____________________________________________________ Video and Radio Frequency Amplifiers
6-19. The actual circuit components are C1, C2, C3, R1, R2, R3, and Q1. C1 is used to
couple the input signal. R1 develops the input signal. R2, the emitter resistor, is used for
proper biasing and temperature stability. C2 is a decoupling capacitor for R2. R3 develops
the output signal. C3 couples the output signal to the next stage. Q1 is the amplifying
6-20. The phantom circuit elements representing the capacitance and inductance of the
wiring are L1, L2, C4, and C5. L1 represents the inductance of the input wiring. L2
represents the inductance of the output wiring. C4 represents the capacitance of the input
wiring. C5 represents the capacitance of the output wiring.
6-21. Figure 6-3, view (A) shows the circuit with a low-frequency input signal.
Remember that if frequency is low, capacitive reactance will be high and inductive
reactance will be low. This is shown by the position of the variable resistors that represent
the reactances. Notice that XL1 and XL2 are low. Therefore, they do not "drop" very much
of the input and output signals. However, XC4 and XC5 are high. Therefore these reactances
tend to "block" the input and output signals and keep them from going to the power
supplies (VBB and VCC). Notice that the output signal is larger in amplitude than the input
6-22. Figure 6-3, view (B) shows that the input signal is a high-frequency signal. Now
XC is low and XL is high. XL1 and XL2 now drop part of the input and output signals. At
the same time XC4 and XC5 tend to "short" or "pass" the input and output signals to signal
ground. The net effect is that both the input and output signals are reduced. Notice that the
output signal is smaller in amplitude than the input signal. Now you can see how the
capacitance and inductance of the wiring affects an amplifier causing the output of an
amplifier to be less for high-frequency signals than for low-frequency signals.
6-23. In addition to the other circuit components, an amplifying device (transistor or
electronic tube) reacts differently to high frequencies than it does to low frequencies.
Transistors and electronic tubes have interelectrode capacitance. Figure 6-4 shows a
portion of the interelectrode capacitance of a transistor and the way in which this affects
high- and low-frequency signals.
Figure 6-4. Interelectrode Capacitance of a Transistor
6-24. Figure 6-4, view (A) shows a transistor with phantom capacitors connected to
23 June 2005