Amplifiers – Signal feedback – In cascade amplifiers
Reexamination Certificate
1999-01-28
2001-03-20
Shingleton, Michael B (Department: 2817)
Amplifiers
Signal feedback
In cascade amplifiers
C330S311000, C330S294000
Reexamination Certificate
active
06204728
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to apparatus and methods of broadband amplification with high linearity and low power consumption.
Broadband signal amplifiers and methods for amplifying signals over a wide range of frequencies have many applications. For example, wireless communication devices, such as cellular telephones, cordless telephones, pagers, television tuners, local area networks, and the like, require wide dynamic range, high linearity amplifiers to boost the strength of received signals without distortion and cross-talk between different frequencies. Unfortunately, all linear amplifiers distort the signals they are required to amplify to some degree. This is particularly undesirable when two or more independent channels are being amplified. Under these circumstances, the amplifier generates unwanted intermodulation products that may cause interference and result in poor performance of the communication device.
Intermodulation distortion is defined in terms of the peak spurious level generated by two or more tones injected into a receiver. A receiver may be characterized by a third-order distortion figure of merit referred to as a “third-order input intercept point” (IIP3), which is defined as the input power (in the form of two tones) required to create third-order distortion products equal to the input two tone power. The linearity of an amplifier, and thus the immunity of the receiver to distortion and cross-talk, improves with the IIP3 of the amplifier. Another amplifier figure of merit is equal to the ratio of the maximum microwave output power (at a specified level of distortion) to the applied DC power. The conventional method of specifying the level of distortion for this figure of merit is called the output intercept point of third order products (OIP3). In accordance with the OIP3 method, two input signals separated only slightly in frequency, and of substantially equal, but adjustable, power are applied to the amplifier input. A plot is made of both the fundamental frequency output power and the power in the third order intermodulation product versus the input power and a linear extrapolation is made of these two plots. The point where these two extrapolations intersect is the OIP3 amplitude, which is read in dB from the output power (ordinate) axis.
Amplifier linearity may be improved by increasing amplifier gain (see, e.g., Wheatley, U.S. Pat. No. 5,732,341). This approach, however, increases the power consumption of the amplifier and, thereby, decreases the length of time a user may operate battery-powered receivers incorporating such amplifiers before the batteries must be recharged.
SUMMARY OF THE INVENTION
In one aspect, the invention features an apparatus configured to amplify a signal, comprising: an input transistor and an output transistor coupled together in a cascode configuration with the input transistor defining an input of the apparatus and the output transistor defining an output of the apparatus; and a feedback network coupled between the input and the output and characterized by an impedance of substantially zero resistance and non-zero reactance.
Embodiments may include one or more of the following features.
The feedback network preferably has a reactance selected to reduce third-order intermodulation distortion produced at the output of the apparatus. The feedback network preferably comprises a feedback capacitor. The feedback network preferably consists essentially of capacitive reactance.
In a preferred embodiment, an inductive feedback network is coupled between the input transistor and a low voltage reference and consists essentially of inductive reactance. An inductor preferably is coupled between the input transistor and the low voltage reference. The inductive feedback network and the capacitive feedback network may have respective reactances selected to achieve a desired impedance at the input of the apparatus.
The reactance of the capacitive feedback network is selected to achieve an impedance at the input of the apparatus that is substantially independent of frequency over a broad frequency range. In one embodiment, the apparatus is configured to amplify a signal having a frequency of at least about 500 MHz with substantial linearity.
In some embodiments, a receiver input, configured to receive a wireless signal, is coupled to the apparatus input, and a receiver output, configured to produce an output signal representative of the wireless signal received at the receiver input, is coupled to the apparatus output.
In another aspect, the invention features a method of making an apparatus configured to amplify a signal, comprising: forming on a substrate an input transistor and an output transistor coupled together in a cascode configuration with the input transistor defining an input of the apparatus and the output transistor defining an output of the apparatus; and providing a feedback network coupled between the input and the output and characterized by an impedance of substantially zero resistance and non-zero reactance.
In yet another aspect, the invention features a method of amplifying a signal comprising: receiving an input signal at an input; amplifying the input signal to provide an output signal at an output; and sampling the output signal at the input through a feedback network characterized by an impedance of substantially zero resistance and non-zero reactance.
The output signal may be sampled at the input by coupling the output to the input through a feedback network that consists essentially of capacitive reactance. The method may further include the step of matching an output impedance by providing a series feedback network that consists essentially of inductive reactance.
In another aspect, the invention features an apparatus configured to amplify a signal, comprising: first and second amplification stages, each comprising an input transistor and an output transistor coupled together in a cascode configuration with the input transistor defining an input of the amplification stage and the output transistor defining an output of the amplification stage, a capacitive feedback network coupled between the input and the output and consisting essentially of capacitive reactance, and an inductive feedback network coupled between the input transistor and a low voltage reference and consisting essentially of inductive reactance; wherein the inputs of the amplification stages are coupled together to form an input of the apparatus, and the outputs of the amplification stages are coupled together by an isolator.
In one embodiment, the isolator comprises a diode.
Among the advantages of the invention are the following.
The invention reduces (or rejects) the intermodulation products inherently present in non-linear amplifier circuits. The OIP3 is improved by at least 3-6 dB at a given supply current; or, the same OIP3 may be obtained at a lower power dissipation (e.g., in one embodiment, a 50% current reduction may be achieved). The low power requirements of the invention makes it particularly useful for wireless applications. The inventive amplifiers and amplification methods described herein also achieve an input impedance and transfer characteristic that is flat over a broad frequency range (at least an octave). Furthermore, the lIP3 is easily adjusted, without changing other circuit parameters or specifications, by simply altering the supply current. Moreover, matching to a real input impedance is accomplished relatively easily. The invention is also substantially insensitive to fabrication process variations. The invention is applicable to a wide variety of applications, including various wireless circuits, without sacrificing important design parameters.
Other features and advantages will become apparent from the following description, including the drawings and the claims.
REFERENCES:
patent: 3371283 (1968-02-01), Rees
patent: 4406990 (1983-09-01), Noro
patent: 4511857 (1985-04-01), Gunderson
patent: 4523057 (1985-06-01), Boeckmann
patent: 4706038 (1987-11-01), Navidi et al.
patent: 5015968 (1991-
Fish & Richardson P.C.
Maxim Integrated Products Inc.
Shingleton Michael B
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