GB2395076A

GB2395076A - Linear high power RF amplifiers

Info

Publication number: GB2395076A
Application number: GB0225436A
Authority: GB
Inventors: Andrew David Vare
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2002-11-01
Filing date: 2002-11-01
Publication date: 2004-05-12
Also published as: GB0225436D0

Abstract

Improved class AB or class B high power transistor amplifiers are described; the amplifiers being intended to operate with good linearity at radio frequency (RF), typically in the 3G UMTS frequency range of 2.14 to 2.17 GHz, and with widely spaced multiple carrier frequencies. The linearity of such amplifiers is known to be adversely influenced by phenomena known as "memory" or "delay", whereby unequal 3<rd> order intermodulation products are produced at respective carrier frequencies, and the invention counters this problem by providing for bias, applied to the drain electrode and/or to the gate electrode, to be influenced by an inductance comprising a transmission line configured to present an impedance of 100 ohms or less at RF envelope frequencies. Typically the characteristic impedance of the transmission line is no greater than 50 ohms, and preferably less than 20 ohms, and its electrical length is less than one quarter wavelength by at least 10%.

Description

LINEAR HIGH POWER RF AMPLIFIERS

This invention relates to class AB or class B high power transistor amplifiers intended to operate with good linearity at radio frequency 5 (RF)

It is well documented that problems arise with such amplifiers due to "memory", observed as amplitude imbalance between intermodulation products, attributable at least in part to amplitude modulation to 10 amplitude modulation (am-am) and amplitude modulation to phase modulation (am-pm) conversion within an amplifying transistor, or "delay", attributable to the re-introduction of am-am products into the

amplifying transistor by way of the biasing networks, causing am-pm distortion. If such effects are not controlled, the intermodulation (imd) products, especially those of the 3r order, can, when viewed in the frequency spectrum, appear different in level at certain two-tone carrier spacings, thereby limiting the usefulness of the amplifier in applications requiring 20 a high efficiency at a given linearity.

The above problems are discussed, inter alla, in "Evaluation and Correction of Time Dependent Amplifier Non-Linearity"; Katz A and Dorval R; 1996 IEEE M'I T-S Digest; pages 839 - 842, and in 25 "Measurement Technique for Characterizing Memory Effects in RF Power Amplifiers"; Vuolevi, Joel H K, Rahkonen, T and Manninen, Jani 1' A; IEkE transacts ou Nlic'uwave Tacky find Tchniqucs, \'ol 19,

No. 8, August 2001. Both papers discuss techniques for measuring effects attributable to the problems outlined above, and the paper by Katz and Dorval discusses, principally on page 841, the possibility that the principal cause of linearity degradation may be due to changes in the bias 5 currents; especially in the circuitry associated with the transistor's drain electrode and, to a lesser extent, in that associated with the gate. In the example discussed by Katz and Dorval (where the amplifier under test comprised a GaAs MESFET), the adverse effects were reduced by adding special low-inductance capacitors on the device side of the drain 10 decoupling circuitry.

The use of such capacitors, however, addresses the problem only to a limited extent, and works over only a limited range of frequency separations, since it depends upon differential resonance effects.

It is an object of this invention to provide a high power RF amplifier in which a useful degree of linearity is preserved over a wide range of operating conditions.

20 According to the invention there is provided a high power RF amplifier including a transistor device configured to operate in class B or in class AB mode, wherein bias applied to the drain electrode and/or to the gate electrode is influenced by an inductance comprising a transmission line configured to present an impedance of approximately lOO ohms or less at 25 RF frequencies. By this means, a very low impedance is presented at RF envelope frequencies whilst maintaining a high impedance at the working carrier frequencies.

Typically, the amplifier may be used in the 3G UMTS frequency range of 2. 14 to 2.17 GHz, but it can find application in any frequency range from about 500 MHz upwards. In the UMTS application, multiple carrier s frequencies may be spaced apart by as much as 5 to 60 MHz, but the invention is also applicable to applications utilising larger spacings.

In one preferred embodiment, the transmission line is implemented as a microstripline with a characteristic impedance no greater than 50 ohms 0 and preferably less than 20 ohms.

In alternative embodiments, the transmission line may comprise a stepped microstripline, a stripline or a stepped stripline.

15 It is preferred that the transmission line is connected in series between the bias source connection and the drain and/or gate electrode of the transistor. Alternatively or in addition however, one or more transmission lines may 20 be connected so as to provide a shunt connection to earth from the drain and/or gate electrodes.

The or each transmission line is preferably electrically shorter, by at least 10%, than one quarter wavelength; the arrangement being configured to 25 present, in conjunction with shunt capacitors, a short circuit to those signal components which could otherwise result in memory or delay problems.

In any of the foregoing configurations, any transmission line may be supplemented by one or more further transmission lines of similar characteristics, connected in parallel therewith, to further reduce the 5 impedance presented thereby to RF signals.

In order that the invention may be clearly understood and readily carried into effect, certain embodiments thereof will now be described, by way lo of example only, with reference to the accompanying drawings of which: Figure I shows, in schematic outline, a portion of the circuitry associated with an amplifier according to one example of the invention; 5 Figure 2 shows, in similar view to Figure 1, circuitry associated with an amplifier according to another example of the invention; and Figure 3 shows, again in similar view to Figure 1, circuitry associated with an amplifier according to a further example of the invention; Figure 4 shows, again in similar view to Figure 1, circuitry associated with an amplifier according to a still further example of the invention; and 2s Figures 5a and 5b show graphs indicative of the improved performance achievable by means of certain embodiments of the invention.

in all figures, equivalent components are designated by the same reference numbers.

