WO1997020385A1

WO1997020385A1 - Doherty-type amplifier and tuning method

Info

Publication number: WO1997020385A1
Application number: PCT/US1996/014269
Authority: WO
Inventors: James Frank Long
Original assignee: Motorola Inc.
Priority date: 1995-11-30
Filing date: 1996-09-04
Publication date: 1997-06-05
Also published as: GB9713888D0; KR19980701804A; GB2313009A; SE9701538D0; DE19681072T1; SE9701538L; FI972345A; JPH10513631A; CA2204409A1; TW322657B; FI972345A0; AU7154696A

Abstract

An amplifier circuit comprising a carrier amplifier (24) producing a carrier amplifier output signal, a peaking amplifier (26) coupled to the carrier amplifier (24) in a Doherty configuration, and a combination circuit responsive to the carrier amplifier (24) and the peaking amplifier (26). The peaking amplifier (26) is voltage-biased to produce an adjusted intermodulation product signal. The combination circuit (35) combines the adjusted modulation product signal with the carrier amplifier output signal to produce a substantially linearized amplifier circuit output signal.

Description

Doherty-Type Amplifier And Tuning Method

Field of the Invention

The present invention relates generally to amplifier circuits, and more particularly to Doherty type amplifier circuits.

Background of the Invention

Conventional Doherty type amplifier circuits are well known to those skilled in the art "A New High Efficiency Power Amplifier for Modulated Waves", Proceedings of the Institute of Radio Engineers, Vol 24, No. 9, pp 1 163-1182. (September 1936). However, it is also well known that conventional Doherty type amplifiers typically have relatively poor linearity. In addition, their linearity is typically inversely proportional to their efficiency. Thus, conventional Doherty type amplifiers that provide good efficiency have poor linearity. Due to their poor linearity, conventional Doherty type amplifier circuits are not well suited to many applications, such as multicarπer power amplifier applications in cellular base station equipment. Accordingly, there exists a need for a Doherty type amplifier circuit with improved linearity.

Summary of the Invention

In order to address this need, the present invention provides an improved amplifier circuit and a method of tuning a Doherty type amplifier circuit. According to one aspect of the present invention, the amplifier circuit comprises a first amplifier having a carrier amplifier and a peak amplifier configured in a Doherty arrangement, a second amplifier having a carrier amplifier and a peak amplifier configured in a Doherty arrangement, and a combination circuit responsive to the first and second amplifier The first amplifier produces a substantially linear first output signal over a first frequency bandwidth. The second amplifier produces a substantially linear second output signal over a second bandwidth. The combination circuit is responsive to the first and second output signal and produces a third output signal that is substantially linear over a third frequency bandwidth. The third frequency bandwidth is greater then either the first or second frequency bandwidths.

According to another aspect of the present invention, the amplifier circuit comprises a carrier amplifier producing a carrier amplifier output signal, a peaking amplifier coupled to the carrier amplifier in a Doherty configuration, and a combination circuit responsive to the carrier amplifier and the peaking amplifier. The peaking amplifier is voltage biased to produce an adjusted intermodulation product signal The combination circuit combines the adjusted modulation product signal with the carrier amplifier output signal to produce a substantially linearized amplifier circuit output signal.

The method of tuning a Doherty type amplifier circuit includes the steps of providing a Doherty type amplifier, measuring intermodulation performance of the Doherty type amplifier as a function of peaking amplifier bias voltage, and selecting a peaking amplifier bias voltage based on the measured intermodulation performance. The invention itself, together with its attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.

Brief Description of the Drawings

FIG. 1 is a circuit schematic of a Doherty type amplifier circuit. FIG 2 is a graph of intermodulation products for the Doherty type amplifier of FIG. 1. FIG. 3 is a circuit diagram of a feedforward amplifier using the

Doherty type amplifier of FIG. 1.

FIG. 4 is a block diagram illustrating a parallel Doherty type amplifier arrangement.

