DOHERTY AMPLIFIER

Abstract

A multistage linear power amplifier receiving an input signal. The multistage linear power amplifier comprises a plurality of Class-AB amplifiers connected in a cascade configuration. The plurality of Class-AB amplifiers amplifies the input signal to generate an amplified input signal. At least one of the plurality of Class-AB amplifiers is biased such that the multistage linear power amplifier emulates a Class-C amplifier.

Claims

1-19. (canceled)

20. A multistage power amplifier comprising: a plurality of amplifiers biased within a bias range associated with an amplifier of a first class, at least one of the plurality of amplifiers biased within the bias range is biased within a first portion of the bias range and at least one other of the plurality of amplifiers biased within the bias range is biased within a second portion of the bias range that differs from the first portion of the bias range such that the multistage power amplifier mimics a behavior of an amplifier of a second class while each of the plurality of power amplifiers is biased within the bias range associated with the amplifier of the first class.

21. The multistage amplifier of claim 20 wherein the plurality of amplifiers are connected in series.

22. The multistage amplifier of claim 20 wherein amplifiers of the first class exhibit a different linearity than amplifiers of the second class.

23. The multistage amplifier of claim 20 wherein amplifiers of the second class exhibit different power added efficiency than amplifiers of the first class.

24. The multistage amplifier of claim 20 wherein the amplifier of the first class is a Class-AB amplifier and the amplifier of the second class is a Class-C amplifier.

25. The multistage amplifier of claim 20 wherein amplifiers of the first class exhibit greater linearity and lower power added efficiency than amplifiers of the second class.

26. A Doherty power amplifier comprising: a first multistage power amplifier including a first plurality of amplifiers of a first class, the first multistage power amplifier configured to amplify a first signal; and a second multistage power amplifier configured to amplify a second signal that is a phase shifted version of the first signal, the second multistage power amplifier including a second plurality of amplifiers biased within a bias range associated with an amplifier of the first class, at least one of the second plurality of amplifiers biased within a first portion of the bias range and at least one other of the second plurality of amplifiers biased within a second portion of the bias range that differs from the first portion of the bias range such that the second multistage power amplifier mimics a behavior of an amplifier of a second class while each of the second plurality of power amplifiers is biased within the bias range associated with the first class of amplifier.

27. The Doherty power amplifier of claim 26 wherein the first signal and the second signal are of equal power and have a phase difference of 90 degrees.

28. The Doherty power amplifier of claim 26 further comprising a splitter configured to split an input signal into the first signal and the second signal.

29. The Doherty power amplifier of claim 26 further comprising a signal combiner configured to combine an output of the first multistage power amplifier and an output of the second multistage power amplifier to obtain an output signal of the Doherty power amplifier.

30. The Doherty power amplifier of claim 26 wherein the first multistage power amplifier is a main amplifier and the second multistage power amplifier is a peaking amplifier.

31. The Doherty power amplifier of claim 30 wherein an amplification of the peaking amplifier is less than an amplification of the main amplifier during back off operation.

32. The Doherty power amplifier of claim 26 wherein the first multistage power amplifier and the second multistage power amplifier include the same number of power amplifier stages.

33. The Doherty power amplifier of claim 26 wherein at least one amplifier of the first multistage power amplifier is biased differently than at least one amplifier of the second multistage power amplifier.

34. The Doherty power amplifier of claim 33 wherein a position within the first multistage power amplifier of the at least one amplifier of the first multistage power amplifier corresponds to a position within the second multistage power amplifier of the at least one amplifier of the second multistage power amplifier.

35. The Doherty power amplifier of claim 26 wherein at least one other of the second plurality of amplifiers is biased within a third portion of the bias range that differs at least in part from the first portion and the second portion of the bias range.

36. The Doherty power amplifier of claim 26 wherein the first plurality of amplifiers are connected in series and the second plurality of amplifiers are connected in series.

37. The Doherty power amplifier of claim 26 wherein amplifiers of the first class exhibit a different linearity than amplifiers of the second class.

38. The Doherty power amplifier of claim 26 wherein amplifiers of the second class exhibit different power added efficiency than amplifiers of the first class.

39. The Doherty power amplifier of claim 26 wherein the amplifier of the first class is a Class-AB amplifier and the amplifier of the second class is a Class-C amplifier.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0022] The following detailed description of the embodiments of the present invention will be better understood when read in conjunction with the appended drawings. The present invention is illustrated by way of example, and not limited by the accompanying figures, in which like references indicate similar elements.

