2-24
Neutralization of RF Amplifiers
The problem of interelectrode capacitance in the transistor of an rf amplifier is solved by
NEUTRALIZATION. Neutralization is the process of counteracting or "neutralizing" the effects of
interelectrode capacitance.
Figure 2-17 shows the effect of the base-to-collector interelectrode capacitance in an rf amplifier.
The "phantom" capacitor (CBC) represents the interelectrode capacitance between the base and the
collector of Q1. This is the interelectrode capacitance that has the most effect in an rf amplifier. As you
can see, CBC causes a degenerative (negative) feedback which decreases the gain of the amplifier. (There
are some special cases in which CBC can cause regenerative (positive) feedback. In this case, the
technique described below will provide negative feedback which will accomplish the neutralization of the
amplifier.)
Figure 2-17.—Interelectrode capacitance in an rf amplifier.
As you may recall, unwanted degenerative feedback can be counteracted (neutralized) by using
positive feedback. This is exactly what is done to neutralize an rf amplifier.
Positive feedback is accomplished by the use of a feedback capacitor. This capacitor must feed back
a signal that is in phase with the signal on the base of Q1. One method of doing this is shown in figure
2-18.
