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A follower of \( 60 \ \Omega \) output impedance replaces a \( 10 \ \mathrm{k\Omega} \) source ahead of a stage with \( 600 \ \mathrm{pF} \) of Miller capacitance. The break moves from

  • A.27 kHz to 4.4 MHz
  • B.27 kHz to 270 kHz
  • C.270 kHz to 4.4 MHz
  • D.4.4 MHz to 27 kHz

Answer: A

Computing the two break frequencies:

\[ \begin{aligned} f_{1} &= \tfrac{1}{2\pi(10^{4})(600\mathrm{p})} \approx 27 \ \mathrm{kHz} \\ f_{2} &= \tfrac{1}{2\pi(60)(600\mathrm{p})} \approx 4.4 \ \mathrm{MHz} \end{aligned} \]

so 27 kHz to 4.4 MHz, a factor of over one hundred and sixty for the price of one transistor. A buffer that takes no voltage gain can still transform a stage's usefulness, which is the clearest possible argument for impedance transformation.

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