Bipolar Junction Transistors in Active Mode

Though the characteristic curves for a transistor are usually generated in a circuit where base current is constant and the collector-emitter voltage (V_CE) is varied, this is usually not how transistor amplifier circuits are constructed. Typically, the base current varies with the input signal, and the collector power supply is a fixed-voltage source:

The presence of a load resistor in the circuit adds another dynamic to the circuit’s behavior. Explain what happens to the transistor’s collector-emitter voltage (V_CE) as the collector current increases (dropping more battery voltage across the load resistor), and qualitatively plot this load line on the same type of graph used for plotting transistor curves:

Calculate and superimpose the load line for this circuit on top of the transistor’s characteristic curves:

Then, determine the amount of collector current in the circuit at the following base current values:

I_B = 10 μA

I_B = 20 μA

I_B = 30 μA

I_B = 40 μA

In this graph you will see three different load lines plotted, representing three different values of load resistance in the amplifier circuit:

Which one of the three load lines represents the largest value of load resistance (R_load)? Which of the three load lines will result in the greatest amount of change in voltage drop across the transistor (∆V_CE) for any given amount of base current change (∆I_B)? What do these relationships indicate about the load resistor’s effect on the amplifier circuit’s voltage gain?