For the amplifier shown in the figure, the BJT parameters are V_BE = 0.7 V, $$\beta $$ = 200, and thermal voltage V_T = 25 mV. The voltage gain (V_o/V_i of the amplifier is ______.

In the voltage regulator circuit shown in the figure, the op - amp is ideal. The BJT has V_BE = 0.7 V and $$\beta $$ = 100, and the Zener voltage is 4.7 V. For a regulated output of 9 V, the value of R (in $$\Omega $$) is ______

A good current buffer has

In the ac equivalent circuit shown in the figure, if i$${_F}$$ is very large, the type of feedback is