Amplifiers – With semiconductor amplifying device – Including frequency-responsive means in the signal...
Reexamination Certificate
1999-07-01
2001-05-15
Lee, Benny (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including frequency-responsive means in the signal...
C330S302000, C330S303000, C330S306000
Reexamination Certificate
active
06232841
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of tuned power amplifiers, particularly to tunable implementations of high efficiency switching power amplifiers.
2. Description of the Related Art
The demand for low-cost, reliable wireless communications continues to increase at a rapid rate, as do the demands on the technologies enabling such communications. Research is being conducted on many fronts to find ways to make the circuitry found inside devices such as cellular phones smaller, cheaper, easier to fabricate, less power-hungry, and more reliable.
A schematic diagram of a class E power amplifier is shown in FIG.
1
. This type of amplifier is not commonly found in battery-powered communications devices in spite of its high efficiency, primarily because of its inherently narrow band nature. It has a theoretical efficiency of 100%, and actual efficiencies of better than 90% are routinely achieved. An RF input signal drives an active device
10
, typically a transistor, which is operated as a switch. The transistor's current circuit is connected to drive a reactive load network
12
made up of a bias inductor L
bias
and a power source
14
connected in series and a shunt capacitor C
sh
connected across the transistor, and a series inductor L
s
and a series capacitor C
s
connected in series with the current circuit. The amplifier's RF output appears at C
s
's other terminal, shown driving a load R
load
.
The class E amplifier shown is efficient only over a narrow frequency range centered around a specific operating frequency; i.e., it is inherently narrowband. The load network
12
is “tuned” to cause a particular phase relationship to exist at the operating frequency which allows the switching to occur at points of either zero voltage or zero current, leading to essentially lossless switching. The design equations used to achieve this condition are as described in U.S. Pat. No. 3,919,656 to Sokal et al.
There is a trade-off between the quality factor (Q) of the load network and the amplifier's bandwidth, with bandwidth being approximately inversely proportional to the network's Q. A very low Q yields an amplifier capable of high efficiencies over a wide bandwidth. However, a low Q also allows significantly higher amounts of harmonic currents to flow to the output load, so that the output requires additional filtering. Such filtering introduces additional losses, and requires additional reactive components which may have to be tuned to achieve acceptable performance
Due to the need for additional output filtering with a low Q load network, a high Q network is preferred. The high Q load and the very low power at which switching occurs combine to keep harmonic content low, simplifying the output filtering. Unfortunately, a high Q load network makes the amplifier efficient over a narrow band of frequencies, and the requisite phase relationship between the load network and the input frequency deteriorates rapidly as the input moves away from the designed operating point.
Similar trade-offs must be made when designing amplifiers from other classes in which an active device drives a reactive load, such as classes C, C-E, and F, with the result being that these amplifiers typically end up being narrowband. Though acceptable for use with inputs near the operating frequency, they are of little use where broad bandwidths, multiple frequencies, or frequency agility is a requirement, including most modern communications systems, both commercial and military.
Tunable inductors and/or capacitors are often incorporated into the design of a power amplifier, which are tuned by a technician to achieve acceptable performance when an amplifier is being assembled. This approach is not useful in a frequency hopping scheme, however, which requires the tuning of the load network to change in the field, to accommodate a change in operating frequency. Reactive components capable of being automatically tuned in the field exist, but either cannot be integrated with the other amplifier components, necessitating the use of hybrid or similar packaging schemes that increase size and weight while lowering efficiency and reliability, or provide low Q and a low efficiency amplifier. Reactive tuning elements are also problematic when designing a system for multiple frequency bands of operation, such as at 900 MHz and 1.9 GHz, or where extremely broad bandwidths are required In the prior art, such systems require lower Q matches, and suffer from reduced efficiency as a result.
SUMMARY OF THE INVENTION
Power amplifiers having reactive networks (classes C, C-E, E and F) are presented that offer both the high Q needed to keep output filtering to a minimum, and the frequency agility needed for broad bandwidth, multiple frequency bands of operation, or modern frequency hopping schemes.
Common to the various amplifier types realizable with the invention is the use of tunable reactive devices which are capable of being adjusted by means of respective control signals. The tunable reactive devices have high Q values across their adjustment range, enabling the amplifier to produce an output with a low harmonic content for a wider range of input signal frequencies than has previously been possible.
One type of tunable reactive device consists of high-Q fixed value components which are interconnected as necessary to provide a particular reactance. The switching needed to accomplish the interconnection is provided by micro-electromechanical (MEM) switches capable of providing low loss switching at RF frequencies, and of being integrated with the amplifier components on a common substrate. This approach can provide a very high Q reactive network at a number of discrete reactance steps. Alternatively, MEM devices having a continuously variable reactance value can be employed to provide the desired tunability. The novel amplifiers preferably include control circuits which generate the control signals needed to operate the MEM devices and thereby tune the amplifier.
In addition to offering a frequency agile, high quality output, the present invention enables an unprecedented degree of integration. The amplifier's active device, its tunable reactive devices, and the control circuitry which generates the necessary control signals to the MEM devices can all be integrated onto a common substrate, conserving space, reducing power consumption, and increasing reliability. The invention can be realized on a number of foundry technologies.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
REFERENCES:
patent: 3919656 (1975-11-01), Sokal et al.
patent: 5276912 (1994-01-01), Siwiak et al.
patent: 5578976 (1996-11-01), Yao
patent: 5872489 (1999-02-01), Chang et al.
patent: 5880921 (1999-03-01), Tham et al.
patent: 6091966 (2000-07-01), Meadows
Alan B. Grebene, Micro-Linear Corporation,Bipolar and MOS Analog Integrated Circuit Design, John Wiley & Sons, pp. 54-120, (1984).
Cheng T. Wang, editor, Device Research Institute,Introduction to Semiconductor Technology: GaAs and Related Compounds, John Wiley & Sons, pp. 123-133, 187-195 and 422-433, (1990).
Yao et al, “A Surface Micromachined Miniature Switch For Telecommunications Applications With Signal Frequencies From DC Up To 4 GHZ”,Tech. Digest, Transducer-95, Stockholm, Sweden, pp. 384-387, (Jun. 25-29, 1995).
Bartlett James L.
Chang Mau Chung F.
Marcy, 5th Henry O.
Mehrotra Deepak
Pedrotti Kenneth D.
Koppel & Jacobs
Lee Benny
Nguyen Patricia T.
Rockwell Science Center LLC
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