[0023] FIG. 1 is schematic diagram of a conventional multistage Class-AB power amplifier;

[0024] FIG. 2 is a schematic diagram of a conventional balanced amplifier;

[0025] FIG. 3 is a schematic diagram of a conventional Doherty amplifier;

[0026] FIG. 4 is a schematic diagram depicting a multistage linear power amplifier in accordance with an embodiment of the invention;

[0027] FIG. 5 is a schematic diagram depicting a Doherty amplifier in accordance with an embodiment of the invention;

[0028] FIG. 6 is a graph illustrating variation in gain of a main amplifier with respect to the output power in accordance with an embodiment of the invention; and

[0029] FIG. 7 is a graph illustrating variation in gain of a peaking amplifier with respect to the output power in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The invention can be best understood with reference to the detailed figures and description set forth herein. Various embodiments are discussed below with reference to the figures. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is simply for explanatory purposes. The disclosed systems or circuits extend beyond the described embodiments. For example, those skilled in the art will appreciate that in light of the teachings presented, multiple alternate and suitable approaches may be realized, to implement the functionality of any detail described herein, beyond the particular implementation choices in the following embodiments described and shown.

Definitions

[0031] The following terms shall have, for the purposes of this application, the respective meanings set forth below.

[0032] Heavy Class-AB biasing refers to 50% or more of Class-A (close to Class-A) biasing.

[0033] Low Class-AB biasing refers to 20% or below of Class-A (close to Class-B) biasing. Further, Low Class-AB biasing is divided into Very Deep Class-AB biasing (below 4% of Class-A biasing) and Deep Class-AB biasing (between 4% and 10% of Class-A biasing).

[0034] Mid Range Class-AB biasing refers to a range between Low Class-AB biasing and Heavy Class-AB biasing (i.e., above 20% and below 50% of Class-A biasing).

[0035] A person having ordinary skill in the art will understand that there exists a continuum of an infinite number of possible bias points between Class-A and Class-B. For the purposes of explanation and readability, descriptive terms have been defined to distinguish different regions of Class-AB biasing. A person having ordinary skill in the art will readily recognize that these are only descriptive terms and will further understand that the appropriate biasing point in the Class-AB continuum is dependent on a wide variety of factors and design/performance trade-offs including, but not limited, to the particular signal characteristics, product application, and the particular semiconductor technology/device technology.

[0036] FIG. 4 is a schematic diagram depicting a multistage linear power amplifier 400 in accordance with an embodiment of the invention. Multistage linear power amplifier 400 includes an input port 402, a multistage amplifier 404, and an output port 412. Multistage amplifier 404 further includes a first Class-AB amplifier 406, a second Class-AB amplifier 408, and a third Class-AB amplifier 410. In an embodiment, input port 402 is connected to first Class-AB amplifier 406 of multistage amplifier 404. First Class-AB amplifier 406, second Class-AB amplifier 408, and third Class-AB amplifier 410 are connected in a cascade configuration to form multistage amplifier 404. Third Class-AB amplifier 410 is connected to output port 412.

[0037] In an embodiment, first Class-AB amplifier 406, second Class-AB amplifier 408, and third Class-AB amplifier 410 are configured as an input stage, a driver stage, and an output stage of multistage amplifier 404, respectively. A person having ordinary skill in the art will understand that scope of the invention is not limited to having three stages in multistage amplifier 404. In an embodiment, numbers of amplifier stages may be determined based on gain requirement of multistage amplifier 404.

[0038] In an embodiment, first Class-AB amplifier 406 (input or pre-driver stage) is low Class-AB biased. Second Class-AB amplifier 408 (driver stage) is deep Class-AB biased. Third Class-AB amplifier 410 (or output stage) is very deep Class-AB biased. A person having ordinary skill in the art will understand that the biasing of first Class-AB amplifier 406, second Class-AB amplifier 408, and third Class-AB amplifier 410 may vary based upon many factors such as the application in which multistage amplifier 404 is to be used and the particular semiconductor technology.

[0039] U.S. Pat. No. 6,515,546, and assigned to the same assignee (Anadigics Inc), which is herein incorporated by reference in its entirety, discloses an exemplary biasing network that can be utilized for biasing first Class-AB amplifier 406, second Class-AB amplifier 408, and third Class-AB amplifier 410. However, the scope of the invention should not be limited to the bias network of U.S. Pat. No. 6,515,546.

[0040] The operation of the multistage linear power amplifier 400 is described later in conjunction with FIG. 5.

