Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2001-06-01
2002-09-03
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S136000
Reexamination Certificate
active
06445247
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to signal amplifiers. More specifically, the present invention relates to a circuit and method for highly-efficient signal amplification over a wide dynamic range wherein the output level and intercept point are determined from the input level and modulation type of the input signal.
II. Description of the Related Art
In the field of wireless telecommunications, such as in various cellular, Personal Communication Services (PCS), and Wireless Local Loop (WLL) communication systems, many different communication standards exist. For example, Code-Division Multiple Access (CDMA) digital communications may be governed by either Telecommunications Industry Association (TIA)/Electronics Industries Association (EIA) Interim Standard IS-95 (series) for cellular systems, or by ANSI J-STD-008 for PCS systems. Additionally, Time-Division Multiple Access (TDMA) digital communications may be governed by the TIA/EIA IS-54, or by the European standard Global System for Mobile Communications (GSM). Furthermore, analog FM-based communications systems may be governed by the Advanced Mobile Phone System (AMPS) standard, or a related standard such as N-AMPS. Other wireless communication standards also exist for both digital and analog modulation.
For each of these communication system standards, a long-felt need exists for an amplifier for a wireless communication device which exhibits the high linearity needed for signal integrity, as well as the high efficiency needed for longer operating time. This is particularly true for dual-mode communication devices that can operate according to two different standards (such as CDMA/AMPS), because each of the standards may have different linearity requirements. For example, the linearity requirements in a CDMA communication device are more stringent than those of an AMPS communication device. Thus, a dual-mode digital/analog communication device would benefit greatly from being able to take advantage of a high linearity amplifier while operating in a digital mode where there may be strict linearity requirements, while still being able to operate with high efficiency while in an analog mode where the linearity requirements are more relaxed.
However, as is known in the art of amplifier design, high linearity and high efficiency are generally mutually exclusive design considerations. That is to say, when one is designing a particular transistor-based amplifier, one must usually make a trade-off between high linearity and high efficiency. The difference between high linearity and high efficiency is manifested by saturation characteristics which are determined by the load impedance in relation to the current capability and the breakdown voltage of the amplifier. In turn, the load impedance, current capability and breakdown voltage of the amplifier are a function of the amplifier device type, construction, periphery (e.g., gate area), and supply voltage. Thus, a designer who wishes to design a highly linear amplifier will generally choose a relatively low load impedance for a given supply voltage. Highly linear amplifiers maintain the integrity of the input signal envelope at the expense of higher average power dissipation. This high average power dissipation which results from overlap of current and voltage in the transistor over time is particularly undesirable in a battery-powered portable transmitter because it reduces battery life, and thus the transmit time, of the portable transmitter between battery charges.
Conversely, a designer who wishes to design a highly efficient amplifier will generally choose a relatively higher load impedance for the same supply voltage. Highly efficient amplifiers maintain a lower average power dissipation at the expense of “clipping” of the input signal at high input amplitudes due to premature saturation of the amplifier. Although clipping the input signal gives rise to high efficiency and longer battery life because the device's power dissipation is minimized during saturation, it results in distortion of the input signal envelope, and consequent generation of in-band spectral sidelobes. Furthermore, clipping generates higher-order harmonics that may be spread outside of the allowed operating bandwidth of the transmitter, causing interference to other RF devices transmitting or receiving on other frequencies.
Although there have been various attempts to create a highly efficient amplifier that is also highly linear, these attempts contain inherent problems which limit their effectiveness. For example, Doherty-type amplifiers are well known in the art as being highly efficient and also highly linear. A Doherty-type amplifier modulates the load impedance in response to the envelope of the input signal. In a Doherty-type amplifier, two amplifiers are connected in parallel, with the output of one of the amplifiers in series with a quarter-wavelength phase shifter. An example of such an amplifier is illustrated in U.S. Pat. No. 5,568,086 to Schuss et al, entitled “LINEAR POWER AMPLIFIER FOR HIGH EFFICIENCY MULTI-CARRIER PERFORMANCE.” However, a significant drawback to the Doherty-type design of Schuss et al is that a quarter-wavelength phase shifter may be difficult and costly to realize at certain frequencies. Additionally, Doherty-type amplifiers are narrowband “tuned” amplifiers that operate best around a single frequency and are ill-suited for use in a broadband application such as digital wireless telephony.
Another example of a highly efficient amplifier is illustrated in U.S. Pat. No. 5,175,871 to Kunkel, entitled “POWER AMPLIFIER FOR A CELLULAR TELEPHONE.” The amplifier of Kunkel uses a non-linear amplifier stage in conjunction with a separate linear amplifier stage. A switch is used to select the non-linear amplifier stage when non-linear behavior is desired, and to select the linear amplifier stage when linear behavior is desired. However, a significant drawback of Kunkel is the increased expense of providing two separate amplifiers, each with its own design characteristics.
In U.S. Pat. No. 5,661,434, entitled “HIGH EFFICIENCY MULTIPLE POWER LEVEL AMPLIFIER CIRCUIT,” issued Aug. 26, 1997 to Brozovich et al, a high efficiency power amplifier is disclosed. The high efficiency power amplifier of Brozovich comprises a plurality of power amplifier stages coupled in a cascade configuration. At least one of the amplification stages includes a signal switching network for switching among one or any combination of the power amplifiers. A signal switch control circuit, external to the amplifier circuit, controls the switches. A drawback of this design is that the signal switch control circuit provides external control to the switches. By using external control to the switches, the complexity of hardware and software is increased.
Thus, there is a resultant need for an amplifier that is both highly efficient and highly linear which avoids the drawbacks inherent in other designs.
SUMMARY OF THE INVENTION
The present invention is a novel and improved circuit and method for amplifying an input signal. Broadly described, the circuit comprises an amplifier circuit having an input for receiving the input signal, and an amplifier control circuit, coupled to the amplifier circuit, for varying a supply power and a device periphery of the amplifier circuit in response to an amplitude envelope of the input signal. In this manner, the amplifier control circuit can control the linearity and the efficiency of the amplifier circuit using the characteristics of the input signal itself, without relying on any external processing.
An exemplary embodiment of the amplifier control circuit comprises an envelope detector that detects the input signal amplitude envelope, and that generates an envelope detection signal in response thereto. At least one threshold detector, coupled to the envelope detector, compares the envelope detection signal to a threshold, and generates a threshold comparison signal in response thereto. An amplitude-modulation (AM) detector, co
Brown Charles D.
Choe Henry
Pascal Robert
Poppas George C.
Qualcomm Incorporated
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