RF directional coupled output from a quadrature combined amplifier

A radio frequency (RF) coupling circuit for coupling an RF output of a quadrature combined amplifier. According to an embodiment, the RF coupling circuit includes a phase shifting component and a coupling network. The phase shifting component provides a predetermined phase shift to an RF signal at a first output terminal of the quadrature combined amplifier. The coupling network combines the phase shifted first output signal with an RF signal at a second output terminal of the quadrature combined amplifier.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. provisional application No. 61/444,000 titled: “RF Directional Coupled Output from a Quadrature Combined Amplifier” filed Feb. 17, 2011 the disclosure of which is hereby incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to the field of a quadrature combined amplifier. More specifically, it relates to a circuit arrangement for obtaining a radio frequency (RF) coupled output from the quadrature combined amplifier.

BACKGROUND

Quadrature combined amplifiers are widely used in power amplification applications in microwave and radio frequency (RF) spectrum. The advantage of the quadrature combined amplifier is that optimum impedance matching can be done for low noise, gain flatness, and power and linearity while the input and output return loss (VSWR) will be good as long as the two single ended amplifiers (within the quadrature amplifier) are identical. Quadrature combined amplifiers are also commonly used rather than single ended amplifiers for multiple cascaded stages due to the low interaction and enhanced stability because of the good input and output return loss. Also, the quadrature combined amplifier is less sensitive to load mismatch and more reliable in operation than the single ended amplifier. Furthermore, there are a variety of other application areas where the quadrature combined amplifier is used, such as, microwave heating, driving another high power source, driving a transmitting antenna, exciting resonant cavity structures, and so forth.

Existing RF amplifying systems based on quadrature combined amplifiers use a directional coupler to obtain an RF coupled output with directivity. The directional coupler is bulky and may have an undesirable impact on the size and the overall cost of the RF amplifying system. In light of the above, there is a need of a system that reduces overall size and cost of the RF amplifying system.

SUMMARY

Various embodiments of the invention provide an RF coupling circuit for coupling an RF output of a quadrature combined amplifier. In an embodiment, the RF coupling circuit includes a phase shifting component having a first terminal and a second terminal. The first terminal of the phase shifting component is connected to a first output terminal of the quadrature combined amplifier. The phase shifting component provides a predetermined phase shift to a first RF signal at the first output terminal of the quadrature combined amplifier. The phase shifted first RF signal is obtained at the second terminal of the phase shifting component.

The RF coupling circuit further includes a coupling network for obtaining an RF coupled output. The coupling network has a first terminal, a second terminal, and a third terminal. The first terminal of the coupling network receives a second RF signal from a second output terminal of the quadrature combined amplifier. The second terminal of the coupling network receives the phase shifted first RF signal from the second terminal of the phase shifting component. The RF coupled output is obtained at the third terminal of the coupling network.

DETAILED DESCRIPTION

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 just for explanatory purposes. The disclosed systems 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.

Various embodiments of the invention provide an RF amplifying system for providing an RF output with directivity with overall reduced size, insertion loss, and cost of the RF amplifying system. The RF amplifying system includes a quadrature combined amplifier and an RF coupling circuit coupled to the quadrature combined amplifier. The RF coupling circuit provides an RF coupled output with directivity.FIG. 1is a block diagram of an RF amplifying system100in accordance with an embodiment of the invention. RF amplifying system100includes a quadrature combined amplifier102, a phase shifting component104, a coupling network106, a first coupling element in the form of a capacitor108, and a second coupling element in the form of a capacitor110.

