Figure 2-9.Low frequency compensation network.
The complete circuitry for Q2 is not shown in this figure, as the main concern is the
signal-developing resistor (R5) for Q2. The coupling capacitor (C2) and the resistor (R5) limit the
low-frequency response of the amplifier and cause a phase shift. The amount of the phase shift will
depend upon the amount of resistance and capacitance. The RC network of R4 and C3 compensates for
the effects of C2 and R5 and extends the low-frequency response of the amplifier.
At low frequencies, R4 adds to the load resistance (R3) and increases the gain of the amplifier. As
frequency increases, the reactance of C3 decreases. C3 then provides a path around R4 and the gain of the
transistor decreases. At the same time, the reactance of the coupling capacitor (C2) decreases and more
signal is coupled to Q2.
Because the circuit shown in figure 2-9 has no high-frequency compensation, it would not be a very
practical video amplifier.
TYPICAL VIDEO-AMPLIFIER CIRCUIT
There are many different ways in which video amplifiers can be built. The particular configuration of
a video amplifier depends upon the equipment in which the video amplifier is used. The circuit shown in
figure 2-10 is only one of many possible video-amplifier circuits. Rather than reading about what each
component does in this circuit, you can see how well you have learned about video amplifiers by
answering the following questions. You should have no problem identifying the purpose of the
components because similar circuits have been explained to you earlier in the text.