Q.What happens to the base current when emitter base junction is reverse biased in CB amplifier?

In a common-base (CB) amplifier the weak input signal is applied to the emitter-base junction and the output is taken across the collector-base junction, with the base held at AC ground — hence 'common base'. The emitter-base junction is forward biased and the collector-base junction reverse biased, with coupling capacitors passing only the AC signal.

Working: on the positive half-cycle the input reduces the forward bias, so the emitter current $i_E$ falls; the collector current follows, $i_C = \alpha\, i_E$; the drop $i_C R_C$ falls, and since $V_{CB} = V_{CC} - i_C R_C$, the output rises. A positive input gives a positive output, so input and output are in phase (0 phase shift). If the emitter-base junction is instead reverse biased, carrier injection stops, the transistor is driven into cut-off, and the base current falls to nearly zero (only a small leakage remains). …