GB2440772A

GB2440772A - A power supply modulator for an RF amplifier

Info

Publication number: GB2440772A
Application number: GB0615762A
Authority: GB
Inventors: Kevin Morris; Paul Warr
Original assignee: Asahi Kasei Microsystems Co Ltd; Asahi Kasei Microdevices Corp; Asahi Kasei EMD Corp
Current assignee: Asahi Kasei Microdevices Corp
Priority date: 2006-08-08
Filing date: 2006-08-08
Publication date: 2008-02-13
Anticipated expiration: 2026-08-08
Also published as: JP2008042917A; GB0615762D0; JP5275590B2; GB2440772B

Abstract

A power supply modulator for an RF power amplifier comprises a combination of a linear amplifier 22 supplying high frequency envelope signals and a switching amplifier 24 supplying low frequency and DC envelope signals. The feedback for the linear amplifier 22 is from the output side of the sense resistor Rsense. The linear amplifier 22 does not need to amplify the LF components of the envelope signal. Efficiency is increased and the output current rating of the linear amplifier can be reduced. The RF amplifier may be for W-CDMA or OFDM signals.

Description

ENVELOPE MODULATOR

This invention relates to a modulator circuit, and in particular to an amplifier circuit that is suitable for use in amplifying a signal envelope, as part of a power amplifier system.

It is known that, in wireless communications devices, it is advantageous to operate the power amplifier in the transmitter at a high efficiency. However, in such transmitters, the amplitude of the input signal, and hence the intended amplitude of the amplified signal for transmission, can vary widely. Since a power amplifier will generally operate most efficiently when the amplitude of the input signal has a particular relationship to its supply voltage, a result of this wide variation is that the power amplifier may not operate efficiently.

It is therefore known to provide an envelope amplifier, which detects an amplitude of the input signal, and modulates a power supply voltage. The modulated power supply voltage is then applied to the power amplifier, to which the input signal is also applied.

The result is that the power amplifier can operate efficiently over a wider range of input signal amplitudes.

One such envelope amplifier is described in the document "Envelope Tracking Power Amplifier with Pre-Distortion Linearization for WLAN 802.1 ig", Wang, et al, IEEE MTT-S Digest 2004, pp 1543-1546. An input envelope signal is applied to a linear stage, comprising an operational amplifier, which produces a first output current. The first output current passes through a resistor, and the voltage across the resistor is supplied as an input to a comparator, with the comparator output being supplied to a switching stage. The switching stage generates a second output current which passes through an inductor. The first and second output currents are combined to generate an overall output current, which is supplied mostly from the second output current at lower envelope frequencies, and mostly from the first output current at higher envelope frequencies.

However, this circuit has the disadvantage that the operational amplifier is forced to amplify the entire input signal.

According to a first aspect of the present invention, there is provided an amplifier circuit, comprising: an operational amplifier, having a first non-inverting input connected to an input of the circuit, and having a second inverting input connected to an output of the circuit; a resistor, connected between an output of the operational amplifier and the output of the circuit; and a switched mode power supply, having first and second inputs connected to the output of the operational amplifier and the output of the circuit respectively, and having an output connected to the output of the circuit.

According to a second aspect of the present invention, there is provided an amplifier circuit, comprising: a first amplifier, having a voltage feedback loop; a resistance element, connected to an output of the first amplifier such that an output current of the first amplifier is connected to a voltage level; and a switched mode power supply, connected to the resistance element to receive the voltage level as an input signal, and having a current feedback loop; wherein an output current of the amplifier circuit is formed from a sum of the output current of the first amplifier and an output current of the switched mode power supply.

This has the advantage that, as the resistance element is within the feedback loop of the first amplifier, the first amplifier is not forced to amplify the low frequency components of the envelope, with the result that the efficiency of the amplifier circuit is improved.

BRIEF DESCRIPTION OF DRAWING

Figure 1 is a block schematic diagram of a transmission amplification system, including an amplifier circuit in accordance with the present invention.

Figure 2 is a circuit diagram of an amplifier circuit in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 is a block schematic diagram, showing a part of a transmitter 10 in a wireless communication system. The transmitter 10 may for example be used in a base station of a wireless communication system, although the high efficiency of the present invention is of particular importance when the transmitter 10 is used in a portable, battery-powered device, such as a mobile phone or other hand-held device.

The transmitter 10 can be used in any wireless communication system, and in the preferred embodiment is intended for use in a W-CDMA system, although it can also be used in other systems, including OFDM systems such as those specified in IEEE 802.11.

