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A coupling capacitor works into \( 10 \ \mathrm{k\Omega} \) total. For a \( 20 \ \mathrm{Hz} \) break it must be at least about

  • A.0.08 microfarads
  • B.80 microfarads
  • C.0.8 microfarads
  • D.8 microfarads

Answer: C

Rearranging the break frequency relation:

\[ \begin{aligned} C &= \frac{1}{2\pi f R} \\ &= \frac{1}{2\pi(20)(10^{4})} \\ &\approx 0.8 \ \mathrm{\mu F} \end{aligned} \]

giving about 0.8 microfarads. The others misplace a decade. Because the capacitance needed is inversely proportional to both frequency and resistance, low-frequency response into a low impedance is what drives capacitor values up. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

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