[0041] FIG. 5 is a schematic diagram depicting a Doherty power amplifier 500 in accordance with an embodiment of the invention. Doherty power amplifier 500 is explained in conjunction with FIG. 4. Doherty power amplifier 500 includes an input port 502, a signal splitter 504, a Doherty amplifier 506, a Doherty combiner 516, and an output port 518. Doherty amplifier 506 includes multistage amplifier 404 and a main amplifier 508. Multistage amplifier 404 includes first Class-AB amplifier 406, second Class-AB amplifier 408, and third Class-AB amplifier 410. Main amplifier 508 includes a fourth Class-AB amplifier 510, a fifth Class-AB amplifier 512, and a sixth Class-AB amplifier 514. Hereinafter, multistage amplifier 404 has been referred as peaking amplifier 404.

[0042] For the purpose of the ongoing description, three amplifier stages have been considered for main amplifier 508. However, it will be apparent to a person having ordinary skill in the art that the number of amplifier stages in main amplifier 508 may vary based on the gain requirement of main amplifier 508. For example, if a plurality of amplifiers stages in main amplifier 508 includes two amplifier stages i.e., a fourth Class-AB amplifier 510 and a fifth Class-AB amplifier 512, then fourth Class-AB amplifier 510 is configured as the driver stage of main amplifier 508 and fifth Class-AB amplifier 512 is configured as the output stage of main amplifier 508.

[0043] Input port 502 is connected to signal splitter 504. Signal splitter 504 is connected to fourth Class-AB amplifier 510 and first Class-AB amplifier 406. First Class-AB amplifier 406, second Class-AB amplifier 408, and third Class-AB amplifier 410 are connected in a cascade configuration to form peaking amplifier 404. Similarly, fourth Class-AB amplifier 510, fifth Class-AB amplifier 512, and sixth Class-AB amplifier 514 are connected in the cascade configuration to form main amplifier 508. Sixth Class-AB amplifier 514 and third Class-AB amplifier 410 are connected to Doherty combiner 516. Doherty combiner 516 is further connected to output port 518.

[0044] In an embodiment, fourth Class-AB amplifier 510 (input or pre-driver stage of main amplifier 508) is heavy Class-AB biased. Fifth Class-AB amplifier 512 (driver stage of main amplifier 508) is heavy Class-AB biased. Sixth Class-AB amplifier 514 (output stage of main amplifier 508) is low Class-AB biased. In an embodiment, peaking amplifier 404 is constructed as an identical fabricated copy of the main amplifier 508 but with differing biasing. First Class-AB amplifier 406, second Class-AB amplifier 408, and third Class-AB amplifier 410 are biased as described above in conjunction with FIG. 4.

[0045] Input port 502 receives the input signal from an external source (not shown). The input signal is transmitted to signal splitter 504. In an embodiment, signal splitter 504 is a 90 degree hybrid power splitter. Signal splitter 504 splits the input signal into a first signal and a second signal. In an embodiment, the first signal and the second signal have equal power (e.g., 3 dB). Further, the first signal and the second signal have a 90 degree phase difference. The first signal and the second signal are transmitted to fourth Class-AB amplifier 510 and first Class-AB amplifier 406.

[0046] For the purpose of ongoing description, it is assumed that Doherty power amplifier 500 achieves peak efficiency at 6 db back-off (i.e., the difference between the increase in power of the output signal with the increase in power of the input signal). However, the scope of the invention is not to be limited to achieving peak efficiency at 6 db back-off. A person having ordinary skill in the art will understand that peak efficiency of Doherty power amplifier 500 can be achieved at a different back-off value other than a 6 db back-off.

[0047] For instance, if the input signal is a low power signal (below 6 db back-off), the first signal and the second signal generated by signal splitter 504 will also be low power signals. As the first signal is a low power signal, main amplifier 508 operates in an ON state due to the biasing of fourth Class-AB amplifier 510, fifth Class-AB amplifier 512, and sixth Class-AB amplifier 514. Further, main amplifier 508 exhibits constant gain as a function of power. FIG. 6 illustrates a graph 600 showing the variation of gain of main amplifier 508 with increasing power of the input signal. From the curve 602, it is observed that the gain of main amplifier 508 is constant with increasing power of the input signal. Therefore, for low power signals, main amplifier 508 outputs a linear amplified replica of the first signal received from signal splitter 504.

[0048] On the other hand, as the second signal is a low power signal and first Class-AB amplifier 406 is low Class-AB biased, the gain of first Class-AB amplifier 406 for a low power signal is very low in comparison to the gain of main amplifier 508. FIG. 7 illustrates a graph 700 showing the variation of gain of peaking amplifier 404 with increasing power of the amplified second signal. From the curve 702, it is observed that peaking amplifier 404 exhibits significantly reduced gain under back off due to the described biasing. Thus, the output of peaking amplifier 404 is a much smaller amplified replica of the second signal in comparison to output of main amplifier 508. Further, the current consumption of peaking amplifier 404 is small (due to low bias currents). Therefore, peaking amplifier 404 emulates the behavior of a Class-C amplifier in a Doherty amplifier configuration (i.e., providing negligible output when a low power signal is applied).

