Biasing for current amplification

Because a small change in the base current, I_B, results in a large collector-current (I_C) variation when the bias is just right, a transistor can operate as a current amplifier. It might be more technically accurate to say that it is a “current-fluctuation amplifier,” because it’s the magnification of current variations, not the absolute current, that’s important.

You’ll see that there are some bias values at which a transistor won’t give current amplification. If the E-B junction is not conducting, or if the transistor is in saturation, the curve is horizontal. A small change (to the left and right) of the base voltage, E_B, in these portions of the curve, will cause little or no up-and-down variation of I_C.
But if the transistor is biased near the middle of the straight-line part of the curve, the transistor will work as a current amplifier.