amplify/increase current from 9v battery
If you use a buck switching regulator instead of a linear regulator like the 7805, you can get more current at 5 V than your battery puts in at 9 V. 

But you can't get more power out than you put in. So to get 850 mA at 5 V, you will need something like 500-550 mA at 9 V, which is more than it's reasonable to expect from a 9 V battery.

You could either use a big honking lantern battery, or switch to using a mains-powered supply (wall wart). If you need to limit yourself to using a single 9 V battery, you'll have to re-design your circuit to use much lower power. That probably means much quieter output.