Radio-frequency high power amplifier with broadband envelope tracking by means of reversed buck converter

Abstract

A radio-frequency power amplifier with envelope tracking, having a power RF amplifying device for amplifying a RF signal and a switching DC/DC converter for providing the power RF amplifying device with a DC power supply at a voltage level (VSUPP) proportional to an envelope of the RF signal, wherein the switching DC/DC converter has a reversed buck topology. Advantageously the switching device is a N-type GaN Field Effect Transistor having its drain connected to the ground.

Claims

1. A radio-frequency power amplifier with envelope tracking, comprising a power RF amplifying device for amplifying a RF signal and a switching DC/DC converter for providing said power RF amplifying device with a DC power supply at a voltage level proportional to an envelope of said RF signal; wherein said switching DC/DC converter has a reversed buck topology, wherein said power RF amplifying device is connected between a first conductor which, during operation, is maintained at a constant positive voltage with respect to a ground reference point, and a second conductor serving as a local voltage reference; and wherein said switching DC/DC converter is connected for setting the voltage level of said second conductor with respect to said ground reference point at a positive value which is lower than the voltage of said first conductor, and in that the radio-frequency power amplifier further comprises a first blocking capacitor connected between an output port of said power RF amplifying device and an RF-load, and a second blocking capacitor connected between an input port of said RF amplifying device and an RF input signal source, and wherein said switching DC/DC converter comprises a switching device having a first terminal connected to said ground reference point and a second terminal connected to said second conductor through an inductor and to said first conductor through a rectifier, said rectifier being configured to allow current flow only from said switch to said first conductor, also comprising filter capacitor connected between said first conductor and said second conductor.

2. A radio-frequency power amplifier according to claim 1, wherein said switching device is a N-type Field Effect Transistor, said first terminal being a source and said second terminal being a drain.

3. A radio-frequency power amplifier according to claim 2, wherein said switching device is a GaN transistor.

4. A radio-frequency power amplifier according to claim 1, further comprising a decoupling capacitor connected between said second conductor and said ground reference point.

5. A radio-frequency power amplifier according to claim 1, further comprising a driving circuit for driving said switching device with a PWM signal representative of the envelope of the RF signal to be amplified.

6. A radio-frequency power amplifier according to claim 5 wherein said driving circuit is configured for driving said switching device in such a way that the voltage difference between said first conductor and said second conductor is proportional to said envelope of said RF signal.

7. A radio-frequency power amplifier according to claim 5, wherein said PWM signal has a frequency greater or equal to 20 MHz, and wherein said constant positive voltage of the first conductor is greater or equal to 25 V.

8. A radio-frequency power amplifier according to claim 5, wherein said PWM signal has a frequency greater or equal to 50 MHz, and wherein said constant positive voltage of the first conductor is greater or equal to 25 V.

9. A radio-frequency power amplifier according to claim 5, wherein said PWM signal has a frequency greater or equal to 50 MHz, and wherein said constant positive voltage of the first conductor is greater or equal to 50 V.

10. A radio-frequency power amplifier according to claim 5, wherein said PWM signal has a frequency greater or equal to 50 MHz, and wherein said constant positive voltage of the first conductor is greater or equal to 100 V.

11. A radio-frequency power amplifier according to claim 5, wherein said PWM signal has a frequency greater or equal to 20 MHz, and wherein said constant positive voltage of the first conductor is greater or equal to 50 V.

12. A radio-frequency power amplifier according to claim 5, wherein said PWM signal has a frequency greater or equal to 20 MHz, and wherein said constant positive voltage of the first conductor is greater or equal to 100 V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Additional features and advantages of the present invention will become apparent from the subsequent description, taken in conjunction with the accompanying drawings, wherein:

(2) FIG. 1 shows a radio-frequency power amplifier with envelope tracking comprising a conventional buck converter supplying a RF amplifier;

(3) FIG. 2 shows a radio-frequency power amplifier with envelope tracking comprising a reversed buck converter supplying a RF is amplifier according to an embodiment of the present invention;

(4) FIGS. 3A to 3D illustrate the operation of the reversed buck converter of FIG. 2;

(5) FIG. 4 illustrates a possible implementation of the conventional envelope-tracking RF power amplifier of FIG. 1;

(6) FIG. 5 illustrates a possible implementation of the inventive envelope-tracking RF power amplifier of FIG. 2;

(7) FIG. 6 shows a control scheme of the reversed buck converter of FIG. 2; and

(8) FIGS. 7A, 7B and 7C show the result of a numerical simulation in both time and frequency domains of the envelope tracking converter of FIG. 2.

(9) FIG. 1 shows the electric scheme of an envelope tracking RF amplifier HPA driven by a conventional buck DC/DC converter (reference SC).

