I was trying to simulate an instrumentaion amplifier in ltspice. 
the problem i am having is the output when measured across C1 is wrong. since i have an amplification factor of 11, i was expecting to get (V2-V3)*11. which should be 11V. but i get 5.5V instead. if i change the power supply for the opamps from single supply (15V,GND) to dual supply +/-15V, then it works perfectly. any help?
Best Answer
A few quick calculations might make it a little more clear what is happening.
When not considering the supply voltages of the op-amps the voltage across R1 should be 1 V. The resulting voltages at the output nodes would be 8 V and -3 V as illustrated in this picture:
The negative voltage at the output of U2 can not be achieved without supplying a negative supply voltage.
As a result, the output of U2 is forced to its negative supply rail which is 0 V. This essentially means that the voltage drop across R1 and R4 is 3 V and an output voltage of 5.5 V is the result of that as can be seen in this picture:
Notice that when the difference between the input voltages is smaller compared to the absolute voltages, the circuit is operating with a gain of 11 even without differential supply voltages. So depending on the input voltage ranges differential supply might be required or not.