Why does the opamp output voltage saturate?
Vout= Vin (1 +r2/r1) is the equation for a non-inverting op-amp with negative feedback.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Non-inverting amplifier configuration.

Your configuration is non-inverting but has positive feedback. This will give a Schmitt trigger effect.

Remember that the op-amp output will be \$ (V_+ - V_-)A \$ where A is the open loop gain and typically > 1M. If we apply V1 = 1 V as shown in your schematic when the output is zero then \$ V_+ \$ goes to 0.5 V. The output will then try to go to \$ (0.5 - 0)1M \$ \$ = 500,000~V \$! It will get as far as the positive supply rail and stop with \$ V_+ \$ half-way between 1 V and the supply voltage. This will hold the opamp in that condition indefinitely.


  Schmitt trigger, ʃmɪt/, noun, ELECTRONICS:
  a bistable circuit in which the output increases to a steady maximum when the input rises above a certain threshold, and decreases almost to zero when the input voltage falls below another threshold.