In what case the out put gain of non inverting amplifiers is one?

The output gain of a non-inverting amplifier is one when the feedback resistor (Rf) is equal to the input resistor (R1). This configuration is known as a unity gain amplifier, where the output voltage is equal to the input voltage.

The output gain of a non-inverting amplifier is one when the feedback resistor (Rf) is equal to the input resistor (Ri). In this case, the amplifier acts as a unity gain buffer, meaning that the output voltage is equal to the input voltage.