In this condition, there is, ideally, an open between collector and emitter, as indicated by the switch equivalent. In part (a), the transistor is in the cutoff region because the base-emitter junction is not forward-biased.
C3 is the emitter decoupling capacitor.The below Figure illustrates the basic operation of a BJT as a switching device. R4 is the emitter stabilizing resistor. R1 and R2 are the base bias resistors, setting the bias point. If the transistors have the same characteristics, then resistor values are the same in both circuits. The polarity of the capacitors is reversed. Small Signal Amplifier The two transistor types have opposite polarity power supplies. For such purpose, Q should be set midway between Qsat and Qcutoff to allow for the maximum possible voltage excursions. The Transistor as an Amplifier When the DC operating point Q is in the active region, the transistor can act as an amplifier to AC signals. The AC equivalent circuit could be drawn as shown here where I The equations for the AC currents are and Of course we also have ie = ib + ic. However, we will assume them to be the same in this course. There are AC parameters and, which are in general slightly different from their DC counterparts. The Transistor as an Amplifier AC Model and Parameters We will also need an AC model for the transistor. Of course, in this mode the transistor is off. The cutoff value of VCE is : VCE(cutoff) = VCC At the other extreme, Qcutoff the transistor is cutoff. In saturation mode, the transistor is on, allowing maximum current to flow. 
The saturation value of the collector current is : In order to achieve saturation, VBBmust be large enough to ensure that IBis greater than which means that VBB must satisfy:
The saturation value of VCE is denoted VCE(sat) and is typically 0.2 V. As IC increases from zero, the voltage drop across RC increases, and so VCE must decrease until the BE junction becomes forward biased and the transistor saturates. The operating point Qsat corresponds to the maximum value of IC. The Transistor as a Switch The two extreme possible operating points Qsat and Qcutoff show how the transistor can act as a switch.
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