OUTPUT CHARACTERISTICS:

Figure shows a "Common Emitter" amplifier stage.

Common Emitter Amplifier Stage

With no base/emitter bias applied, there will be no collector current flowing.  The output voltage V_CE will be equal to the supply voltage (40V).  As the collector current  I_C flows through resistor R_L there will be a voltage drop (I_C x R_L) across the resistor.  The collector voltage will depend upon the collector current i.e.  40V minus I_C x R_L  volts.

If  I_C  is zero, then V_CE must be 40v. V_CE  could be reduced to zero by a value of  I_C  of 40/R_L amperes.  If we assume R_L to have a value of 18 ohms, then V_CE will be zero for an IC of approximately 2.2 amperes.

Figure shows a graph plotting collector current against collector/emitter voltage
for different values of base current.

The straight line from the collector/emitter voltage axis to the collector current axis is the "Load Line" and indicates the only values of V_CE and I_C which are possible for that circuit.  Obviously, if a different value of R_L is used, we would get a different load line with a different slope.

For amplifiaction to take place we must first apply an initial bias voltage to the base.  With 1.3 V_BE applied the I_B will be 25mA giving an I_C of 1.4A which would produce an output of 15V (V_CE).  If we now apply a sinusoidal input signal of 0.6V, the base current would vary between 15 --35mA, which in turn would vary the IC between 1.1 - 1.7A, giving a voltage output between 10 - 20V.

This produces an amplifier gain of 0.6A/25mA = 30.  Figure shows the voltage/current waveforms of these variations.