2-12
Q-5. What type of feedback is usually caused by interelectrode capacitance?
Q-6. What happens to capacitive reactance as frequency increases?
Q-7. What happens to inductive reactance as frequency increases?
VIDEO AMPLIFIERS
As you have seen, a transistor amplifier is limited in its frequency response. You should also
remember from chapter 1 that a VIDEO AMPLIFIER should have a frequency response of 10 hertz (10
Hz) to 6 megahertz (6 MHz). The question has probably occurred to you: How is it possible to "extend"
the range of frequency response of an amplifier?
HIGH-FREQUENCY COMPENSATION FOR VIDEO AMPLIFIERS
If the frequency-response range of an audio amplifier must be extended to 6 megahertz (6 MHz) for
use as a video amplifier, some means must be found to overcome the limitations of the audio amplifier.
As you have seen, the capacitance of an amplifier circuit and the interelectrode capacitance of the
transistor (or electronic tube) cause the higher frequency response to be limited.
In some ways capacitance and inductance can be thought of as opposites. As stated before, 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.
Since frequency affects capacitive reactance and inductive reactance in opposite ways, and 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.
As in all electronic circuits, nothing comes free. The use of peaking coils WILL increase the
frequency response of an amplifier circuit, but it will ALSO lower the gain of the amplifier.
Series Peaking
The use of a peaking coil in series with the output signal path is known as SERIES PEAKING.
Figure 2-6 shows a transistor amplifier circuit with a series peaking coil. In this figure, R1 is the
input-signal-developing resistor. R2 is used for bias and temperature stability of Q1. C1 is the bypass
capacitor for R2. R3 is the load resistor for Q1 and develops the output signal. C2 is the coupling
capacitor which couples the output signal to the next stage. "Phantom" capacitor COUT represents the
output capacitance of the circuit, and "phantom" capacitor CIN represents the input capacitance of the next
stage.
