pi fC}$ — The capacitive reactance representing opposition to AC flow; $f = 0$ — The frequency condition where the capacitor blocks DC current

Key Points: Coupling capacitors are essential in RC-coupled amplifiers.; They maintain the independence of biasing networks between amplifier stages.; Improper sizing of the capacitor results in a lower cutoff frequency ($f_L$), causing signal attenuation at low frequencies.; DC blocking ensures that the transistor $V_{CE}$ and $V_{BE}$ are not altered by subsequent stages.

Advantages: Provides DC isolation between stages.; Simple and cost-effective design.; Allows cascading multiple amplifier stages.

Disadvantages: Limited low-frequency response.; Increased component count.; Phase shift introduced at lower frequencies.

Applications: Multi-stage audio amplifiers.; Signal processing chains.; Pre-amplifiers.

Additional Info: In integrated circuits, direct coupling is often preferred over capacitive coupling because large-value capacitors are difficult to fabricate on-chip.. Option B is incorrect because while it 'protects' the bias stability, its primary electrical purpose is signal transfer; 'protecting' is a byproduct of DC blocking.. Option D is incorrect because biasing is provided by resistors and voltage dividers, not capacitors.

Answer: C is correct — The coupling capacitor acts as a high-pass filter element that passes AC signals while creating an open circuit to block DC potentials.

Exam Tip: Always remember that in the frequency domain, a coupling capacitor creates a high-pass filter; hence, if the capacitor is too small, your amplifier will fail to amplify bass/low-frequency signals.

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