Three basic procedures can be used to obtain a high-power AM signal: base-bias modulation, AM signal amplification, and collector modulation [1, 2, 4, 5, 9, 25]. Combined modulation techniques can also be used to improve circuit performance.

In base-bias modulation, the low-frequency modulating signal is used to control the base-bias of a Class C operated transistor. A change in the base-bias voltage affects the conduction angle, and thus the amplitude of the output signal, generating an AM signal. Base-modulation is not generally used due to several disadvantages, such as low efficiency (a typical maximum value is about 40 percent), significant nonlinearity of the modulation characteristics, and adverse operating conditions for the amplifier providing the modulating signal. (It operates into the base-emitter capacitance whose value varies with the signal level.) Although somewhat better results can be obtained using FETs instead of BJTs, this method is used only in very low-cost, simple transmitters. Base-bias modulation is also sometimes used in collector-modulated stages to improve the linearity of the modulation characteristics [4, 5].

AM signal amplification is widely used in SSB transmitters. The SSB signal is generated easily at low power levels and must be amplified linearly to the desired power level. This is usually accomplished with Class AB push-pull linear amplifiers [4, 5].

The main form of amplitude modulation used in solid-state RF amplifiers is collector modulation [2, 4, 5, 9, 25, 56]. This technique can be used directly to obtain an AM double sideband (DSB) signal. In an...