You should give attention to DC floating node when you use SPICE simulator.
For examples, The node “A” of the figure (a) is floating.
(Floating Node Example)
In this case, some SPICE program can’t determine the DC operating point of node “A”. If you want to simulate, you have to add a resistor having very resistance between node “A” and ground like figure (b).
There are two type of SPICE simulator in DC floating case.
(1) You have to add the resistor which is placed between floating node and ground.
(2) You don’t have to edit schematics. SPICE simulator automatically add the resistor which is placed between floating node and ground.
One of the type (1) program is Pspice, on the other hand, LTspice is one of type (2). You may think it seems that type (2) simulator is more useful than type (1), but it should be noted that you may also overlook floating mistakes, if you use the type (2) simulator.