Quadrature combined amplifier102has an input terminal103, a first output terminal112, and a second output terminal114. A first terminal of phase shifting component104is connected to first output terminal112through a first coupling capacitor108. It is apparent to a person of ordinary skill in the art that the first terminal of phase shifting component104may also be directly connected to first output terminal112, without departing from the scope of the ongoing description. Coupling network106has three terminals, in which, a first terminal116is connected to second output terminal114through second coupling capacitor110, a second terminal118is connected to a second terminal of phase shifting component104, and a third terminal120is used to obtain an RF coupled output. It is also apparent to a person of ordinary skill in the art that second terminal118of coupling network106may also be directly connected to second output terminal114of quadrature combined amplifier102, without departing from the scope of the ongoing description.

Quadrature combined amplifier102receives an RF signal at input terminal103. The RF signal is internally divided into an in-phase component and a quadrature component. The in-phase component and the quadrature component are amplified by quadrature combined amplifier102. The amplified in-phase component (which is hereinafter referred to as first RF signal) is obtained at first output terminal112of quadrature combined amplifier102. The amplified quadrature component (hereinafter referred to as a second RF signal) is obtained at second output terminal114of quadrature combined amplifier102. It is understood by any person of ordinary skill in the art that the second RF signal is ninety degrees out of phase with respect to the first RF signal. Further, the functionality of quadrature combined amplifier102will be explained in detail in conjunction withFIG. 2.

Phase shifting component104receives the first RF signal from first output terminal112. Phase shifting component104provides a predetermined phase shift to the first RF signal. In an embodiment, phase shifting component104provides a ninety degrees phase shift to the first RF signal. The phase shifted first RF signal is obtained at the second terminal of phase shifting component104. Various examples of phase shifting component104, may include, but are not limited to a transmission line, a low pass filter, an all pass filter, or a lumped equivalent of the transmission line. The lumped equivalent of the transmission line may include one or more inductors and/or one or more capacitors.

Coupling network106receives the second RF signal at first terminal116from second output terminal114. Coupling network106also receives the phase shifted first RF signal at second terminal118from the second terminal of phase shifting component104. In an embodiment of the invention, coupling network106combines the phase shifted first RF signal and the second RF signal to provide the RF coupled output at third terminal120. In another embodiment of the invention, coupling network106sums the phase shifted first RF signal and the second RF signal to provide the RF coupled output at third terminal120. In an embodiment, coupling network106includes a plurality of resistors. Coupling network106will further be explained in conjunction withFIG. 2.

FIG. 2is a block diagram of an RF amplifying system200in accordance with another embodiment. The RF amplifying system200includes various elements in the RF amplifying system100(refer toFIG. 1). Quadrature combined amplifier102includes a first amplifying device such as an amplifier202a, a second amplifying device such as an amplifier202b, a first quadrature coupler204, a second quadrature coupler206, and a resistor218. Coupling network106includes a resistor228, a resistor230, and a resistor232.

First quadrature coupler204has four terminals (222,224,208, and210). Similarly, second quadrature coupler206also has four terminals (236,238,216, and220). Various examples of quadrature couplers, such as, first quadrature coupler204and second quadrature coupler206, include, but are not limited to coupled transmission lines, a branch-line coupler, a micro-strip “3-dB” coupler, a strip-line broadside coupler, a Lange coupler, such as, MLANG, and the like. In an embodiment, first quadrature coupler204and second quadrature coupler206are 3-dB Lange couplers. In first quadrature coupler204, terminal222corresponds to an input terminal, terminal224corresponds to an isolated terminal, terminal208corresponds to a coupled terminal, and terminal210corresponds to a through terminal. Further, it is apparent to a person of ordinary skill in the art that a signal at terminal210is ninety degree out of phase with reference to a signal at terminal208.

An RF input signal is applied at terminal222of first quadrature coupler204. Terminal224is grounded (terminated) through resistor226. Terminal208is connected to an input terminal of amplifier202aand terminal210is connected to an input terminal of amplifier202b. A first output terminal212of amplifier202ais connected to terminal236(corresponds to a through terminal) of second quadrature coupler206. Similarly, a second output terminal214of amplifier202bis connected to terminal238(corresponds to coupled terminal) of second quadrature coupler206. Terminal216(corresponds to an isolated terminal) of second quadrature coupler206is grounded (terminated) through resistor218. An RF output is obtained at terminal220. Various examples of the amplifying devices, such as, amplifier202aand amplifier202b, include, but not limited to any amplifier that includes a transistor, such as, a bipolar junction transistor (BJT), or field effect transistor (FET), gallium arsenide HBT, MESFET, pHEMPT, etc, gallium nitride FET, and the like.

