Patent Description:
A power amplifier (PA) is used to amplify a radio-frequency (RF) signal for radio transmission. The PA is commonly found in a wireless communication device for driving antenna (s) of a transmitter. The power consumption of a PA is critical to a wireless communication device that is battery operated. Traditionally, the PA is biased with a fixed supply voltage. Peak RF output power conditions generally occur when the RF input signal input to the PA is at a maximum level. However, when the PA is backed-off from the peak RF output power conditions, the excess input power must be dissipated by the PA because it is not being transformed into useful RF output power. That is, the traditional fixed PA supply voltage results in significant amount of power loss as heat. Envelope tracking is a technique that requires the supply voltage of the PA to be modulated dynamically with the envelope of the RF input signal. This would make the PA operate closer to the peak level at all times and dramatically improve the efficiency of the PA. That is, the envelope tracking technique modulates the PA supply voltage to track the envelope of the RF input signal for reducing the amount of power dissipated as heat.

In wireless communications, bandwidth is the frequency range occupied by a modulated carrier signal. With the advance of wireless communication technology, a wider bandwidth is used by one modulated carrier signal. For example, the bandwidth requirement increases rapidly in <NUM> New Radio (NR) applications. Hence, a wide bandwidth linear amplifier is needed by an envelope tracking supply modulator that is used to supply a modulated supply voltage to a PA that has high peak to average power ratio (PAPR) output signals. However, a typical linear amplifier generally consumes large quiescent current for achieving a wide envelope tracking bandwidth. As a result, a typical wide-bandwidth envelope tracking design is power-hungry.

Thus, there is a need for an innovative amplifier design which achieves wide-bandwidth envelope tracking with reduced quiescent current consumption.

<CIT> discloses an amplification stage comprising: a current mirror circuit comprising a reference transistor arranged to receive a current associated with an input signal and an output transistor providing a current source for an output signal line; a current sink to the output signal line, under the control of the input signal; circuitry arranged to maintain equality between the drain/collector voltages on the transistors of the current mirror circuit. <CIT> discloses a voltage-current converter including a first input stage and a second input stage with a first transistor and a second transistor driven by the first input stage and by the second input stage, respectively. First and second current generators are coupled to current lines of the first transistor and of the second transistor. At least one resistor couples the current lines of the first transistor and of the second transistor, where the ends of the aforesaid resistor are coupled to feedback terminals of the input stages so that an input voltage applied between voltage input terminals of the input stages is converted into a current on respective current output terminals of the converter. The converter includes switching circuits for coupling the first and second current generators alternately to the current line of the first transistor and to the current line of the second transistor.

One of the objectives of the claimed invention is to provide an envelope tracking supply modulator using an amplifier circuit using voltage-to-current conversion to achieve unity feedback factor and input common-mode rejection for a linear amplifier.

Certain terms are used throughout the following description and claims, which refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not in function. In the following description and in the claims, the terms "include" and "comprise" are used in an open-ended fashion, and thus should be interpreted to mean "include, but not limited to. Also, the term "couple" is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.

<FIG> is a block diagram illustrating an envelope tracking supply modulator (ETSM) according to an embodiment of the present invention. The ETSM <NUM> is arranged to generate a modulated supply voltage VPA according to an envelope input SENV, and provide the modulated supply voltage VPA to a power amplifier (PA) <NUM>. The PA <NUM> is powered by the modulated supply voltage VPA for amplifying a radio-frequency (RF) signal SRF to generate an RF output PA_OUT with the desired TX power, where the RF output PA_OUT is transmitted to the air via an antenna <NUM>. In this embodiment, the ETSM <NUM> employs hybrid ETSM architecture, and includes a switching converter (SWC) <NUM> and an amplifier circuit <NUM>. The SWC <NUM> is a DC-DC converter arranged to generate a regulated direct current (DC) voltage output VDC to an output port N_OUT of the ETSM <NUM> via an inductor LDC. For example, the SWC <NUM> may be implemented by a buck converter. The amplifier circuit <NUM> is arranged to receive the envelope input SENV, and generate an amplifier output (which is an output voltage signal) VAC according to the envelope input SENV. The envelope input SENV is derived from processing a transmit (TX) baseband signal generated from a modulator/demodulator (Modem) in a wireless transceiver. For example, the TX baseband signal (digital signal) is fed into an envelope tracking digital baseband circuit (which may include an envelope detection block, a power scaling block, a lookup table, an upsampling block, etc.), a processing result of the TX baseband signal is output from the envelope tracking digital baseband circuit and converted into an analog signal (voltage signal) by a digital-to-analog converter, and then the analog signal (voltage signal) is processed by an analog filter to act as the envelope input (voltage signal) SENV. However, this is for illustrative purposes only, and is not meant to be a limitation of the present invention. Since the present invention focuses on the amplifier design, further description directed to generation of the envelope input SENV is omitted for brevity.

