GB2535180A - AC amplifier output impedance reduction - Google Patents

Abstract

A modulated power supply 28 for an RF PA stage 2 includes a switch-mode stage 10 and a correction stage 16 which both receive an input signal 21. The outputs of the switch-mode stage 10 and the correction stage 16 are combined in a combiner circuit 30, in which current from the switch mode stage 10 passes through an inductor 14. The correction stage 16 receives a feedback signal on line 50 containing information about the switching current flowing in the inductor 14. The feedback allows the signal generated by the correction stage 16 to be adjusted to reduce the current ripple due to the switching of the switch mode stage 10, and hence to lower the output impedance of the power supply 28. The feedback signal 50 may be taken from a shunt network 44 comprising a capacitor 42 and a resistor 40. The voltage at the node between these components provides information that is not otherwise available in the modulated power supply, and may be used to improve the switched power supply. The circuit 44 may be provided for other purposes, e.g. to suppress resonance.

Description

AC AMPLIFIER OUTPUT IMPEDANCE REDUCTION

BACKGROUND TO THE INVENTION:

Field of the Invention:

The present invention relates to power supplies incorporating switching stages, and partioularay but not exclusively to envelope tracking power supplies for radio frequency power amplifiers,

Description of Related Art:

An envelope tracking power supply for a radio frequency power amplifier is an example of a power supply incorporating switches. The switches in a switching stage of such a power supply switch between different supply levels in order to aenerate an output modulated power supply, The switching introduces noise into the signal at the output of the power supply.

For efficient power supplies it. is advantageous in at least. some scenarios to minimise any noise associated with such switchincf, in order to avoid such noise being present. on the modulated power supply itself, It is an aim of the present invention to provide a 25 technique to reduce the switching noise caused fly switching in a switched power supply,

SUMMARY OF THE INVENTION:

There is provided a power supply including $0 swtchihg stage and configured to adapt the generated power supply signal to remove at least some of the currents in that s-ignel.

The power supçi.yniay further comerise a circuit configured to sense the switching currents in the power sueoly.

The power supply may include a switching stage, a correction stage and a combiner, the combiner configured to combine the outputs of the switching stage and the correction stage to provide the generated power supply, wherein the circuit is connected to the output of the correction stage, The combiner may include an inductor, the circuit being configured to sense the switching currents in the indugt0r, The circuit may include a series connection of a resistor and a capacitor between the combiner and ground, The power supply may further comprise a connection 20 from the Junction of thr-') resistor and capacitor to the correction stage.

The correction stage may include an amplifier. and is configured to modify the correction stage output in 25 dependence on the connection, The correction stage may he configured to modify the correction stage output further in dependence on n feedback connection from the output of the correction 30 stage, The combiner may comprise an inductor having one terminal connected to the output of the switching stage and a second terminl connected to the output of the correction stage, the circuit being connected to the second terminal of the inductor and the power supply signal b5iiig generated at the second terminal, of the inductor.

The combiner may comprise an inductor and a capacitor, the inductor having. one terminal connected to the output of the switching stage and another terminal connected to one terminal of the capacitor, and the capacitor having another terminal connected to the output of the correction stage, the circuit being connected to the second terminal of. the capacitor and the power supply signal being. generated at the second terminal of the inductor.

BRIEF DESCRIPTION OF THE FIGURES:

The invention is now described by way of reference to the drawings, in which: FIG. 1 illustrates an example power supply including a switching stage; FIG. 2 illustrates a clot of inductor current for an arrangement of FIG. I using an inductor in a combiner; FIG. 3 illustrates an arrangement of FIG. 1 to 95' obtain information on switchihq currents in the inductor; FIG. 4 illustrates an improved power supply including a switching stage; FIG. 5 illustrates a further improved power supply including a switching step,' and FIG. 6 illustrates a yet further improved power supply including a switching stage.

DES CR I' ON OF PREkERREU EMBODIMENTS: The invention is now described with reference to particular examples and with reference in particular to an example of a switched power supply stage of an envelope tracking modulated power supDly for radio frequency amplifier. The invention is not limited to such an example implementation.

With reference to FIG. I an exempla modulated power supply stage 28 provides a. modulated supply signal on lino 8 to a power amplifier (PA) 2, The PA 2 receives 10 a radio frequency (P.F.) input signal on line 4 and generates an RE' output signal on line 6. The PA. 2 represents a load being driven by the modulated power supply stage 28.

