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.