2-15
Figure 2-8.—Combination peaking.
The "phantom" capacitor CT represents the total capacitance of the amplifier circuit. "Phantom"
capacitor CIN represents the input capacitance of the next stage. Combination peaking will easily allow an
amplifier to have a high-frequency response of 6 megahertz (6 MHz).
Q-8. What is the major factor that limits the high-frequency response of an amplifier circuits?
Q-9. What components can be used to increase the high-frequency response of an amplifier?
Q-10. What determines whether these components are considered series or shunt?
Q-11. What is the arrangement of both series and shunt components called?
LOW-FREQUENCY COMPENSATION FOR VIDEO AMPLIFIERS
Now that you have seen how the high-frequency response of an amplifier can be extended to 6
megahertz (6 MHz), you should realize that it is only necessary to extend the low-frequency response to
10 hertz (10 Hz) in order to have a video amplifier.
Once again, the culprit in low-frequency response is capacitance (or capacitive reactance). But this
time the problem is the coupling capacitor between the stages.
At low frequencies the capacitive reactance of the coupling capacitor (C2 in figure 2-8) is high. This
high reactance limits the amount of output signal that is coupled to the next stage. In addition, the RC
network of the coupling capacitor and the signal-developing resistor of the next stage cause a phase shift
in the output signal. (Refer to NEETS, Module 2, for a discussion of phase shifts in RC networks.) Both of
these problems (poor low-frequency response and phase shift) can be solved by adding a parallel RC
network in series with the load resistor. This is shown in figure 2-9.
