Transfer and Output Characteristics:

The transfer characteristics are plot of the collector current i_C  as a function of the

base-to-emitter voltage v_BE along the collector-to-emitter voltage v_CE held constant. From

 i_S and ic  equations we can write

                           i_C  = I _SO  (1 +  (υ_CE / V_A ))  exp (υ _BE  / V_T )

It follows that i_C  varies exponentially with v_BE ,the plot of which is given in Figure It may be seen from the plot that the collector current is nesessary zero until the base-to-emitter voltage reaches a threshold value. Above this particular value, the collector current enhanced rapidly. The threshold value is typically in the range of 0.5 to 0.6 V. For high current transistors, usually it is smaller. The plot illustrates a single curve. If v_CE is enhanced, the current for a given vBE is larger. Though, the displacement between the curves is so small that it may be difficult to distinguish between them. The small-signal transconductance g_m is described as the slope of the transfer characteristics curve evaluated at the quiescent or dc operating point for a device.

Figure: BJT Transfer and Output Characteristics

The output characteristics are a plot of the collector current i_C as a function of the collector-to-emitter voltage v_CE with the base current i_B held constant. Through  i_c and  β eq we may write

i_C  = β₀ (1 +  (υ _CE  /V_A)  iB

It follows that i_C varies linearly with v_CE. A plot of this variation is given in Figure. For small v_CE such 0 ≤ v_CE < v_BE, Eq. (9) does not hold. In the region in Figure  where this holds, the BJT is saturated. The small-signal collector-to-emitter resistance ro is described as the reciprocal of the slope of the output characteristics curve evaluated at the quiescent or dc operating point for the device.