Interview Question

Due to a miscommunication during design, you thought your circuit was supposed to have a supply voltage of 2.1 volts (threshold voltage is 0.7 volts) and a 25 ns cycle time, and you designed it to meet those specifications. Now your boss tells you you were supposed to have a 20 ns cycle time. To avoid redesigning the whole circuit, a co-worker suggests increasing the voltage of the circuit to decrease the delay to 20 ns. The same co-worker suggests picking some arbitrary number like 3.5 volts.
  1. Determine the new cycle time of your circuit with a 3.5 volt input voltage.
  2. Your boss is worried about the additional power consumption - calculate the increase in power consumption of your circuit at 3.5 volts, assuming activity factor and capacitance remain the same and neglecting short circuit and leakage power.
  3. To satisfy your boss, calculate the minimum voltage you would increase the supply voltage to, in order to allow your circuit to run at 20 ns. You may leave your answer in non-simplified numeric terms, but not in the form of an equation to solve.

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