Digital Electronics - Signals and Switches - Discussion

From figure, when base current is maximum in the positive direction,collector emmiter voltage become max'm in the negative direction and vise-versa.

Thus, input and output voltages are in phase opposition ie.the transister has produced a phase reversal of output voltage w.r.t. the input signal voltage.

I think here should be only reason behind it an application of CE configuration of transistor that exhibits phase reversal characteristics.

If the input signal is high there will flow current through 1 megohm and the transistor's base-emitter junction (base, not gate). This current will be amplified, and the collector current through 1 kohm will cause a voltage drop so that the output will be low. Input high, output low.

If the input signal is low there won't be any base current, and no collector current. No current through R1 means no voltage drop, so that the output will be at +V. Input low, output high.