FIG. 5 is a flow chart of a method of tuning a Doherty tpe amplifier. FIG. 6 is a particular embodiment of a matching circuit. Detailed Description

Referring to FIG. 1 , an amplifier circuit 20 including a carrier amplifier 24 and a peaking amplifier 26 configured in a Doherty arrangement is illustrated. The amplifiers 24 and 26 each receive a bias voltage. The amplifier circuit 20 has an input 22 and an output 38 The amplifier circuit includes a delay line 28, preferably providing a 90 degree delay, and a transformer line 30. The carrier amplifier 24 produces an output signal that is transmitted over a phasing line 32 and over the transformer line 30. The peaking amplifier 26 provides an output signal that is transmitted over a second phasing line 34. The output signals from the carrier and peaking amplifiers 24 and 26 are joined in a combination circuit 35 such as a common node, transmitted over a transformer line 36, and finally outputted at the amplifier circuit output 38.

The carrier amplifier 24 preferably a metal oxide semiconductor field effect transistor (MOSFET) type amplifier, such as a MRF183 Series amplifier available from Motorola operating in a class AB mode. The peaking amplifier 26 is preferably a MOSFET type amplifier such as a MRF183 Series amplifier available from Motorola operating in a class C mode The MRF 183 Series amplifiers are available from Motorola at 5008 E. McDowell Road, Phoneix, Arizona, 85008 The delay line 28 is preferably implemented with microstπp or stπpline technology in a manner known to those of ordinary skill. The transformer line 30 has an impedance of about fifty ohms and is a quarter wavelength. In the preferred embodiment, the transformer line 36 is also quarter wavelength and has an impedance of about thirty five ohms. The peaking amplifier 26 is responsive to the delay line 28 and is coupled to the phasing line 34. The transformer line 30 is responsive to the carrier amplifier 24 and interconnects the outputs from the carrier and peaking amplifiers 24 and 26 During operation, the carrier amplifier 24 is voltage biased for linear operation while the peaking amplifier 26 is voltage biased for nonlinear operation Over a predetermined frequency range, the peaking amplifier 26 produces intermodulation products such as third order intermodulation products, that destructively combine with intermodulation products from the carrier amplifier 24 such that the entire amplifier circuit 20 operates substantially linearly. However, due to fluctuations in individual amplifiers, the amplifier circuit 20 should be tuned to improve linearity of performance over the desired frequency range.

A preferred method of tuning the amplifier circuit 20 to be substantially linear over a certain frequency range will now be described. First, determine baseline intermodulation (IM) product performance by subjecting the amplifier circuit 20 with a two tone excitation signal. Second, based on the measured IM performance, voltage bias the carrier amplifier 24 based on application specific design considerations such as gain, IM performance, and efficiency. Third, sweep IM performance of the amplifier circuit 20 as a function of the peaking amplifier 26 bias voltage. An illustration of an exemplary peaking amplifier sweep is shown in FIG. 2. If good IM cancellation is observed, adjust the bias voltage of the peaking amplifier 26 to finely tune amplifier circuit 20 to further reduce IM products.

However, If no IM cancellation is observed, then rematch the carrier amplifier 24 and/or the peaking amplifier 26, and/or adjust the length of phasing lines 32 and 34. After adjusting components within amplifier circuit 20, repeat steps one to three above until satisfactory IM performance is achieved. A flow chart of the preferred method is illustrated in FIG. 5, and an example of a Doherty amplifier that has been tuned is disclosed in FIG. 6.

Referring to FIG. 3, another preferred embodiment of an amplifier circuit 150 is illustrated. The amplifier circuit 150 includes first 154, a second 156, and a third 158 Doherty type amplifiers that are preferably in a parallel arrangement. Each of the amplifiers 154, 156, and 158 is responsive to a driver amplifier 152 that receives an input signal 164 and produces a driver signal 160. The driver signal 160 is fed into the input of each of the amplifiers 154, 156, and 158. Each of the amplifiers 154, 156, and 158 produces an amplified output that is joined at a common node 162 and sent to an output 166 of the amplifier circuit 150. Each of the Doherty type amplifiers 154, 156, and 158 is preferably substantially similar in construction to the amplifier 20 illustrated in FIG. 1 and tuned to operate substantially linearly as described by the preferred tuning method set forth above.

