NPN Transistor Load Line Analysis

Self-bias Operating Point

traditional transistor load line

The plot above is a textbook transistor load line plot for a simple test circuit shown below.

The collector current Ic of this NPN transistor is plotted as a function of its VCE voltage, while its base (gate) current Ib is swept.

However, the effect of R1 isn’t very obvious in such a plot. Rather a new way of looking at the circuit is to plot the external voltage source V1 itself as the x-axis, as shown in the following plot

V1 on the x-axis ranges from 0 to 3V and the collector current Ic is still on the y-axis. There are two bundle of curves, one from R1 = 1k-Ohm, the other from R1 = 2k-Ohm.

Two key points from these bundles of curves:

1. There are two easy to distinguish straight lines. They look like that the voltage source V1 is simply driving a resistor R1. For example, a 2k-Ohm driven with 3V should give a straight line up to 1.5mA. The current is a bit smaller, because the…