The amplifier circuit <NUM> transmits the amplifier output VAC to the output port N_OUT of the ETSM <NUM>. In accordance with the hybrid ETSM architecture, the regulated DC voltage VDC and the amplifier output VAC jointly control the modulated supply voltage VPA of the PA <NUM>. More specifically, the regulated voltage VDC decides a DC part (i.e., low-frequency part) of the modulated supply voltage VPA, and the amplifier output VAC decides an AC part (i.e., high-frequency part) of the modulated supply voltage VPA.

In this embodiment, the amplifier circuit <NUM> employs the proposed wideband amplifier architecture, and thus uses voltage-to-current conversion to achieve unity feedback factor and input common-mode rejection for a linear amplifier. As shown in <FIG>, the amplifier circuit <NUM> includes a voltage-to-current conversion circuit (labeled as "V/I Conv") <NUM> and a current-to-voltage conversion circuit (labeled as "I/V Conv") <NUM>. The voltage-to-current conversion circuit <NUM> is arranged to generate a current signal IENV according to an input voltage signal (e.g., envelope input SENV). The current-to-voltage conversion circuit <NUM> is arranged to generate an output voltage signal (e.g., amplifier output VAC) according to the current signal IENV. The output voltage signal (e.g., amplifier output VAC) is generated at an output port NA of the current-to-voltage conversion circuit <NUM>, and the output port NA of the current-to-voltage conversion circuit <NUM> is coupled to the SWC <NUM> via the inductor LDC. It should be noted that the current-to-voltage conversion circuit <NUM> does not need an input resistor for converting the current signal IENV into an input voltage of a linear amplifier. Further details of the proposed wideband amplifier architecture are described as below.

<FIG> is a diagram illustrating an amplifier circuit according to an embodiment of the present invention. The amplifier circuit <NUM> shown in <FIG> is implemented by the amplifier circuit <NUM> shown in <FIG>, and the PA <NUM> shown in <FIG> may act as a load <NUM> of the amplifier circuit <NUM>, where the load <NUM> may be modeled by a resister RPA and a capacitor CPA connected in parallel, and is coupled to the amplifier circuit <NUM> via a printed circuit board (PCB) trace inductor LPCB. The amplifier circuit <NUM> includes a voltage-to-current conversion circuit <NUM> and a current-to-voltage conversion circuit <NUM>. The voltage-to-current conversion circuit <NUM> includes an input filter bank <NUM> and an operational transconductance amplifier (OTA) <NUM> with transconductance Gm. In this embodiment, the envelope input SENV acts as an input voltage signal of the amplifier circuit <NUM>, and is a differential signal consisting of a positive signal VP and a negative signal VN. The input filter bank <NUM> applies noise filtering to the envelope input SENV before the envelope input SENV is processed by the OTA <NUM>. The OTA <NUM> generates a current signal IENV at an output port N3 by applying voltage-to-current conversion to the envelope input SENV passing through the input filter bank <NUM>, where IENV=Gm*SENV.