DETAILED DESCRIPTION

(10) The RF amplifier HPA comprises a N-type Depletion mode or Enhanced mode GaN transistor T.sub.A, having its drain connected to a first conductor C1 through an inductor L.sub.D, its source directly connected to a second conductor C2 and the gate biased at a voltage V.sub.GS through an inductor L.sub.G.

(11) The RF signal to be amplified, V.sub.RF, is applied to the gate of the transistor through a DC blocking capacitor C.sub.G. The amplified signal is extracted from the transistor drain through a blocking capacitor C.sub.out to be applied to a load Z.sub.L, e.g. an antenna.

(12) The second conductor C2 is connected to the ground (GND) to serve as a reference voltage. The first conductor C1 is at a positive voltage V.sub.SUPP whose value is set by the buck converter SC.

(13) The converter SC comprises, in a conventional way, a switching device T.sub.S selectively connecting and disconnecting the first conductor C1 to a DC power supply (battery in parallel to a filtering capacitor C.sub.DC) at a voltage V.sub.DC which is higher or equal to the maximum allowed value of V.sub.SUPP. The converter also comprises an inductor L.sub.SC connected between the switching device T.sub.S and the first conductor C1, a capacitor C.sub.SC connected between the first and the second conductor and a rectifier (diode or, as disclosed in EP 2 432 118, transistor connected as a two-terminal device) having an anode connected to the second conductor (i.e. the ground) and a cathode connected between the switching device and the inductor.

(14) The operation of this conventional buck converter is known in the art. See e.g. ST Application Note AN513/0393 Topologies for switched mode power supplies.

(15) As explained above, however, switching device T.sub.S has to work at a very high frequency (e.g. 100 MHz) and at a quite high voltage level (e.g. V.sub.DC=100V, V.sub.SUPP comprised between 8 and 50 V), which requires the use of a RF transistor, hence N-type, for switching. As it can be seen, this means that the source S of the transistor is floating between the ground (0 V) and the input voltage (V.sub.DC), while the drain D is at a constant voltage V.sub.DC. As a consequence, the gate G must also be floating. This implies the use of an insulating transformer IST between the gate G and the driving circuit DRV which generates a pulse width modulation signal V.sub.G for opening and closing the switch.

(16) As mentioned above, however, at very high switching frequency the parasitic capacitance between transformer coils induces peak currents during switching, whose amplitudes can be incompatible with proper converter operation. For example, a 1 pF parasitic capacitance sinks a 1 A current peak to switch from 0 V to 100 V in 1% of the switching period at 100 MHz. Such a current level is far above the level of current circulating in the drive circuitry.

(17) Other known schemes for driving floating gates are even less suitable for use at high frequency/high voltage because they are more complex, therefore showing higher losses and/or having higher parasitic capacitances. See e.g. International Rectifier Application Note AN-978 HV Floating MOS-Gate Driver ICs.

(18) FIG. 2 shows the electric scheme of an envelope tracking RF amplifier driven by a reversed buck converter SC, according to the invention.

(19) As it can be seen, in the reversed buck converter the switching device T.sub.S and the inductor L.sub.SC are disposed between the second conductor C2 and the ground GND. When the switching device is implemented by a N-type transistor, its source S is connected to the ground and it is the drain D which is floating, being connected to the second conductor C2 via the inductor L.sub.SC. This allows the driving circuit DRV to be directly connected to the gate G of the transistor, without the need for a transformer. As a consequence the problem discussed above, due to the parasitic capacitance of the transformer, does not arise.

(20) FIGS. 3A, 3C and 3D show that, when the switch T.sub.s is closed (between time t.sub.0 and time t.sub.1) a linearly increasing electric current flows from the battery through C1 and C2 to power the HPA, the excess/deficit of current charging/discharging capacitor C.sub.SC. The positive terminal of this capacitor is at a constant voltage V.sub.1=V.sub.DC, while the negative terminal is at a voltage V.sub.2 slightly oscillating according to the charge/discharge cycle. At t=t.sub.2, the switch opens. As illustrated on FIG. 3B, a linearly decreasing current keeps flowing through the inductor L.sub.SC to continue powering the HPA, the excess/deficit of current charging/discharging the capacitor C.sub.SC. V.sub.1 remains equal to V.sub.DC while V.sub.2 slightly oscillates according to the charge/discharge cycle. The average value of V.sub.2 depends on the duty cycle =(t.sub.1t.sub.0)/(t.sub.2t.sub.0), and so does the supply voltage level of the amplifier, V.sub.SUPP=V.sub.2V.sub.1=V.sub.2()V.sub.DC.

(21) The reversed buck topology is known in the art of power electronics. See: P. Jacqmaer et al. Accurately modelling of parasitics in power electronics circuits using an easy RLC-extraction method IEEE International Instrumentation and Measurement Technology Conference (I2MTC), 13-16 May 2012, pages 1441-1446; and On Semiconductor Application Note NCL30100-D Fixed Off Time Switched Mode LED Driver Controller.

