TC 9-62
6-25. For simplicity, Figure 6-4 (views (B) and (C)) shows the capacitive reactance of
these capacitors 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
affect 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 (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 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.
VIDEO AMPLIFIERS
6-26. You have seen that a transistor amplifier is limited in its frequency response.
Remember from chapter 5 that a VIDEO AMPLIFIER should have a frequency response
of 10 Hz to 6 MHz. The following describes how it is possible to "extend" the range of
frequency response of an amplifier.
HIGH-FREQUENCY COMPENSATION FOR VIDEO AMPLIFIERS
6-27. If the frequency-response range of an audio amplifier must be extended to 6 MHz
for use as a video amplifier, some means must be found to overcome the limitations of the
audio amplifier. You have seen that the capacitance of an amplifier circuit and the
interelectrode capacitance of the transistor (or electronic tube) cause the higher frequency
response to be limited.
6-28. In some ways capacitance and inductance can be thought of as opposites. As
already stated; as frequency increases, capacitive reactance decreases and inductive
reactance increases. Capacitance opposes changes in voltage and inductance opposes
changes in current. Capacitance causes current to lead voltage and inductance causes
voltage to lead current.
6-29. Frequency effects capacitive reactance and inductive reactance in opposite ways.
Since it is the capacitive reactance that causes the problem with high-frequency response,
inductors are added to an amplifier circuit to improve the high-frequency response. This is
called HIGH-FREQUENCY COMPENSATION. Inductors (coils), when used for high-
frequency compensation, are called PEAKING COILS. Peaking coils can be added to a
circuit so they are in series with the output signal path or in parallel to the output signal
path. Instead of only in series or parallel, a combination of peaking coils in series and
parallel with the output signal path can also be used for high-frequency compensation. The
6-8
TC 9-62
23 June 2005
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