The current-to-voltage conversion circuit <NUM> includes a linear amplifier (LA) <NUM>, a feedback network <NUM>, and an optional AC coupling capacitor CAC. An input port of the LA <NUM> may include a first input node N1 and a second input node N2, where a voltage signal VDAC provided by a digital-to-analog converter (DAC) is coupled to the first input node N1, and the current signal IENV generated from the voltage-to-current conversion circuit <NUM> (particularly, OTA <NUM>) is coupled to the second input node N2. Specifically, the input port of the LA <NUM> (particularly, second input node N2 of the LA <NUM>) is directly connected to the output port N2 of the OTA <NUM>, such that there is no input resistor for the LA <NUM>. The feedback network <NUM> is coupled between the input port of the LA <NUM> (particularly, second input node N2 of the LA <NUM>) and an output port N4 of the LA <NUM>, and includes at least one resistor RFB and at least one optional capacitor CFB. The resistor RFB is a feedback resistor used to return part of the output signal (output voltage) LA_OUT from the output port N4 of the LA <NUM> to the second input node N2 of the LA <NUM>. In this embodiment, the resistor RFB of the feedback network <NUM> further deals with current-to-voltage conversion of the current signal IENV. In a case where the capacitor CFB is implemented in the feedback network <NUM>, the capacitor CFB can be used for noise filtering. However, this is not meant to be a limitation of the present invention. Alternatively, the capacitor CFB may be omitted from the feedback network <NUM>.

As shown in <FIG>, the LA <NUM> generates the output signal LA_OUT at the output port N4, where an amplifier output VAC of the amplifier circuit <NUM> is derived from the output signal (output voltage) LA_OUT. In a case where the AC coupling capacitor CAC is implemented in the current-to-voltage conversion circuit <NUM>, the amplifier output VAC is obtained by passing the output signal LA_OUT through the AC coupling capacitor CAC. The AC coupling capacitor CAC is capable of applying DC smoothing to the output signal LA_OUT, thereby allowing the LA <NUM> to operate under a lower voltage range for additional quiescent current reduction. However, this is not meant to be a limitation of the present invention. In some embodiments of the present invention, the AC coupling capacitor CAC may be omitted from the current-to-voltage conversion circuit <NUM>, and the output signal LA_OUT may directly act as the amplifier output VAC.

Due to inherent characteristics of the OTA <NUM>, the output impedance ROUT of the OTA <NUM> (i.e., the impedance (or resistance) looking into the OTA <NUM> from the current-to-voltage conversion circuit <NUM>) is large. Hence, a feedback factor β of the feedback network <NUM> may be regarded as having a value equal to <NUM>. That is, the feedback network <NUM> may have a unity feedback factor (β=<NUM>) due to large output impedance ROUT possessed by the OTA <NUM>. The feedback factor β of the feedback network <NUM> may be expressed by the following formula.

Compared to a conventional LA design with a feedback factor β smaller than one, an operational amplifier needs to consume larger quiescent current to have a larger open-loop gain for meeting a target closed-loop gain requirement. To address this issue, the present invention proposes using the voltage-to-current conversion circuit <NUM> with large output impedance ROUT to make the feedback factor β equal to one, thus allowing the LA <NUM> to meet the same target closed-loop gain requirement under lower quiescent current consumption. In other words, the present invention proposes an amplifier circuit using an LA with unity-gain feedback for better power efficiency.

As mentioned above, the amplifier output VAC decides an AC part (i.e., high-frequency part) of the modulated supply voltage VPA. Hence, a conventional LA design may suffer from input common-mode (CM) swing due to envelope tracking swing at the modulated supply voltage VPA that is fed back to the voltage input of the LA. To address this issue, the present invention proposes using the voltage-to-current conversion circuit <NUM> to provide the LA <NUM> with a current-mode input rather than a voltage-mode input, where the current signal IENV is immune to the envelope tracking swing at the modulated supply voltage VPA. To put it simply, the voltage-to-current conversion circuit <NUM> offers input CM rejection in the current mode, and therefore ensures less CM voltage swing for the LA <NUM>. Since the LA <NUM> has a fixed CM voltage level at its inputs, the LA <NUM> can have improved linearity as well as lower quiescent current consumption.

