NP= number of turns in the primary
NS= number of turns in the secondary
ZP= impedance of the primary
As you can see, impedance matching between stages can be accomplished by a
combination of the amplifier configuration and the components used in the amplifier
5-60. Turning down the volume of a public address system, when a squeal or high-
pitched noise comes from the speaker, will cause the noise to stop. That noise was an
indication that the amplifier (at least one stage of amplification) had begun oscillating. All
you need to know for now is that oscillation is caused by a small part of the signal from the
amplifier output being sent back to the input of the amplifier. The signal is amplified and
again sent back to the input where it is amplified again. This process continues and the
result is a loud noise out of the speaker. The process of sending part of the output signal of
an amplifier back to the input of the amplifier is called feedback.
5-61. The two types of feedback in amplifiers are called POSITIVE FEEDBACK (also
called regenerative feedback) and NEGATIVE FEEDBACK (also called degenerative
feedback). The difference between these two types is whether the feedback signal is "in
phase" or "out of phase" with the input signal.
5-62. Positive feedback occurs when the feedback signal is in phase with the input
signal. Figure 5-14 shows a block diagram of an amplifier with positive feedback. Notice
that the feedback signal is in phase with the input signal. This means that the feedback
signal will add to or "regenerate" the input signal. The result is a larger amplitude output
signal than would occur without the feedback. This type of feedback is what causes the
public address system to squeal as described above.
5-63. Figure 5-15 is a block diagram of an amplifier with negative feedback. In this case
the feedback signal is out of phase with the input signal. This means that the feedback sign
will subtract from or "degenerate" the input signal. This results in a lower amplitude output
signal than would occur without the feedback.
5-64. Sometimes feedback that is not desired occurs in an amplifier. This happens at
high frequencies and limits the high-frequency response of an amplifier. Unwanted
feedback also occurs as the result of some circuit components used in the biasing or
coupling network. The usual solution to unwanted feedback is a feedback network of the
opposite type. For example, a positive feedback network would counteract unwanted
23 June 2005