As is well known, the transmitter 10 includes signal processing circuitry 12, for generating a radio frequency signal, containing the required information for transmission, and complying with the format required by the relevant communications system. The radio frequency signal therefore includes a data signal, within an envelope, the amplitude of which is determined by the signal processing circuitry 12.

For example, when the transmitter 10 is known to be close to the intended receiver, the amplitude of the envelope can be smaller than when the transmitter 10 is known to be distant from the intended receiver. The radio frequency signal is passed to a power amplification block 14, and the amplified signal is passed to an antenna 16, for transmission to the intended receiver or receivers.

In this case, the power amplification block 14 includes a power amplifier 18, and also, in order to improve the efficiency of the transmitter 10, an envelope detector 19 and an envelope modulator 20. The function of the envelope detector 19 is to detect the amplitude of the envelope of the radio frequency signal, and the function of the envelope modulator 20 is to modulate the power supply to the power amplifier 18 based on the detected amplitude of the envelope, so that the power amplifier 18 can operate efficiently. That is, when the radio frequency signal has a large amplitude, it is necessary to supply a relatively high power supply voltage to the power amplifier 18.

However, if the same power supply voltage is supplied to the power amplifier 18 when the radio frequency signal has a smaller amplitude, the power amplifier will not operate efficiently. An amplitude limiter may be placed at the input to the power amplifier 18 in order to remove the amplitude variation at this point.

Although Figure 1 shows the envelope detector 19 as a block within the power amplification block 14, it is equally possible to provide an envelope detection function within the signal processing block 12, providing an additional output directly to the envelope modulator 20.

Figure 2 is a circuit diagram of the envelope modulator 20, in one embodiment of the invention. In this embodiment the envelope modulator 20 includes a linear amplifier stage 22 and a switched-mode power supply stage 24. The switched-mode power supply stage 24 can efficiently amplify the lower frequency components of the received signal, but is not able to react to the higher frequency components, while the linear amplifier stage 22 is provided in order to amplify these higher frequency components.

More specifically, the input RF envelope signal is applied to a first non-inverting input terminal of a linear amplifier A1. The amplifier A1 has a voltage feedback loop 23, with its output terminal connected to a first end of a relatively low value sense resistor Rsense, in this case a 0.10 resistor, and the second end of the sense resistor Rsense being connected to the second inverting input terminal of the linear amplifier A1 through a resistor 26. The inverting input terminal of the linear amplifier A1 is connected to ground through a further resistor 28. The linear amplifier A1 is powered from a voltage source VLIN, which is grounded through a capacitance 30.

In this illustrated embodiment, the gain of the amplifier A1 is set by the values of the resistors 26, 28, and can be set to any desired value by a suitable choice of the resistors. For example, in other embodiments, the amplifier A1 can be configured as a voltage follower.

A second amplifier A2 acts as a comparator. A first non-inverting input terminal of the comparator A2 is connected through a resistor R81, in this case a 3000 resistor, to the first end of the sense resistor Rsense. A second inverting input terminal of the comparator A2 is connected through a further resistor Ra2, of the same resistance value as the resistor Rai, to the second end of the sense resistor Rsense.

The output of the comparator A2 is connected back to the inverting input terminal through a resistor Rbl, in this case a 56k0 resistor, while the non-inverting input terminal of the comparator A2 is connected to ground through a further resistor Rb2, of the same resistance value as the resistor Rb1, and through an RC network, comprising a resistor R3 and variable resistor R4, connected in parallel with a capacitor C3.

The comparator A2 is powered from a battery voltage source VBAn, which is grounded through capacitances 32, 34. The voltage source VLIN, mentioned above, is also derived from the battery voltage source VBATT.

The output of the comparator A2 is connected through a resistor 36, which in turn is connected to ground through a capacitor 38, to an input terminal of a switch mode power supply (SMPS) integrated circuit (IC), in the form of a buck converter 40. In this illustrated embodiment of the invention, the SMPS device 40 is a MAX182O device, available from Maxim Integrated Products. In the illustrated embodiment, this known SMPS device 40 is powered from the battery voltage source VBAU, and has an earth connection through a capacitor 42 and a resistor 44.

The SMPS device 40 produces an output current through an inductor 46 to an output node 48. This output node 48 is connected to ground through a capacitor 50, and to an output node 52 of the envelope modulator 20 through an inductor L2.

