For a certain logic-circuit family, the standard inverter, when loaded by a similar circuit, has an average propagation

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For a certain logic-circuit family, the standard inverter, when
loaded by a similar circuit, has an average propagation delay tP
equal to 20 ps. a) If the current available to charge a load
capacitance is twice as large as that available to discharge the
capacitance, what do you expect tPHL [5 points] and tPLH [5 points]
to be? b) When an external capacitive load of 15 fF is added at the
inverter output, its propagation delay increases by 70% (i.e. by a
factor of 1.7). What do you estimate the combined capacitance of
inverter input and output to be [5 points]? c) If without the 15 fF
load connected, and with the load inverter removed, the propagation
delay is reduced by 50% (i.e. by a factor of 0.5), estimate the two
components of capacitance found in (b) [10 points].