First output terminal212of amplifier202ais connected to the first terminal of phase shifting component104through first coupling capacitor108. The second terminal of phase shifting component104is connected to second terminal118of coupling network106(shown as a dotted line shape). Second output terminal214of amplifier202bis connected to first terminal116of coupling network106through second coupling capacitor110.

First terminal116is connected to third terminal120through resistor230. Second terminal118is connected to third terminal120through resistor228. Third terminal120is grounded through resistor232. It is apparent to a person of ordinary skill in the art that coupling network106may also include more than three resistive elements connected in a suitable fashion to obtain the RF coupled output without limiting the scope of the invention. In an embodiment, coupling network includes one or more impedance elements.

The RF input signal applied at terminal222is divided into an in-phase component and an out-of-phase component at terminal208and terminal210, respectively. The RF signals at terminals (208,210) are amplified by amplifiers (202a,202b). An amplified output RF signal at second output terminal214is ninety degree out of phase with respect to an amplified output RF signal at first output terminal212. Further, an RF output signal of quadrature combined amplifier102is obtained at terminal220.

Phase shifting component104provides a ninety degree phase shift to the amplified output RF signal appearing at first output terminal212. The phase shifted RF signal is available at the second terminal of phase shifting component104. As discussed earlier, the amplified output RF signal at second output terminal214is ninety degree out of phase with respect to an amplified output RF signal at first output terminal212. Coupling network106combines the phase shifted RF signal from phase shifting component104and the amplified output RF signal at second output terminal214. Thus, the RF coupled output is obtained at third terminal120of coupling network106.

In an embodiment, RF input signal of magnitude one volt and phase zero (1, 0°) is applied at terminal222. Further, Table-1 illustrates the relative magnitude and phase of RF forward voltages at different terminals inFIG. 2in accordance with an embodiment.

‘A’ denotes amplitude of an RF signal at first output terminal212and second output terminal214assuming amplifiers202aand202bare identical and having the same gain value.

‘K’ denotes amplitude of an RF signal at third terminal120of coupling network106. In an embodiment, the value of ‘K’ depends on the values of resistors228,230, and232. In another embodiment, the value of ‘K’ depends on the values of coupling capacitors108and110, and of resistors228,230, and232.

It is observed from the Table-1 that the RF coupled output at third terminal120of coupling network106is proportional to the RF output of quadrature combined amplifier102at terminal220.

Table-2 illustrates the relative magnitude and phase of RF reflected voltages at different terminals inFIG. 2in accordance with an embodiment. For simplicity of the ongoing description, voltage (1, 0°) is assumed at terminal220to calculate reflected voltages at other terminals and amplifiers202aand202bhave identical electrical properties.

It is observed from Table-2 that no reflected voltage appears at third terminal120, hence, RF coupled output with directivity is obtained. This is further explained in conjunction withFIG. 3.

In an embodiment, a load resistor (hereinafter referred to as a load RL) may be connected at terminal220. Further, the RF input signal to terminal222may be provided from an RF voltage source. Further, Table-3 discloses various exemplary elements and corresponding values of various parameters of the elements ofFIG. 2in accordance with an embodiment.

It is apparent to a person of ordinary skill in the art that suitable elements with values other than the values shown in the table above may also be used without departing from the scope of the ongoing description.

FIGS. 3a,3b, and3care graphical representations300a,300b, and300cdepicting power output at various terminals inFIG. 2with specific values of parameters of the elements in conjunction with Table-3.

