Adapter option; match wattage, connector and your original adapter. HP guide description: "45-W HP Smart adapter (non-PFC, RC, 2-wire, 4.5-mm)". OEM documentation only; not bench-tested.
Source notes
HP EliteBook 820 G2 Notebook PC and / HP EliteBook 720 G2 Notebook PC Maintenance and Service Guide, printed p29 (PDF p39): "45-W HP Smart adapter (non-PFC, RC, 2-wire, 4.5-mm) 742437-001".