Circuit for efficiently producing low-power radio frequency...

Details Circuit for efficiently producing low-power radio frequency... Circuit for efficiently producing low-power radio frequency...

2000-06-27

2001-11-13

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including plural amplifier channels

C330S302000

Reexamination Certificate

active

06317002

ABSTRACT:

The present invention relates to the low-power radio frequency communications art. Specifically, a power amplifier is disclosed which can provide multiple levels of modulated radio frequency signals at peak efficiency.
Cellular telephone technology requires the efficient generation and transmission of radio frequency signals in order to conserve battery power. A backed off power transmission mode has been implemented in the cellular telephone art so that the transmit power can be reduced when signal levels received at the base station are adequate using low power. Further, a reduced transmit power by the cellular telephone is necessary when operating near the base station to avoid overloading the base station receive circuit.
The use of multiple power levels in cellular telephone transmitting circuits is complicated by the fact that the active amplification devices, typically bipolar transistors, provide a peak efficiency at a single output power level. As the power is reduced the transistor efficiency dramatically decreases. Thus, under full power conditions, the power output transistors may operate close to 50% efficiency, their maximum, and in a backed-off power mode, the efficiency may drop to 10% or less.
The approach to provide multiple transmit power levels has been to operate the output transistors at different input signal drive and bias levels. When varying the bias or drive levels, the output impedance value remains for full power operations. At low power operations, the impedance is thus not optimal for reduced power operations and maximum efficiency. Accordingly, power transfer from the power output transistors to the antenna is not optimum, further reducing the net radio frequency power efficiency for the output transistors.
SUMMARY OF THE INVENTION
The present invention provides a circuit for generating multiple levels of low-power radio frequency signals. A plurality of amplifying transistors are connected to amplify a common radio frequency signal. Multiple output power levels are selectively obtained by selectively applying the signal for amplification to one or more of the transistor input terminals. Each of the transistors has a matching network connected to a single output terminal. The matching networks provide a substantially constant impedance to each collector when any number of the transistors are selected to amplify the signal. Each transistor may be operated at its maximum power efficiency generation level when selected for amplifying the input signal, thus efficiently producing a radio frequency signal for any selected power level.


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