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A tail current of \( 20 \ \mathrm{\mu A} \) charges a \( 30 \ \mathrm{pF} \) compensation capacitor. The slew rate is about

  • A.0.67 V/us
  • B.1.5 V/us
  • C.0.067 V/us
  • D.6.7 V/us

Answer: A

Dividing the available current by the capacitance:

\[ \begin{aligned} \frac{20 \times 10^{-6}}{30 \times 10^{-12}} \\ &\approx 0.67 \ \mathrm{V/\mu s} \end{aligned} \]

giving about 0.67 volts per microsecond, close to the figure a general-purpose operational amplifier quotes. That such a familiar specification falls out of two internal quantities is worth noticing, since it explains why faster devices run more tail current or use less compensation.

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