Analyzing an Amplifier

28 Up to now, the emphasis has been on designing a simple amplifier and an emitter follower. This section shows how to “analyze” a circuit that has already been designed. In this case, to “analyze” means to calculate the collector DC voltage (the bias point) and find the AC gain. This procedure is basically the reverse of the design procedure.

Start with the circuit shown in Figure 8.27.

Following are the steps you use to analyze a circuit:

1. Find VB by using the following equation:
2. Find VE by using VE = VB − 0.7 volt.
3. Find IC by using the following equation:
Note that IC = IE.
4. Find VR by using VR = RC × IC.
5. Find VC by using VC = VS2VR. This is the bias point.
6. Find AV by using the following equation:

When you use the second formula, you must find the value of Rin (or hie) on the data sheets for the transistor from the manufacturer.

Use the circuit shown in Figure 8.28 for the following questions. For these questions, use β = 100, Rin = 2 kΩ and the values given in the circuit drawing.


Calculate ...

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