However, each of the amplifiers 154, 156, and 158 are designed to operate in a substantially linear mode over a different frequency band. For example, the first amplifier 154 may be designed to operate substantially linearly between about 865 MHz and about 875 MHz, the second amplifier 156 may be designed to operate substantially linearly between about 875 and about 885 MHz, and the third amplifier 158 may be designed to operate substantially linearly from about 885 MHz to about 895 MHz. In the preferred embodiment of FIG. 4, the first amplifier 154 has a center frequency of about 870 MHz, the second amplifier 156 has a center frequency of about 880 MHz, and the third amplifier 158 has a center frequency of about 890 MHz. A Doherty type amplifier may be tuned to operate substantially linearly over a narrow frequency range. The specific frequency bandwidth of linear operation may be determined by adjusting a matching circuit within the Doherty amplifier, by adjusting the lengths of phasing lines, such as phasing lines 32 and 34 in amplifier 20, or by adjusting bias voltages of the carrier or peaking amplifiers 24 and 26. Alternatively, each of the amplifiers 154, 156, and 158, may be operating at a different transition voltage leading to varying frequency bands of linearity.

The Doherty amplifier architecture has an intrinsic bandwidth limitation. The limitation is due to circuit loading of the carrier amplifier by the peaking amplifier. The degree of circuit loading is determined by the peaking amplifier output matching circuit reactance, as well as the intrinsic reactance of the device, and the associated parasitic reactance of the device package. Feedforward amplifiers generally require broadband main amplifiers to minimize time delays through active devices and to facilitate broadband carrier cancellation. In the preferred embodiment where several Doherty amplifiers are parallel combined, the intrinsic bandwidth limitation can be overcome by using a tuning methodology which extends Doherty amplifier bandwidth and substantially maintains intermodulation performance, gain flatness, and high efficiency. The tuning methodology to achieve a total system bandwidth of X MHz consists of several parts.

Each carrier amplifier and peaking amplifier stage (for N total Doherty stages in parallel) are matched for a desired intermodulation, efficiency, and gam flatness over a bandwidth of X/N MHz. Matching circuits are composed of conventional discrete reactive elements such as capacitors, inductors and/or distributed transmission lines, in both series and parallel configurations for RF circuits. An example of a tuned matching circuit is shown in FIG. 6. By matching carrier and peaking amplifier stages for desirable performance over a narrower X/N MHz bandwidth, intermodulation performance and efficiency for the total Doherty configuration is enhanced. For example, if there are three Doherty stages in parallel, and the total system bandwidth requirement is 30 MHz, then each of the peaking and carrier amplifiers should be matched for a 10 MHz fractional bandwidth (X = 30 MHz, N ^•*. 3). If the band center of the amplifier were 855 MHz, then one Doherty stage would be matched over the 840-850 MHz band, the second Doherty stage would be matched for the 850-860 band, and the final Doherty stage would be matched for the 860-870 MHz band. When the stages are paralleled, the gain responses overlap, resulting in a flat gam response over the full X MHz bandwidth. A similar bandwidth extension mechanism is used in developing wideband filter designs.

Each carrier amplifier and peaking amplifier in a Doherty circuit is preferably coupled to provide proper power combining between the amplifiers. This coupling is often achieved using a transmission line of approximately λ/4 wavelengths. Since the transmission line (or phasing line) is frequency sensitive, desirable coupling of the carrier and peaking amplifier for maximum power combining occurs at a single frequency. Therefore, Doherty efficiency (dependent on peaking amplifier circuit loading) and intermodulation performance (dependent on carrier amplifier output loading) are enhanced when phasing line optimization is performed over a X/N MHz bandwidth, rather than the entire X MHz bandwidth. The tuning methodology thus provides that the phasing line length of each N Doherty amplifier uses a phasing line matched for a different X/N MHz fractional bandwidth. Using the above example, three different phasing line lengths would be used. Referring to the above example again, the 840-850 MHz Doherty stage would have λ/4 phasing line length of λ845 MHz''⁴. The 850-860 MHz Doherty stage would have a }JA phasing line length of λ855 MHz-⁷⁴. The 860-870 MHz Doherty stage would have a λ/4 phasing line length of λ865 MHz/⁴.