The inductor 46 and the capacitor 50 together form a low pass filter.

Thus, the integrated circuit 40, together with the capacitor 42 and resistor 44, and the inductor 46 and capacitor 50, form a buck SMPS converter with LC output filtering. It will be recognized by the person skilled in the art that other configurations can be used to provide this function.

The second end of the sense resistor Rsense is also connected to the output node 52 of the envelope modulator 20.

Thus, the output current provided through the output node 52 of the envelope modulator 20 is the sum of the output current produced by the SMPS device 40 and the output current produced by the linear amplifier A1. The inductor L2 prevents the high frequency output current from the linear amplifier A1 entering the output of the SMPS device 40.

A load can therefore be connected to the output node 52, such that the output current can be supplied to the load. The output current will be an accurately amplified version of the input envelope signal, and so, in the embodiment shown in Figure 1, the amplified output signal can be used as a modulated power supply to a power amplifier, which is to be used to amplify a signal for transmission.

In the case of a WCDMA (Wideband Code Division Multiple Access) signal, the large majority of the energy is located in the DC component, while a significant component is located in the frequency range from 500 kHz to 4 MHz, and only a very small proportion is found in the range from DC to 500 kHz. The envelope modulator 20 described above takes advantage of these properties, by using the SMPS device 40 to amplify the DC and low frequency components of the input signal, while using the linear amplifier A1 to amplify the higher frequency AC components.

Moreover, the output from the SMPS device 40 is effectively fed back into the feedback path 23 of the linear amplifier A1, because the sense resistor Rsense is located in the feedback loop 23. This has the result that any amplification of the DC or low frequency components of the input signal by the linear amplifier A1 is cancelled by the feedback, and so the linear amplifier A1 is only used to amplify the higher frequency AC components. This means that the circuit as a whole can operate with a higher efficiency, and the output current drive/sink demands of the linear amplifier A1 are reduced.

In connection with the feedback action, it should be noted that the action of the resistor 36 and the capacitor 38 is to control the loop bandwidth of the feedback into the buck converter 40.

ft should also be noted that the action of the feedback into the buck converter 40 is to perturb an average level of current. This current level is determined by a DC offset voltage, which in turn is a function of the selected resistance value of the variable resistor R4. Thus, adjusting this selected resistance value can improve the efficiency of the amplification and the stability of the system.

It will be apparent that there are many possible variations in the illustrated embodiment, with the preferred configuration of the circuit depending on how the circuit is to be used, and on the properties of the available integrated circuit components.

There is therefore provided an envelope modulator circuit that can amplify an envelope signal, with DC and AC components, at a high efficiency.

Claims

CLAIMS

1. An amplifier circuit, comprising: an operational amplifier, having a first non-inverting input connected to an input of the circuit, and having a second inverting input connected to an output of the circuit; a resistor, connected between an output of the operational amplifier and the output of the circuit; and a switch mode power supply, having first and second inputs connected to the output of the operational amplifier and the output of the circuit respectively, and having an output connected to the output of the circuit.

2. An amplifier circuit as claimed in claim 1, wherein the switch mode power supply comprises a comparator, having first and second inputs connected to the output of the operational amplifier and the output of the circuit respectively.

3. An amplifier circuit as claimed in claim 2, wherein the switch mode power supply further comprises a buck converter, connected to an output of the comparator.

4. An amplifier circuit as claimed in claim 1, further comprising an inductor, connected between the output of the switch mode power supply and the output of the circuit.

5. An amplifier circuit as claimed in claim 1, further comprising a variable resistor, connected to the switch mode power supply, for setting an average level of an output current thereof.

6. An amplifier circuit, comprising: a first amplifier, having a voltage feedback loop; a resistance element, connected to an output of the first amplifier such that an output current of the first amplifier is converted to a voltage level; and a switch mode power supply, connected to the resistance element to receive the voltage level as an input signal, and having a current feedback loop; wherein an output current of the amplifier circuit is formed from a sum of the output current of the first amplifier and an output current of the switch mode power supply.

7. A transmitter circuit, comprising: an amplifier circuit, as claimed in any preceding claim; and a power amplifier, wherein an output of said amplifier circuit is supplied to said power amplifier to modulate a power supply thereto.

8. A transmitter circuit as claimed in claim 7, further comprising: an envelope detector, for detecting an amplitude of a received signal, and for supplying an input signal to said amplifier circuit, corresponding to said amplitude.