Referring now to Figure 1, a field effect transistor 11 of any convenient

5 kind, such as a GaAs MESFFf, has source, drain and gate electrodes 12, 13 and 14 respectively. Bias voltage +V is applied to the drain electrode 13 via a terminal 15 and by way of a low-inductance transmission line 16, of low characteristic impedance, connected in series between the terminal 15 and the drain electrode 13. The characteristic impedance of 0 the transmission line 16 is no more than 50 ohms; preferably no more than 20 ohms, and in this example it is selected to be around 15 ohms.

Moreover, the length of the line 16 is shorter, by at least 10%, than one quarter wavelength at the RF frequency. This configuration, in conjunction with the shunt capacitors shown at 30 and 31, presents an 5 impedance of about 100 ohms or less at RF, and very low impedance at the RF envelope frequency (which may extend up to 50 MHz). This ensures that the envelope voltages at the drain electrode 3 of transistor 1 are not significantly modified by the source resistance of the power supply, including the circuit used to supply do bias to the transistor 11.

The circuit configuration shown in Figure 1 differs significantly from the conventional wisdom associated with the design of such circuitry, which contends that, for a circuit intended for the application of tic to a transistor device, it is necessary to present a very high RF impedance to 25 the device; a provision usually contrived, at microwave frequencies, by means of a relatively narrow microstripline, of characteristic impedance greater than S() chins an1 elf line. quarter wavelength.

Most high power microwave transistors, such as LDMOSFETs and GaAsFETs, are formed with internal circuitry which worsens the performance of the transistor itself as regards memory and delay. The s use of a very low inductance line, as shown in Figure 1, minimises the adverse effects of such internal circuitry in this context.

Figure 2 shows a circuit according to another example of the invention, wherein the transmission line 16 of low characteristic impedance (c.f.

0 Figure 1) is replaced by a standard transmission line 20 of characteristic impedance 50 ohms or more and a quarter wavelength long. However, a transmission line 21, having the low characteristic impedance exhibited by the line 16 of Figure 1, and being shorter by at least 10% than one quarter wavelength, is connected so as to shunt the drain electrode 13 to 15 earth via capacitors 22 and 23.

Figures 3 and 4 show circuits according to further examples of the invention, which include transmission lines 16 and 21, both of low characteristic impedance, in both series and shunt configurations. The 20 two circuits differ only in their application of bias, as illustrated.

Transmission lines such as 16 and/or 21 car' be connected to the biasing circuit for the gate electrode 14 instead, of or as well as, that for the drain electrode 13.

In all embodiments, transmissio-ii liaises sucl' as 16 or 21 may be impiell-ie'ittU 5 iiiiliipliii., s4iipli.nc: Or steFFed ctripline

Moreover, in embodiments such as that shown in Figure 3, where two or more transmission lines are employed, one may take the form of a transmission line of low characteristic impedance and the other may take an alternative form, such as a microstripline, or a stepped stripline, s stripline or stepped stripline.

in any of the foregoing configurations, any transmission line such as 16 lo or 21 may be supplemented by one or more further transmission lines of similar characteristics, connected in parallel therewith, to further reduce the impedance presented thereby to RF signals. Where necessary, when such parallel connections are used, decoupling capacitors may also be I duplicated, or their capacitance values may be adjusted appropriately.

Figures 5a and 5b show graphically the levels of third order intermodulation as the spacing between the carriers is varied, for a conventional amplifier and an amplifier in accordance with an embodiment of the invention respectively. Comparison of the graphs 20 readily reveals that, by means of the invention, the carrier spacings over which the same transistor amplifier can be used is significantly extended, from about 12MHz to about 37MHz.

Claims

CLAIMS:

1. A high power RF amplifier including a transistor device configured to operate in class B or in class AB mode, wherein bias s applied to the drain electrode and/or to the gate electrode is influenced by an inductance comprising a transmission line configured to present an impedance of lOO ohms or less at RF frequencies.

2. An amplifier according to claim 1 configured for use in the 3G 0 UMTS frequency range of 2.14 to 2.17 GHz.

3. An amplifier according to claim l or claim 2 wherein the transmission line has a characteristic impedance no greater than 50 ohms.

5

4. An amplifier according to claim 3 wherein the transmission line has a characteristic impedance of less than 20 ohms.

5. An amplifier according to claim l or claim 2 wherein the transmission line comprises one of a microstripline, a stepped 20 microstripline, a stripline or a stepped stripline.

6. An amplifier according to any preceding claim wherein the transmission line is connected in series between a bias source connection and said drain and/or gate electrode.

7. An amplifier according to any preceding claim wherein the

() transmission line is connected so as to provide a shunt connection to earth froth said drain and/or gate electrodes.

8. An amplifier according to any preceding claim wherein said 5 transmission line is supplemented by one or more further transmission lines of similar characteristics, connected in parallel therewith.

9. An amplifier according to any preceding claim wherein said transmission line is electrically shorter, by at least 10%, than one quarter 10 wavelength at the RF envelope frequency.

lO. A high power RF amplifier substantially as herein described with reference to, and/or as shown in, the accompanying drawings.