A Stable Amplifier

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 RE and R2 have been added. R2, along with R1 (labeled RB in the previous circuits), ensures the stability of the DC bias point.

By adding the emitter resistor RE, 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.


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


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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