The exemplary modulated power supp stage 23 15 includes a switching stage or switcher lu,a correction stage 16, and a combiner 30. The combiner,t) includes an inductor 14 and a capacitor. 26. The switching stage 10 generates a switcher output signal on line 12. Line 12 15 connected to one terminal of the inductor 14. The other terminal of the inductor 14 is connected to the output line 8. The correction stage 16, which includes correction amplifier. 18, generates a correction signal on line 24. Line 24 is connected to one terminal of a capacitor 28. The other terminal of the capacitor 26 is connected to the output signal on line 3. The inductor 14 and capacitor 26 tocether form. the combiner 30 which. combines the output signals from the switching stage and the correction. stage to produce the modulated supply signal on. line 8. A reference signal representing the desired output signal. level is provided on line 21 to the switching stage 1.0 and to the correction. stage 16. The reference signal on line 21 is provided to the correction stage on line 20 and forms One input to the correction amplifier 18, The other input to the correction amplifier 18 is a feedback signal, with the output signal of the correction amplifier on line 24 being fed back to its input as denoted by line 19.

Modulated power supply stages are designed to have low impedance outputs in order to minimise the noise current in t.heir output signal, A switcher current, denoted stIcherr flowing. in the inductor 14 contributes to the noise current in the output: signal, FIG. 2 illustrates a clot 7 or the total current flowinc in inductor 14, plotted against. frequency.. The plot 7 includes a. portion of wanted signal, a portion associated with the self-resonance of the inductor, and a portion as denoted by reference numeral 9 which is distorted due to the ripple current associated with the switching current dashed.

from the switching stage line 5 continuing from the Th,+, represents the ideal wanted signal in which the ripple current and the current due to the self-resonance are 20 eiiminated, A modification is proposed to reduce the ripple current.

FIG. 3 shows a modification to the arrangement (-I FIG, 1, in which. a circuit 44 is connected to the output of the: correction stage 16. The circuit 44 includes a capacitor. 42 having a. first terminal connected to the line 24, and a resistor 40 having a first terminal connected to the second terminal of the capacitor 42 and a second terminal connected to electrical ground.

The current flowing in the circuit 44 provides information on the switching current flowing in the inductor 14. This information can be used to improve the switched power supply of FIG. 1 to remove at least some of the switching current in the generated power supply signal.

FIG. 4 illustrates a further modification to the arrangement of FIG, 3. A signal. line 50 is connected from S the circuit 44 -the connection point of the resistor 40 witfl the capacitor 42 -to the correction stage 16. The correction stage 15 thus receives, with this connection, information about the current flowing in the conductor 14 from the circuit. 44. The signal on 50 is a feedback signal to the correction stage.

The feedback signal to the correction stage utilises the characteristic. that the voltage at the node between the resistor 40 and capacitor 42 in the circuit 14 provides information that is not otherwise available in the modulated bower supply.

The feedback provided by the signal on line 50 can be made fast and can operate at high frequencies.

FIG. 5 shows a specific: arrangement in which the feedback of the correction stage is used together. with the feedback.. of the information. on the switching current flowing in the inductor 14. The correction stage 16 is shown, as additionally including an amplifier 54 and a feedback stage 52.

The correction ampfier 34 receives the feedback. 25 signal on line 50, amd receives the output of the amplifier 54. The amplifier. 54 receives the reference signal on line 20, and the cutout of the correction amplifier 34 fed back through a feedback stage 52. The feedback stage 52 applies any necessary at to the feedback signal before it is applied to the ampl.if.ter 54. The feedback stage 52 and the amplifier 51 provide, in the arrangement of FIG. 5, the functionality of the feedback provided by the signal on line 19 in the figures.

The feedback provided on signal line 50 allows the signel generated by the correction stage lc, in the FIG. 5 arrangement the signal generated by the amplifier. 34, to he adjusted to reduce the current rioule disc, to switching in the output signal,: and hr-snce lower the output impedance of the modulated power supply stage 28, FIG, 6 shows sueltennative arrangement. The combiner 36 of the FIG. 6 arrangement does not include a capacitor, The capacitor of the combiner of any previous figures may be eliminated in certain implementations, and. yt, 6 illustrates how the advantageous feedback connection may still be utilised even when the cacacjtor 15 of the combiner is not imc.demented.

The switching stage and the correction stage are sig ncl. a 0

embodiments they may receive different reference In embodiments they may not both recei.vc reference signals, The reference signal may indicate the ideal cutout signal. The reference signal may be the envelope of the RE input signal on line 4.

The circuit. 44 may be provided for other purposes, and the invention may take advanLage of the provision of this circuit for other purposes, to provide information. which can be used to improve the switched power supply without adding additional, circuit components, and just adding the connection of line 50. For example the circuit 44 may be provided to suppress resonance as 30 disclosed in WO 2005/081399.

The capacitance value of the capacitor 42 and the: resistance value of the resistor 40 will be implementation dependant.

each shown receiving a common reference * The invention has been described with reference to example implementntions, The invention is not limited to the. specifics of any described embodiment, and any described element may be used incH. viduallu or in combination within the scope of the appended claims,