Telecommunications – Transmitter
Reexamination Certificate
1998-12-10
2001-10-30
Le, Thanh Cong (Department: 2684)
Telecommunications
Transmitter
C455S042000, C455S102000, C455S013400, C455S127500, C455S522000
Reexamination Certificate
active
06311046
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to power amplifiers and amplifying methods, and more particularly to high-efficiency power amplifiers and related methods
BACKGROUND OF THE INVENTION
Power amplifiers are widely used in communication systems, for example in radiotelephone base stations and radiotelephones. In radiotelephone communication systems, power amplifiers typically amplify high frequency signals for transmission.
A major consideration in the design of power amplifiers is the efficiency thereof. High efficiency is generally desirable so as to reduce the amount of power that is dissipated as heat. Moreover, in many applications, such as in satellites and portable radiotelephones, the amount of power that is available may be limited. An increase in efficiency in the power amplifier is therefore important, in order to allow an increase the operational time or capacity for the satellite or portable radiotelephone.
A conventional power amplifier such as a class-B amplifier generally only provides maximum efficiency at or near to its maximum saturated power output level. In order to accurately reproduce a signal of varying amplitude, the peak output signal level should be equal to or less than that maximum saturated power level. When the instantaneous signal output level is less than the peak, a conventional class-B power amplifier generally operates at less than maximum efficiency.
The efficiency generally reduces as the square root of the output power. This is because, using the class-B example, the output power reduces as the square of the output current but the power consumption from the battery or other DC supply reduces only proportional to the output current. Therefore, the efficiency, which is the ratio of output power to battery power, reduces proportional to the current, i.e., proportional to the square root of the output power.
Accordingly, a power amplifier that has 60% efficiency at a peak output of 2 watts will generally have no more than 42% efficiency at an output of 1 watt (3 dB reduced output). Moreover, when amplifying a signal of varying amplitude, a conventional amplifier may not produce an output signal amplitude proportional to the input signal amplitude, thereby causing nonlinear distortion and intermodulation.
With a varying output signal power P(t)=A
2
(t), the average efficiency can be estimated to be:
Max
efficiency
×
average
of
(
P
(
t
)
/
Pmax
)
average
of
square
root
(
P
(
t
)
/
Pmax
)
Max
efficiency
×
average
of
(
A
(
t
)
/
Amax
)
2
average
of
(
A
(
t
)
/
Amax
)
.
Nonlinearities in conventional amplifiers can be reduced by various techniques, such as by an inverse predistortion of the input signal, or by feedback including Cartesian feedback in radio frequency power amplifiers for linearly amplifying signals with a bandwidth much less than the center frequency. Unfortunately, linearization generally does not alter the above efficiency formula, which in fact already assumes that the output amplitude can be made to faithfully follow the desired varying amplitude waveform. In effect, the average efficiency calculated above already assumes perfect linearization.
The loss of efficiency comes about because current I(t) is drawn from the battery at a constant voltage Vcc, but is supplied to the load at a varying voltage I(t)·RL which is less than Vcc. The voltage difference Vcc−I(t)·RL is lost across the output device (e.g. collector junction), causing power dissipation in the device.
In U.S. Pat. No. 2,210,028 to Doherty (Aug.1940), an arrangement of two vacuum tube power amplifiers coupled by a single quarter-wave line is described. The first amplifier is operated up to an output level of Pmax/4, at which it achieves maximum practical class-B efficiency. For powers above this level, the second amplifier is caused to contribute. The second amplifier affects the load impedance of the first amplifier one quarter wave away such that the first amplifier can increase its power up to Pmax/2, while the second amplifier also contributes up to Pmax/2, making Pmax in total, at which point both amplifiers are once more achieving maximum practical class-B efficiency. Thus, efficiency is preserved over a 6 dB range of output levels from Pmax/4 to Pmax. A semiconductor version of the Doherty amplifier is described in a more recent U.S. Pat. No. 5,420,541 entitled “Microwave Doherty Amplifier” to Upton et al.
In the prior art Doherty amplifier, the “normal” power amplifier amplifies a signal from 0 power to ¼ the peak power level, achieving maximum class-B efficiency at that power level. The peak power amplifier then begins to contribute to the output power and by reducing the effective load impedance seen by the “normal” power amplifier, enables it to generate a greater power output up to half the peak power level. The peak power amplifier also generates half the peak power level so that the two amplifiers jointly produce the desired peak power level. The “peak” power amplifier in this prior art is not operated in antiphase so as to detract from the output power level, and thereby increasing the effective load impedance seen by the “normal” power amplifier and allowing it to generate less power efficiently. Thus the “peak” power amplifier does not operate symmetrically as a “trough” power amplifier.
In Proc. IRE, Vol. 23 No. 11 (1935), pages 1370-1392, entitled “
High Power Outphasing Modulation
”, Chireix describes producing a transmitter giving a modulated amplitude output signal by combining two constant output amplitude amplifiers with a variable phase difference so that their outputs can be varied in relative phase from additive to subtractive. The Chireix and Doherty techniques were not combined to obtain an amplifier of good linearity and high efficiency, as the Doherty amplifier relied on the two constituent amplifiers being co-phased while the Chireix amplifier relied upon them being out-of-phase. When two amplifiers are out-of-phase, as they were in the prior art, they are preferably isolated from one another using a hybrid coupler or directional coupler to combine them. The directional coupler combines the two amplifier's output signals to produce a sum signal and a difference signal, the sum signal being used as the desired output and the difference signal being terminated in a dummy load. Since all the amplifier power ends up at either the sum or the difference port and is not reflected to either amplifier, the amplifiers are isolated from one another and do not affect each other's load line.
In U.S. Pat Nos. 5,568,088; 5,574,967; 5,631,604; and 5,638,024 to applicant Dent, all entitled “Waste Energy Control and Management in Power Amplifiers ”, various arrangements of coupled power amplifiers are disclosed in which a varying amplitude signal may be produced using constant amplitude power amplifiers. In one arrangement, two constant power amplifiers are driven with a relative phase shift as in Chireix such that their outputs add more or less constructively or destructively to produce a varying output. The amplifiers were coupled at their outputs using a hybrid coupler or directional coupler which forms both a sum signal and a difference signal. An improvement over the prior art described therein comprises recovering the normally wasted energy at the difference port using a rectifier circuit. The Doherty patent, the Chireix paper and the above referenced Dent patents are hereby incorporated by reference herein.
In applicant's 1964 graduate thesis project, an amplifier was built and reported in which the value of Vcc was selected to be either Vcc or 0.7 Vcc based on whether the desired output amplitude was greater or less than 0.7 Vcc. With a pure sine wave drive, this raised the peak efficiency from the theoretical value of &pgr;/4 (~78.5%) for a class-B amplifier to 85.6%
Cong Le Thanh
Ericsson Inc.
Myers Bigel & Sibley & Sajovec
Woldetatios Yemane
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