2-19Figure 2-12.—Semiblock diagram of rf amplifier.In this "semi-block" diagram, C1 and C2 are the input and output coupling capacitors. R1 representsthe impedance of the input circuit. The blocks marked FDN represent the frequency-determiningnetworks. They are used as input-signal-developing and output impedances for Q1.FREQUENCY-DETERMINING NETWORK FOR AN RF AMPLIFIERWhat kind of circuit would act as a frequency-determining network? In general, a frequency-determining network is a circuit that provides the desired response at a particular frequency. Thisresponse could be maximum impedance or minimum impedance; it all depends on how the frequency-determining network is used. You will see more about frequency-determining networks in NEETS,Module9—Introduction to Wave-Generation and Shaping Circuits. As you have seen, the frequency-determining network needed for an rf amplifier should have maximum impedance at the desiredfrequency.Before you are shown the actual components that make up the frequency-determining network for anrf amplifier, look at figure 2-13, which is a simple parallel circuit. The resistors in this circuit are variableand are connected together (ganged) in such a way that as the resistance of R1 increases, the resistance ofR2 decreases, and vice versa.Figure 2-13.—Parallel variable resistors (ganged).If each resistor has a range from 0 to 200 ohms, the following relationship will exist between theindividual resistances and the resistance of the network (R_{T}). (All values are in ohms, R_{T} rounded off totwo decimal places. These are selected values; there are an infinite number of possible combinations.)

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