[0049] Additionally, for low power input signals, as output of peaking amplifier 404 is negligible, peaking amplifier 404 provides negligible load modulation emulating the effect of a Class-C peaking amplifier that offers an open circuit to Doherty combiner 516. Therefore, the load presented to the main amplifier 508 approaches twice that of optimum output load impedance. In an embodiment, the optimum load impedance for main amplifier 508 and peaking amplifier 404 is 50 ohms.

[0050] As the power of the input signal increases (region between 0 db and 6 db back-off), peaking amplifier 404 exhibits smooth gain expansion (due to the self biasing of first Class-AB amplifier 406, second Class-AB amplifier 408 and third Class-AB amplifier 410). From curve 702, it can be observed that the gain of peaking amplifier 404 increases smoothly with increase in the power of the input signal. The smooth increase in the gain of peaking amplifier 404, leads to smoother AM-AM and AM-PM distortion characteristics with the absence of sudden changes or discontinuities.

[0051] Additionally, as the power of the input signal increases the efficiency of main amplifier 508 reaches a peak and the gain of peaking amplifier 404 starts increasing. Due to the increase in the gain of peaking amplifier (in the region between 0 dB and 6 dB back-off), the parallel connection of the main and peaking amplifier 404 across the load through Doherty combiner 516 results in load modulation.

[0052] As power of the input signal increases, the power of the first signal and the second signal (generated by signal splitter 504) also increases. As the power of the first signal and second signal increases, the gain and output power of peaking amplifier 404 both rises until both the peaking amplifier 404 and the overall Doherty amplifier 506 reaches saturation and peak power added efficiency at the zero back-off condition. As main amplifier 508 and peaking amplifier 404 are driven into the saturation region, the gain of main amplifier 508 and peaking amplifier 404 is reduced. From curves 602 and 702, it is seen that the gain of main amplifier 508 and peaking amplifier 404 shows a dip after 30 db power of the output signal.

[0053] Post the amplification, signal combiner 508 combines the amplified second signal and the amplified first signal to generate an output signal. The out signal is outputted from output port 518.

[0054] A person having ordinary skill in the art will understand that single peaking amplifier 404 in Doherty amplifier 500 has been shown for illustrative purposes. In an embodiment, Doherty amplifier 500 may include more than one peaking amplifiers connected in parallel with main amplifier 508. Various such Doherty amplifier configurations having more than one peaking amplifiers have been disclosed in the U.S. patent application Ser. No. 13/548,774, filed Jul. 10, 2012 entitled INTEGRATED OUTPUT COMBINER FOR AMPLIFIER SYSTEM, assigned to the same assignee (ANADIGICS, INC.), which is herein incorporated by reference in its entirety. It should be apparent to a person having ordinary skill in the art that the biasing as disclosed in various embodiments of the present invention can be applied to the one or more peaking amplifiers to achieve further improvements.

[0055] The embodiments of the present invention provide several advantages. As main amplifier 508 and peaking amplifier 404 include a plurality of Class-AB amplifiers, main amplifier 508 and peaking amplifier 404 can be constructed as identical fabricated copies. Constructing identically fabricated copies of main amplifier and peaking amplifier prevents the problems caused by manufacturing/process variations due to the inherent matching of MMIC (Monolithic microwave integrated circuit) dies fabricated side by side on a semiconductor wafer. Identically fabricated copies leads to closely matched gain and phase in the Doherty power amplifier containing two (or more) multistage amplifier paths without the use of costly and space consuming circuitry for calibration or compensation of the gain and phase balance. The conversion of an existing linear multistage amplifier into a Doherty-like amplifier has strong positive business and manufacturing implications such as time to market, stability over temperature range and relative immunity to process/manufacturing variation. In addition, this technique enables an existing linear amplifier to be converted to a Doherty power amplifier with higher power and higher efficiency. Further, peaking amplifier 404 exhibits a smoother gain expansion in comparison to sudden turn-on in Class-C amplifier (as used in the prior art). Therefore, distortions, such as the AM-AM distortion and the AM-PM distortion, due to sudden turn-on are reduced.

[0056] While various embodiments of the present invention have been illustrated and described, it will be clear that various multistage amplifiers (the main amplifier and one or more peaking amplifiers) of the Doherty amplifier can be fabricated as a single integrated circuit, or as discrete circuit components connected together. Further, various other possible combinations of electronic components of the multistage amplifiers may also be used without departing from the scope of the invention.

[0057] While various embodiments of the present invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art, without departing from the basic scope and spirit of the invention, as described in the claims that follow.