By virtue of the open-loop gain being infinity for an ideal op-amp, the two input terminals are said to be virtually shorted.
Yet, the virtual short concept is only applied to op-amps in negative feedback configuration, and not in case of positive feedback.
Is there any plausible reason for this?
Some insight in this regard would be really helpful.
Best Answer
Positive feedback does not result in stable, linear operation. In a positive feedback amplifier the output adds to the input, resulting in a larger input, resulting in a larger output. In other words, the output accelerates quickly in 1 direction until it can't anymore. In a circuit the limitation is the power supply and the output of the amplifier will be saturated at/near the power supply voltage.
When the output of the amplifier is saturated at/near the power supply voltage, it is not operating as a linear amplifier. The output voltage can no longer be described as GAIN*INPUT. No matter what the input is, the output is the power supply rail voltage.
The reason why you get a virtual short with negative feedback is because negative feedback stabilizes the op amp in linear operation, and the output voltage can be described as GAIN*INPUT. Since the gain is very high, the input is very small. This is the condition of the virtual short.
This is a general explanation, and is overlooking details such as closed-loop vs open-loop gain. Maybe some one else can provide a more detailed answer.