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Difficulty: MediumCapacitors

In a multi-stage audio amplifier circuit, a capacitor is placed in series between the output of one transistor stage and the input of the next stage. What is the primary purpose of this coupling capacitor?

  1. A
    To store energy within a magnetic field to oppose sudden changes in circuit current
  2. B
    To convert the direct current power supply voltage into an alternating current audio signal
  3. To pass the alternating current audio signal while blocking the direct current bias voltageAnswer
  4. D
    To conduct direct current continuously while insulating against alternating signal frequencies

Answer

To pass the alternating current audio signal while blocking the direct current bias voltage
In multi-stage amplifiers, each transistor stage requires specific direct current (DC) biasing voltages to operate in its linear region. A coupling capacitor placed in series between stages acts as an open circuit to steady DC (XC=∞X_C = \infty at 0 Hz0\text{ Hz}), preventing the DC bias of one stage from disturbing the next, while exhibiting low capacitive reactance to alternating current (AC) audio signals, allowing the desired signal to transfer unimpeded.

Step-by-Step Solution

1
Analyze the capacitive reactance formula in relation to frequency
XC=12πfCX_C = \frac{1}{2\pi f C}, meaning capacitive reactance is inversely proportional to frequency.
Determining how the component reacts to different signal types establishes its filtering and coupling characteristics.
2
Evaluate the behavior of the capacitor at zero frequency (direct current)
For DC (f=0 Hzf = 0\text{ Hz}), reactance XC→∞X_C \to \infty, effectively acting as an open circuit that blocks DC voltage.
Amplifier stages require separate DC operating bias voltages that must not interfere with adjacent stages.
3
Evaluate the behavior of the capacitor for audio frequencies (alternating current)
For AC (f>0 Hzf > 0\text{ Hz}), XCX_C drops to a low value, allowing the alternating signal voltage to pass freely to the next stage.
Coupling requires the time-varying signal information to transfer smoothly across the circuit without attenuation.

Key Concept

DC Blocking and AC Coupling Behavior of Capacitors
Estimated Time:1m 0s
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Topics

Capacitors

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