Tuned Power Amplifiers

The basic circuit of a tuned power amplifier is substantially that of the single-tuned direct-coupled type discussed in Sec. 10-1. The essential differences are in the magnitude of the grid-bias supply voltage Ecc, the corresponding value of the grid input signal eg, and the amount of power involved. A schematic diagram of a tuned power amplifier is given in Fig. 11-1.

Owing to the negative bias on the tube, which is adjusted approximately to plate-current cutoff in the class B amplifier and which is adjusted beyond plate-current cutoff in the class C amplifier, harmonic currents are generated in the plate which are comparable in amplitude with the fundamental component. However, if the Q of the tuned plate circuit has a value of 10 or more, the impedance of the tank circuit to the second or higher harmonics will be very low. As a result, the higher harmonic potentials across the tank will be very small compared with the fundamental potential. That is, the effect of the harmonic generation in the tube plate current is largely suppressed by the tuned plate load.

But the requirement that the Q of the tank circuit must be high in order to suppress harmonics in the output imposes a limitation on the frequency-response characteristics of the amplifier, since the gain is constant over a very narrow band of frequencies. Consequently such amplifiers are confined in their operation to narrow frequency bands. In fact, the class B amplifier may be used to amplify a narrow band of frequencies of differing amplitudes, whereas the class C amplifier is confined to a narrow ■ band of frequencies of constant amplitudes. Despite these severe restrictions, both classes of amplifier are extensively used in restricted applications, the class B amplifier to amplify an a-m r-f carrier wave, the class C amplifier as a frequency multiplier or as a source for the production of an a-m carrier wave.

Fig. 11-1. Schematic diagram of a tuned power amplifier.

ll-l. Properties of the Tank Circuit. The tuned plate load in the diagram of Fig. 11-1 is drawn as a simple parallel resonant circuit. Ordinarily the load is coupled inductively to the plate tank, and a more typical coupling network is that shown in Fig. 11-2. The capacitor C2

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