(22) However, the reverse buck topology is usually not used for power conditioning because its output voltage is referenced to the input voltage (V.sub.DC) and not to the ground. In particular, in spacecrafts it is required that all the equipments are referenced to a common ground, which is not compatible with the use a reverse buck.

(23) However, this feature is not a drawback in the application considered here because the blocking capacitors C.sub.G and C.sub.out isolate the RF signal source and the RF load from the DC potential difference between the amplifier HPA and the rest of the payload. Moreover, one or more decoupling capacitors (C.sub.dec, C.sub.dec on FIG. 2) are provided between the second conductor C2 and the ground. In the embodiment of FIG. 2, both decoupling capacitors C.sub.dec, C.sub.dec are necessary because they allow grounding at different physical locations. Moreover, C.sub.dec operates at radio-frequency, while C.sub.dec is used to filter the current flowing through the passive capacitance of inductor L.sub.SC, and operates at the switching frequency of the TS transistor. Therefore, the two capacitors are technologically different.

(24) FIG. 4 illustrates a physical implementation of a conventional RF high-power amplifier, of the kind represented under the reference HPA on FIG. 1. The amplifier is mounted on a printed circuit board PCB having a back-side metallization BSM which is in direct contact with a wide metal ground plane MGP constituting a reference physical ground to which the microstrip lines MSG and MSD, feeding the gate and drain of the amplifying transistor T.sub.A, are referenced. The DC voltages V.sub.GS and V.sub.SUPP (the latter being generated by the switching converter SC, not represented on FIG. 4) are also referenced to the metal ground plane MGP through vias connecting the front face of the printed circuit board to the metalized back-side. The RF input signal is fed from the external source V.sub.RF, referenced to the metal ground plane.

(25) FIG. 5 illustrates a physical implementation of a RF high-power amplifier according to an embodiment of the present invention, of the kind represented under the reference HPA on FIG. 2. Like in the case of FIG. 4, the amplifier is mounted on a printed circuit board PCB having a back-side metallization BSM. However, said metallization is separated from the metal ground plane MGP by an insulating layer IL, so as to constitute an internal ground reference (corresponding to the second conductor C2 of FIG. 2) floating with respect to the external ground reference MGP (which corresponds to the GND reference on FIG. 2). The internal and external ground references are connected together at radio-frequency by decoupling capacitors C.sub.dec. The decoupling capacitor C.sub.dec is not represented because it is internal to the switching converter SC.

(26) The source of the amplifying transistor T.sub.A is connected to back-side metallization BSM (not visible on the figure) and the microstrip line MSG is referenced to it, while the input RF voltage V.sub.RF is referenced to the external ground reference MGP, the necessary voltage shift being ensured by capacitor C.sub.G. Similarly, the load Z.sub.L is referenced to the external ground MGP while the microstrip line MSD is referenced to the local ground BSM, the necessary voltage shift being ensured by the output capacitor C.sub.out. The DC voltages V.sub.GS and V.sub.SUPP (the latter being generated by the switching converter SC, not represented on FIG. 5) are also referenced to the local ground through vias connecting the front face of the printed circuit board to the metalized back-side BSM.

(27) An advantageous feature of the invention is that buck converters can easily be controlled in closed loop, which is not the case for boost converters. FIG. 6 shows a block diagram of a feedback control for the envelope tracking amplifier of FIG. 2. A high bandwidth differential amplifier DA built around an operational amplifier with a series RC feedback network extracts, integrates and amplifies the output voltage V.sub.SUPP of the reverse Buck converter in order to perform Proportional-Integral (PI) regulation, and provides it to the a pulse-width modulation driver PWMD in charge of controlling the ON/OFF operation of the converter switch T.sub.S.

(28) On FIG. 7A, V.sub.env, made of straight line segments, represents the envelope of the RF signal i.e. the reference signal for the control of the converter. The output of the buck converter SC, V.sub.SUPP, features a limited amplitude ripple at the switching frequency and follows the reference signal quite well thanks to the control scheme. The V.sub.S.sup.F signal represents the converter output signal where the ripple has been eliminated by an additional filtering (represented by C.sub.dec on FIG. 2 but which can be of different topology).

(29) FIGS. 7B and 7C show, respectively, the open loop gain and phase of the control system of FIG. 6. It can be seen that the phase response is reasonably flat up to about 30 MHz: this means that the envelope tracking will work as expected for envelope bandwidths up to 30 MHz.

(30) The invention has been disclosed with reference to a specific technology (GaN transistors, or more generally N-type transistors) and a specific application (space telecommunications). However, the scope of the invention is more general and also applies to different technologies and applications. In particular, the switching transistor T.sub.S can be either of the enhancement (normally off) type or of the depletion (normally on) type, and of GaAs or LDMOS technologies. Same applies to the RF transistor T.sub.A.