Totem pole rf amplifiers with phase and amplitude correction

Details Totem pole rf amplifiers with phase and amplitude correction Totem pole rf amplifiers with phase and amplitude correction

2002-06-20

2003-11-18

Choe, Henry (Department: 2817)

Amplifiers

With plural amplifier channels

Redundant amplifier circuits

C330S295000

Reexamination Certificate

active

06650180

ABSTRACT:

STATEMENT RE FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
REFERENCE TO SEQUENCE LISTING
Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to apparatus and method for splitting, amplifying, and combining radio frequency (rf) or microwave rf signals. More particularly, the present invention pertains to rf power amplifiers in which field-effect devices are connected in series to dividingly share a dc supply voltage, and in which both apparatus and method are provided for phase correcting and power equalizing rf outputs prior to combining.
2. Description of the Related Art
Gallium arsenide field-effect transistors (GaAsFETs) are the primary solid state devices used for amplification of high frequency signals in the range of 3 Ghz and higher. GaAsFETs have the advantages of being readily available and relatively inexpensive. However, a major disadvantage of GaAsFETs is that the maximum operating voltage is commonly +10.0 volts dc.
For many transmitter/amplifier applications, particularly airborne applications, the dc supply voltage is 28 volts dc, plus or minus 4.0 volts dc. Since gallium arsenide FETs have an operative voltage of +10 volts dc, the use of gallium arsenide FETs has presented a problem.
Traditionally, there have been two solutions to this problem. One is to use a linear voltage regulator. The other is to use a switching regulator.
In linear voltage regulators, the voltage is linearly regulated from the supply of 28 volts to approximately 10 volts with the power difference being dissipated in heat by the regulator. This type of regulation has the disadvantages of excessive heat and low power efficiency.
Switching regulators, on the other hand, are power converters that transfer the power of a higher voltage supply to lower voltage with increased current capacity. This type of regulation has the advantage of low heat dissipation and high power efficiency, but has the disadvantages of increased costs, space inefficiency due to their large size, and the creation of a spurious signal on the rf carrier (EMI problems) due to the switching action of the regulator. A high-attenuation filter is required to suppress this spurious switching signal.
A third approach to solving the problem of disparity between the operating voltage of solid-state current devices and a source voltage has been to connect the solid-state current devices in series, thereby dividingly sharing the source voltage and utilizing the same current flow two or more times. This third approach was presented in
IEEE Transactions on Microwave Theory and Techniques
, Volume 46, Number 12, of December 1998, in an article entitled, “
A
44-
Ghz High IP
3
InP
-
HBT Amplifier with Practical Current Reuse Biasing.”
This type of circuit solves the problem of the disparity between the operating voltage of solid-state current devices and a higher supply voltage by stacking the solid-state current devices in a totem pole fashion so that the source voltage is divided between the solid-state current devices. Two, or more, solid-state current devices are used in series for dc operation, but they are used in parallel for rf operation.
Thus, current that flows in series through the solid-state current devices is used two, or more times, in the production of the rf output. It is used once in each of two, or more, series-connected solid-state current devices, thereby increasing the rf output for a given current flow, as compared to rf amplifiers connected in the conventional fashion.
However, totem pole, voltage-dividing, or current-sharing circuits, have been used only at low rf powers, as in the above-referenced article wherein the power was in the order of 10 milliwatts. At higher rf powers, problems associated with inadequate rf decoupling have included low power efficiency, oscillation, a decrease in reliability of the circuits, and destruction of the solid-state current devices. This problem of inadequate decoupling was solved by Lautzenhiser et al. in U.S. patent application Ser. No. 10/028,844 which was filed on Dec. 20, 2001.
BRIEF SUMMARY OF THE INVENTION
In the rf power amplifiers of the present invention, an rf input signal is supplied to a power splitter, which in the present invention is a quadrature power splitter. The quadrature-split rf signals are separately amplified in solid-state current devices that preferably are field-effect transistors, and that more preferably are gallium-arsenide field-effect transistors (GaAsFETs). The separately-amplified rf signals are supplied to a power combiner, which in the present invention is a quadrature combiner. Optionally, a high-power rf amplifier can be interposed between the drain terminals of the GaAsFETs and rf inputs to the power combiner.
The GaAsFETs are connected in series between positive and negative terminals of a supply voltage, as taught by Lautzenhiser et al. in the aforementioned patent. Drain-to-source voltages of the GaAsFETs dividingly share a supply voltage.
In the present invention, apparatus and method are provided for phase correcting rf output power prior to combining, and for power equalizing the rf outputs prior to combining, thereby further increasing rf power efficiency over the gains made by Lautzenhiser et al. in the aforementioned patent.
Considering further the background of the present invention, in order for a splitter, such as a quadrature splitter used in the subject invention, to operate optimally, three conditions must be met. First, assuming a 50 ohm splitter, the drive impedance as well as the load impedance should be 50 ohms. Second, the splitter should be operating in its design bandwidth. And third, the splitter should be operating in its power range. In the present state of the art, these factors do not present design problems, so the present invention does not address any problem related to any of these three conditions.
In order for a combiner, such as the quadrature combiner that is used in the present invention, to operate optimally, three conditions must be met. First of all, the combiner must output to a 50 ohm load in its design bandwidth. Second, the rf inputs to the combiner should be at 90 degrees. And finally, the rf inputs to the combiner should be at the same power level.
The present invention relates to these last two conditions, namely: the present invention optimizes efficiency of rf amplifiers of the type shown and described herein by correcting the phase angle of the rf inputs to the combiner. And, the present invention optimizes the efficiency of rf amplifiers of the type shown and described herein by equalizing amplitudes of the rf inputs to the combiner.
Phase correction is achieved by measuring a voltage that is a function of the phase error and then by trimming a line length to correct a phase angle of one of the two rf inputs to the combiner. More particularly, an rf mixer is connected across the two rf inputs to the combiner to measure a dc voltage that is a function of any phase-angle error.
That is, when quadrature signals are applied to the RF and LO inputs of an rf mixer, since the two rf inputs have equal frequencies, if no phase error exists, a zero dc voltage at the IF output reflects a zero variation from quadrature phase angles. However, any phase error in the quadrature frequencies will be indicated by a dc voltage. This dc voltage will be plus or minus polarity depending upon whether the actual phase angle is more than, or less than, ninety degrees.
Correction in the phase angles is achieved by a slider that adjustably lengthens or shortens a tuning loop, and by subsequently removing a redundant length of the tuning loop.
Since rf power losses are dissipated in the resistor that connects the combiner to an electrical ground, rf power losses may be measured by replacing the grounding resistor with a power meter. Since this rf power loss may represent a phase error and/or unequal rf power amplitudes, subsequent to phase correcting, the remaining rf power loss is the result of unequal rf power amplitudes deliver

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