Base Voltage Controls Collector Current Difference</h2>

Base Voltage Controls Collector Current Difference

Figure 1 Collector current comparison circuit.

Figure 2 The current, I₁, is greater than the current I₂ at low Vbe₂.

The circuit in figure 1 was simulated. Vbe₂ was varied and the collector currents were observed. Collector currents depend on base emitter voltage.

I_c = n I_se^Vbe/V_T

where n is the emitter area multiplication factor.

At low currents the Vbes of the two transistors are nearly equal, because the drop across R is small, and I₁ is greater than I₂ because N1 is bigger. As the base voltage increases, currents increase. The current in N₁ is limited by R to increase linearly, while the current in N₂ increases exponentially with Vbe₂.

There is an input voltage at which the currents are equal(about 0.65V in this simulation). If Vbe₂ increases above that value, I₂ > I₁. If it drops below it, I₂ < I₁.