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In this letter, the tradeoff between sensing and throughput is studied for cognitive radios by considering the energy consumed in the circuit. The total energy is minimized with respect to the power fractions of the power amplifier and the low noise amplifier, the bit resolution of the analog-to-digital converter, the input backoff of the power amplifier and the sensing time in spectrum sensing, for a fixed achievable number of bits per packet. Numerical results show that the optimal sensing time increases when the circuit energy is considered. They also show that the analog-to-digital converter consumes more energy and that it is advantageous to operate the power amplifier at high input backoffs in cognitive radios.