Electronics W8 Lab help

For the circuit of example 10-6

Without the AC signal

The AC voltage drop applied to the base is given by

Figure 1

Figure 1 shows the value of the AC signal fed to the base of transistor. It varies greatly from the theoretical value because while the theoretical calculation procedure only puts into consideration the resistor in the path between the signal source and ground, the signal experiences a circuit comprising several resistors connected in parallel. It is also affected by the relatively large DC current flowing from the DC source to the base. The signal experiences low resistance paths in some cycles where the current flowing from the DC source tends to cancel out the signal current, giving rise to the low signal voltage at the voltage-divider junction

The coupling capacitor also offers some reactance which was neglected in finding the theoretical value and thus contributed to the error.

Figure 2

The experimental value of output voltage is slightly larger than the theoretical value. This can be explained by the fact that the capacitors were treated as short-circuits while in real sense they offer reactance to the flow of the AC current. The feedback capacitor contributes very little to this since it is in parallel with a low resistance component which brings the effective impedance of that section close to zero. The decoupling capacitor on the other hand is connected in series with a resistor. If included, the reactance would have contributed to a larger impedance and thus a larger volt drop in the output circuit.