** 11 ** You can overcome the instability of the transistor amplifier discussed in the first ten problems of this chapter by adding two resistors to the circuit. Figure 8.5 shows an amplifier circuit to which resistors R_{E} and R_{2} have been added. R_{2}, along with R_{1} (labeled R_{B} in the previous circuits), ensures the stability of the DC bias point.

By adding the emitter resistor R_{E,} you ensure the stability of the AC gain.

The labels in Figure 8.6 identify the DC currents and voltages present in the circuit. These parameters are used in the next several problems.

Question

In designing an amplifier circuit and choosing the resistor values, there are two goals. What are they? _____

Answer

A stable DC bias point, and a stable AC gain

** 12 ** Look at the gain first. The gain formula for the circuit shown in Figure 8.6 is as follows:

This is a slight variation on the formula shown in problem 10. (The complex mathematical justification for this is not important right here.) Here, the AC gain is not affected by transistor β and transistor input resistance, so the AC gain will be constant, regardless of variations in these parameters.

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