Each Doherty amplifier achieves improved gain flatness and intermodulation performance with an adjustment to the peaking amplifier bias. Therefore, each Doherty amplifier of bandwidth X/N MHz has its bias set for a desired gain flatness and intermodulation performance. However, some parasitic loading effects due to module paralleling may occur, perturbing the parallel configuration intermodulation and/or gain flatness The preferred embodiment for the paralleled Doherty configuration includes a final adjustment of each Doherty amplifier's peaking amplifier bias voltage to simultaneously adjust the Doherty main amplifier intermodulation performance, efficiency, and gain flatness. Since the bias adjustment involves the simultaneous optimization of three parameters (gain, flatness, IM, efficiency), a bias adjustment algorithm is typically used. The bias adjustment algorithm is best described in terms of a flow chart.

Improved feedforward main amplifier Doherty amplifier performance is realized when IM performance, bandwidth, gain, efficiency, and group delay targets are all met substantially simultaneously.

By providing a plurality of Doherty type amplifiers that each operate substantially linearly over a different frequency band, the amphfier circuit 150 may operate substantially linearly over a greater frequency band then any of the individual Doherty amplifiers. In the particular example of FIG. 3, the amplifier circuit 150 operates substantially linearly over the frequency band of about 865 MHz to about 895 MHz. Accordingly, the amplifier circuit 150 has the benefit of operating efficiently by using Doherty type amplifiers and advantageously operates substantially linearly over a relatively wide bandwidth.

The above described preferred embodiment provides many benefits. For example, the group delay through a Doherty amplifier will be higher than in a conventional amplifier due to the inherent bandlimited nature of the Doherty circuit The preferred embodiment reduces the group delay through the Doherty amplifier. Also, in multicarner amplifier applications, it is important to "randomize" the phase relationships as much as possible between the multiple intermodulation products which add vectoπally at a given frequency. A phase offset (randomization) is introduced between intermodulation products generated in each of the parallel Doherty stages. The phase offset occurs because each Doherty stage has a unique matching structure, a unique phasing line length, and a unique peaking amplifier bias set point. The result is the multicarner intermodulation products add vectonally to a peak value less often than in a conventional parallel amplifier design, producing a lower average intermodulation level. In addition, the preferred X/N MHz design method increases the bandwidth of an inherently bandlimited Doherty amplifier, which substantially reduced impact on gam, efficiency and intermodulation performance

FIG 4 illustrates a preferred embodiment of a feedforward amplifier circuit 100. The amplifier circuit 100 includes a main amplifier 106 and an error amplifier 1 14 The amplifier circuit 100 includes an input 102, a first coupler 104, a second coupler 108, a third coupler 1 12, and a fourth coupler 1 16 The amplifier circuit 100 further includes a first delay line 1 10 and a second delay line 116. The first coupler 104 samples an RF input signal received at the input 102 and produces a clean signal that is delayed by delay line 1 10. The second coupler 108 samples the output 120 of the main amplifier 106. The third coupler 1 12 receives the sampled output signal from coupler 108 and combines the output signal from the output 120 of mam amplifier 106 with the delayed version of the input signal sampled by the first coupler 104. The output of the third coupler is preferably an error signal that is amplified by error amplifier 1 14 to produce an amplified error signal 118. The amplified error signal 118 is combined by the fourth coupler 1 16 with a delayed output signal 122 that is produced by the second delay line 1 16. By combining the delayed output signal 122 with the amplified error signal 1 16, the resulting output 1 18 has a reduced level of error relative to the output signal 120. In this manner, at least a portion of the error due to nonlineaπty due to the main amplifier 106 is cancelled by the fourth coupler 1 16 to produce a more linear output 1 18. In the preferred embodiment, the mam amplifier 106 is a Doherty type amplifier, such as the amplifier circuit 20 illustrated in FIG. 1 , that has been tuned according to the above-described tuning method.

