The negative fed back OPamp, means the scheme where a "portion" of output voltage of OPamp is fed to its inverting input, shows a remarkable behaviour: The OPamp tries everything to keep the input voltage minimal, ideally to zero volt! (By "input voltage" the difference of "+" input and "-" input is meant, just because an OPamp always amplifies this input difference voltage.)

This can very easily be understood from the following:

Assume the "+" input is forced to GND (signal ground, 0V). When the potential at "-" input is negative now, the input voltage of OPamp is positive and the output voltage, as being a highly amplified copy of the input voltage, is also positive. On the other hand, when the input voltage of OPamp is negative, because the "-" input is positive, then the output voltage is also negative.

So, when the "-" input is positive, then the output is negative, and, vice versa, when the "-" input is negative, then the output is positive. That's no surprise, of course, just because that's the reason why the "-" input is called "-": The "-" input causes a change of polarity of output.

When now a portion of output voltage is fed back to the "-" input, then, due to the polarity change, the input voltage of OPamp is automatically controlled to nearly zero volt! The higher the internal gain of OPamp, the closer the input voltage goes to zero.

So, what happens in your circuit?

When the input current, flowing to the "-" input and then further through Rfb, is zero, then the voltage drop across Rfb is also zero. So, to control the input voltage ("+" input - "-" input) to zero volt, the output of OPamp has to emit 0V too.

But when the input current is 1mA, for instance, and Rfb is 10kOhm, for instance, then the voltage drop across Rfb will be 10V. So, to control the input voltage of Opamp to zero volt, the output of OPamp must emit -10V. This, to compensate for the voltage drop across Rfb.

Kai