FIG. 3ais a graphical representation300adepicting a power output of a quadrature combined amplifier with varying load impedance (load-pull) in accordance with an embodiment.

In graphical representation300a, the vertical axis represents the output power on a logarithmic scale. The horizontal axis represents the phase φ (in degrees) of the load reflection coefficient (rho) varied all phases from magnitude=0.429, phase=0° (corresponding to Load RL=125 Ohms real) to magnitude=0.429, phase=180° (corresponding to Load RL=20 Ohms real). The output load impedance (Load RL) is varied with a constant VSWR of 2.5:1 all phases or a return loss of about 7.36 dB. The change in the output load impedance (Load RL) results in to the change in the phase φ. A curve302depicts various values of an RF power at terminal220of second quadrature coupler206, for different values of the phase φ.

FIG. 3bis a graphical representation300bdepicting an RF coupled output with varying load impedance (load-pull) in accordance with an embodiment. In graphical representation300b, the vertical axis represents the RF coupled output on a logarithmic scale. The horizontal axis represents the phase φ (in degrees) of the load reflection coefficient (rho) varied all phases from magnitude=0.429, phase=0° (corresponding to Load RL=125 Ohms real) to magnitude=0.429, phase=180° (corresponding to Load RL=20 Ohms real). The output load impedance (Load RL) is varied with a constant VSWR of 2.5:1 all phases or a return loss of about 7.36 dB. The change in the output load impedance (Load RL) results in to a change in the phase φ. A curve304depicts various values of an RF power at an output of coupling network106, i.e. at third terminal120of coupling network106, for different values of the phase φ.

Further, the load reflection coefficient (rho) is explained below, during the calculation of directivity, in conjunction withFIG. 3c.

It is seen fromFIG. 3aandFIG. 3bthat curve304closely resembles curve302, i.e. the RF coupled output is proportional to the power output of quadrature combined amplifier102.

FIG. 3cis a graphical representation300cdepicting the difference in the quadrature combined amplifier102output and the RF coupled output with varying load impedance (load-pull) in accordance with an embodiment. In graphical representation300c, the vertical axis represents error values on a logarithmic scale. As explained earlier, the horizontal axis represents the phase φ (in degrees) of the load reflection coefficient (rho). A curve306represents difference or error in the power output of quadrature combined amplifier102(at terminal220) and coupling network106(at third terminal120) for different values of the phase φ. As disclosed earlier, curve306is based on specific values of parameters of the elements in conjunction with Table-3.

In an embodiment, the error may be due to signal losses or signal reflections due to various elements ofFIG. 2as listed in Table-3. In accordance with an embodiment, the value of error is about 0.137 dB.

In an embodiment, in conjunction with Table-3, directivity of the RF coupled output may be approximately calculated using following equation:
Directivity=−20 LOG(10(dBErr/20)−1)−dBRL
Where, dBErr=0.137; and
dBRL may be calculated using following equations:
dBRL=−20 LOG(rho)
Where, rho is the reflection coefficient. In an embodiment of the invention, the value of rho may be calculated approximately using following formula:
rho=MAG((Load RL−50)/(Load RL+50))
Based on the above equation, for Load RL=20 Ohm (real), the value of rho is 0.429. Further, for Load RL=125 Ohm (real), the value of rho is 0.429.

In conjunction with above equations and Load RL=20 Ohms, the values of Directivity, dBRL, and dBErr is shown in a Table-4.

It is observed that the RF coupled output with directivity 28.61 is obtained at terminal third120of coupling network in the disclosed embodiment.

The embodiments of the invention provide several advantages. The coupling network according the embodiments of the invention provides the RF coupled output with directivity. The requirement of an additional directional coupler can be avoided hence reducing the overall cost and the size of the RF amplifying system. In an embodiment of invention, the coupling network includes a network of resistors which further reduces an overall weight of the RF amplifying system.