AM-FM transmitter power amplifier using class-BC

A power amplifier circuit for efficiently generating an output signal into a load at more than one alternative power level, comprising:

• at least two amplifying means;
• drive signal means, coupled to the at least two amplifying means, for selectively activating any one of the at least two amplifying means and for deactivating all amplifying means that have not been selected for activation, wherein the activated amplifying means generates the output signal to be supplied to the load;
• input means for receiving power from a common power source;
• and
• coupling means, coupled to the input means, to each of the at least two amplifying means and to the load, for supplying power from the common power source to each of the at least two amplifying means, and for coupling the at least two amplifying means to the load in a manner such that each of the at least two amplifying means has optimum efficiency at a correspondingly different power output level and such that signal power flowing from the activated amplifying means to the load is not impeded by the deactivated amplifying means,
• wherein the coupling means comprises at least two impedance matching networks, each coupled to the input means and to the load, and each further coupled to a corresponding one of the at least two amplifying means, wherein an impedance transformation of a first one of the impedance matching networks that is coupled to a first one of the amplifying means is different from an impedance transformation of a second one of the impedance matching networks that is coupled to a second one of the amplifying means, whereby optimum efficiency of the first amplifying means occurs at a first power output level and optimum efficiency of the second amplifying means occurs at a second power output level, the first and second power output levels not being equal to one another,
• and wherein further:
• each of the at least two impedance matching networks comprises:
• a quarter-wavelength line characterized by a matching ratio;
• and
• a balun having a secondary winding coupled to the quarter-wavelength line and a primary winding having a center tap coupled to the input means and remaining taps for coupling to a corresponding one of the at least two amplifying means;
• and
• a first one of the at least two impedance matching networks has a first matching ratio and a second one of the at least two impedance matching networks has a second matching ratio, the first matching ratio not being equal to the second matching ratio.

A method for operating a power amplifier circuit to generate an output signal into a load at more than one alternative power level, the power amplifier circuit comprising at least two amplifying means, each coupled to the load and each having optimum efficiency at different power output levels, the method comprising the steps of:

• generating a control signal that is independent of the output signal;
• using the control signal to select any one of the at least two amplifying means;
• activating the selected amplifying means to produce an amplified signal;
• concurrent with the activating step, deactivating all amplifying means that have not been selected for activation;
• supplying the amplified signal from the activated amplifying means to the load;
• and
• preventing the deactivated amplifying means from consuming signal power from the amplified signal,
• wherein the step of generating a control signal comprises generating a control signal based on a modulating signal having a frequency that is slower than that of a carrier signal being supplied to the power amplifier circuit.