These devices are also available in a TO-5 package. C3 is the emitter decoupling capacitor.This family of 2N2904 and 2N2905A switching transistors are military qualified up to the JANS level for high-reliability applications.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.
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