2-11
In addition to the other circuit components, an amplifying device (transistor or electronic tube),
itself, reacts differently to high frequencies than it does to low frequencies. In earlier NEETS modules you
were told that transistors and electronic tubes have interelectrode capacitance. Figure 2-5 shows a portion
of the interelectrode capacitance of a transistor and the way in which this affects high- and low-frequency
signals.
Figure 2-5.—Interelectrode capacitance of a transistor.
In view (A) a transistor is shown with phantom capacitors connected to represent the interelectrode
capacitance. CEB represents the emitter-to-base capacitance. CBC represents the base-to-collector
capacitance.
For simplicity, in views (B) and (C) the capacitive reactance of these capacitors is shown by variable
resistors R1 (for CEB) and R2 (for CBC). View (B) shows the reactance as high when there is a
low-frequency input signal. In this case there is very little effect from the reactance on the transistor. The
transistor amplifies the input signal as shown in view (B). However, when a high-frequency input signal
is applied to the transistor, as in view (C), things are somewhat different. Now the capacitive reactance is
low (as shown by the settings of the variable resistors). In this case, as the base of the transistor attempts
to go positive during the first half of the input signal, a great deal of this positive signal is felt on the
emitter (through R1). If both the base and the emitter go positive at the same time, there is no change in
emitter-base bias and the conduction of the transistor will not change. Of course, a small amount of
change does occur in the emitter-base bias, but not as much as when the capacitive reactance is higher (at
low frequencies). As an output signal is developed in the collector circuit, part of this signal is fed back to
the base through R2. Since the signal on the collector is 180 degrees out of phase with the base signal,
this tends to drive the base negative. The effect of this is to further reduce the emitter-base bias and the
conduction of the transistor.During the second half of the input signal, the same effect occurs although the
polarity is reversed. The net effect is a reduction in the gain of the transistor as indicated by the small
output signal. This decrease in the amplifier output at higher frequencies is caused by the interelectrode
capacitance. (There are certain special cases in which the feedback signal caused by the interelectrode
capacitance is in phase with the base signal. However, in most cases, the feedback caused by
interelectrode capacitance is degenerative and is 180 degrees out of phase with the base signal as
explained above.)
Q-4. What are the factors that limit the frequency response of a transistor amplifier?
