I have to calculate current gain of the following circuit and to model it in Spice. I arbitrarily set Is = 0.0002 A: Here are the results provided by LTSpice: Also, in order to find an equation for current gain, my first idea was to transform Is and its parallel 5k resistor to a combination of voltage source (Vs = Is*5000 Ohm = 1V) and series 5k resistor. In this case the voltage at inverting input is Vy = Vx = 0.33*Vout, and the output voltage itself can be found according to following equation: (Vs - Vy)/5000 = (Vy - Vout)/20000. Strangely, the results provided by LTSpice become quite disturbing when I increase the voltage to 2V (or current to 0.0004 A): The same happens when I increase the value of 5k or 2k resistors. As you can see, I had a good luck arbitrarily choosing Is value as 0.0002 A. Otherwise, I could have very hard times. In general, if I keep all the resistors as they are and don't increase the current above 0.0004 A, then the output voltage can be calculated following these equations:
(5k*I1 - 1/3Vout)/5k = (1/3Vout - Vout)/20k; 6R*Is = Vout; 6*5000*0.0002 = 6 V;
My question is: how to correctly select voltage/current source and resistor values in order to avoid op-amp saturation?