____________________________________________________________________________ Amplifiers
Figure 5-17. Positive Feedback in a Transistor Amplifier
Figure 5-18. Positive Feedback in Two Stages of Transistor Amplification
5-72. Sometimes positive feedback is used to eliminate the effects of negative feedback
that are caused by circuit components. Figure 5-19 shows one way in which a circuit
component can cause negative feedback.
5-73. Figure 5-19, view (A) shows a CE transistor amplifier. An emitter resistor (R2) has
been placed in this circuit to provide proper biasing and temperature stability. An
undesired effect of this resistor is the development of a signal at the emitter in phase with
the input signal on the base. This signal is caused by the changing current through the
emitter resistor (R2) as the current through the transistor changes. You might think that this
signal on the emitter is a form of positive feedback since it is in phase with the input
signal; however, the emitter signal is really negative feedback. Current through the
transistor is controlled by the base-to-emitter bias. If both the base and emitter become
more positive by the same amount at the same time, current will not increase. It is the
23 June 2005
TC 9-62
5-19
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