Dynamic Current Amplification:

The other way of identifying current amplification is as the ratio of a difference in I_C to a small incremental difference in I_B which generates it. This is the dynamic current amplification, also termed as current gain. It is usual to abbreviate the words the difference in by the uppercase Greek letter delta (Δ) in mathematical expressions. After that, according to this definition,

Current gain = ΔI_C/ΔI_B

The ratio ΔI_C/ΔI_B is maximum where the slope of the characteristic curve is steepest. Geometrically, ΔI_C/ΔI_B at any specified point on the curve is the slope of a line tangent to the curve at that point.

Whenever the operating point of a transistor is on the steep portion of the characteristic curve, the device has the major possible current gain, as time-consuming as the input signal is small. This value is close to H_FE. Since the characteristic curve is a straight line in this area, the transistor can serve as a linear amplifier when the input signal is not so strong. This means that the output signal waveform is a truthful reproduction of the input signal waveform, apart from that the output amplitude is bigger than the input amplitude.

Since the operating point is shifted into the portion of the characteristic curve where the graph is not straight, the current gain reduces, and the amplifier becomes non-linear. The similar thing can occur if the input signal is strong sufficient to drive the transistor into the non-linear portion of the curve during any part of the signal cycle.