The slight difference in the measured and calculated values is because of the non-ideal character of an Op-Amp, the connecting wires etc. The moisture component present in the atmosphere, the wiring arrangement of the library, some High Tension (High Voltage) public supply electricity line going around the laboratory also affects the measurements. Sometimes if the connecting wires make a loop then the induction effect also comes in picture.
Once again the non-ideal nature of electronic gadgets comes into picture. In fact the inherent resistive component of the power supply too causes some variations in the output parameters. The moisture present in the atmosphere or less than ideal nature of oscilloscope probes too causes variations.
The ideal output voltage should be the product of the dc input signal and the amplifier's closed-loop voltage gain. However, the output voltage has an added error component. If the ideal value of output voltage is large with respect to the error component, then we can usually ignore the op amp characteristic that causes it. But if the error component is comparable to or even larger than the ideal value, we must try to minimize the error. The closed loop gain mainly depends upon the external resistors. Op amp characteristics that add error components to the dc output voltage are:
When we disconnect the feedback loop (Resistor R), then it is no more 'closed loop gain' for the op amp. There is no negative feedback. And input voltages at oscilloscope terminals are different. The measured values are;
Now to get a gain of 6 in figure 5 (above), the calculated value of resistance R is;
6 = 1 + (R/10000)
i.e. R = 60000-10000 = 50K
Measured voltages : . . . . . . Channel1: 3V
Calculated gain : . . . . .6 . . . . . . . . . . .
Changing the potentiometer setting to + 2 V,
Measured voltages : . . . . Channel1: 2V