The Doherty configured main amplifier 106 provides a significant increase in direct current (DC) to RF conversion efficiency in the feedforward amplifier circuit 100. The efficiency improvement over conventional feed forward amplifier circuits may be about 40%, far exceeding other conventional efficiency enhancement techniques such as harmonic termination. For small fractional bandwidths (typically less than 1 %), the Doherty configured main amplifier 106 may also improve intermodulation performance. Further, Doherty configured mam amplifiers may be employed with large fractional bandwidths.

Further advantages and modifications of the above described apparatus and method will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described above. Various modifications and variations can be made to the above specification without departing from the scope or spirit of the present invention, and it is intended that the present invention cover all such modifications and variations provided they come within the scope of the following claims and their equivalents.

Claims

ClaimsWhat is claimed is:

1 . An amplifier circuit comprising: a first amplifier having a carrier amplifier and a peak amplifier configured in a Doherty arrangement, the first amplifier producing a substantially linear first output signal over a first frequency bandwidth; a second amplifier having a carrier amplifier and a peak amplifier configured in a Doherty arrangement, the second amplifier producing a substantially linear second output signal over a second bandwidth, and a combination circuit responsive to said first and second amplifiers and responsive to said first and second output signal and producing a combined output signal that is substantially linear over a combined frequency bandwidth, said combined frequency bandwidth being greater than one of said first and second frequency bandwidths.

2. The amplifier circuit of claim 1 , further comprising a third amplifier having a carrier amplifier and a peak amplifier configured in a Doherty arrangement, the third amplifier producing a substantially linear third output signal over a third bandwidth, said combination circuit being further responsive to said third output signal.

3. The amplifier circuit of claim 2, wherein said first amplifier is operating at a first transition voltage and said second amplifier is operating at a second transition voltage.

4. The amplifier of claim 2, wherein said first amplifier comprises a delay line in communication with said peaking amplifier, a transmission line coupled to the carrier amplifier, a phasing transmission line coupled to said peaking amplifier, and an output transmission line responsive to said peaking and carrier amplifiers.

5. An amplifier circuit comprising. a carrier amplifier producing a carrier amplifier output signal; a peaking amplifier coupled to the carrier amplifier in a Doherty configuration, said peaking amplifier having a voltage biased to produce i an adjusted intermodulation product signal; and a combination circuit responsive to said carrier amplifier and said peaking amplifier, said combination circuit combining said adjusted intermodulation product signal with said carrier amplifier output signal to produce a substantially linearized amplifier circuit output signal.

6. The amplifier of claim 5, further comprising a phasing transmission line coupled to said peaking amplifier.

7. An amplifier circuit comprising^* a mam amplifier having an input and an output comprising: a carrier amplifier producing a carrier amplifier output signal; and a peaking amplifier coupled to said carrier amplifier in a

Doherty configuration, said peaking amplifier voltage biased to produce an adjusted intermodulation product signal; and a combination circuit responsive to said carrier amplifier and said peaking amplifier, said combination circuit combining said adjusted intermodulation product signal with said carrier amplifier output signal to produce a substantially linearized amplifier circuit output signal; a first coupler sampling an input signal received at the input of the mam amplifier; a second coupler coupled to the output of the main amplifier; a third coupler responsive to said first and second couplers; an error amplifier having an input responsive to said third coupler and producing an error output, and a fourth coupler responsive to said second coupler and said main amplifier, said fourth coupler producing an error reduced amplified output signal.

8. A method of tuning a Doherty type amplifier circuit comprising the steps of: providing a Doherty type amplifier; measuring intermodulation performance of the Doherty type amplifier as a function of peaking amplifier bias voltage; and selecting a peaking amplifier bias voltage based on the measured intermodulation performance.

9. The method of claim 8, further comprising adjusting a matching circuit within the Doherty type amplifier.

10. The method of claim 8, further comprising adjusting a phasing line length within the Doherty type amplifier.