Cryogenic solid-state quantum processors require classical control and readout electronics; to achieve compactness and scalability, cryogenic integrated circuits have been recently proposed for this goal. Circulators are widely used in readout circuits, however they are typically discrete bulky devices, thus preventing miniaturization. To address this issue, we propose a fully integrated 40-nm CMOS 6.5-GHz circulator operating from 300K to 4.2 K. At 300 K, it achieves a 2.2-dB insertion loss, an 18-dB isolation, and a 2.4-dB noise figure over the 1-dB bandwidth from 5.6GHz to 7.4 GHz, with a core power of only 2.5mW. This improves to 2.1mW core power at 4.2 K, while showing 1.3-dB insertion loss and 17-dB isolation over the 1-dB bandwidth from 5.8 GHz to 7.6 GHz. The circuit achieves a record-low core power and a 1.6 wider fractional bandwidth than the state-of-the-art, thus allowing its use for multiple channels in power-constrained cryogenic refrigerators. These advances are enabled by a fully-passive architecture based on LC all-pass filters, allowing the use of a lower clock frequency than in prior art.