Complete Electronics Self-Teaching Guide with Projects

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.

Figure 8.27

Following are the steps you use to analyze a circuit:

1. Find V_B by using the following equation:

2. Find V_E by using V_E = V_B − 0.7 volt.

3. Find I_C by using the following equation:

Note that I_C = I_E.

4. Find V_R by using V_R = R_C × I_C.

5. Find V_C by using V_C = V_S2V_R. This is the bias point.

6. Find A_V by using the following equation:

When you use the second formula, you must find the value of R_in (or h_ie) 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, R_in = 2 kΩ and the values given in the circuit drawing.

Figure 8.28

Questions

Calculate ...