The closed loop gain G of the amplifier circuit <NUM> can be expressed using the following formula.

Since the resistance of the feedback resistor RFB is fixed, variation of the transconductance Gm provided by the OTA <NUM> affects the stability of the closed loop gain G. To achieve stable transconductance Gm, the present invention proposes using a source degenerated amplifier. <FIG> is a diagram illustrating a source degenerated amplifier. For brevity and simplicity, <FIG> only shows a part of the source degenerated amplifier <NUM>. The source degenerated amplifier <NUM> includes a differential pair with source degeneration, where the differential pair consists of two P-channel metal-oxide-semiconductor (PMOS) transistors MP1 and MP2, a source terminal of the PMOS transistor MP1 is series-connected to one end of a resistor R1 (R1=Rdeg), a source terminal of the PMOS transistor MP2 is series-connected to one end of a resistor R2 (R2=Rdeg), and the other end of the resistor R1 and the other end of the resistor R2 are both coupled to a supply voltage VDD via a current source <NUM>. The transconductance Gm of the source degenerated amplifier <NUM> may be expressed using the following formula.

In above formula (<NUM>), gm represents the transconductance of each PMOS transistor MP1/MP2. If one or both of gm and Rdeg are properly set to make gm · Rdeg >><NUM>, the transconductance Gm of the source degenerated amplifier <NUM> may be expressed using the following formula.

Thus, under a condition where gm · Rdeg >> <NUM>, the closed loop gain G of the amplifier circuit <NUM> can be expressed using the following formula.

Since the closed loop gain G is determined by a ratio of resistance of the feedback resistor to resistance of the source degeneration, the closed loop gain G is a fixed value, regardless of operations of the amplifier circuit <NUM>.

In some embodiments of the present invention, a transconductance boosting technique is employed to ensure that the condition of gm · Rdeg >> <NUM> is met. <FIG> is a diagram illustrating a source degenerated amplifier with transconductance boosting according to an embodiment of the present invention. The OTA <NUM> shown in <FIG> is implemented by the source degenerated amplifier <NUM> shown in <FIG>. The source degenerated amplifier <NUM> includes PMOS transistors MP1, MP2, NMOS transistors MN1, MN2, amplifiers A1, A2, resistors R1, R2, and a current source <NUM>. The source degenerated amplifier <NUM> has a differential pair with source degeneration that consists of two NMOS transistors MN1 and MN2, where a source terminal of the NMOS transistor MN1 is series-connected to one end of the resistor R1 (R1=Rdeg), a source terminal of the NMOS transistor MN2 is series-connected to one end of the resistor R2 (R2=Rdeg), and the other end of the resistor R1 and the other end of the resistor R2 are both coupled to a ground voltage GND via the current source <NUM>. The amplifiers A1 and A2 are used to boost the transconductance of the differential pair. Specifically, the amplifier A1 is used to boost the transconductance gm of the NMOS transistor MN1, and the amplifier A2 is used to boost the transconductance gm of the NMOS transistor MN2.

With the advance of wireless communication technology, a wider bandwidth is used by one modulated carrier signal. For example, the bandwidth requirement increases rapidly in <NUM> New Radio (NR) applications. Hence, a wide bandwidth linear amplifier is needed by an envelope tracking supply modulator that is used to supply a modulated supply voltage to a power amplifier. When the amplifier circuit <NUM> using a source degeneration amplifier with transconductance boosting is employed by a wideband application, the present invention further proposes using a two-stage amplifier with at least one compensation capacitor as a transconductance boosting amplifier (e.g., amplifier A1 or A2 shown in <FIG>), so as to ensure that the condition of gm · Rdeg >> <NUM> is still met under higher frequencies.

<FIG> is a diagram illustrating a two-stage amplifier with at least one compensation capacitor according to an embodiment of the present invention. Each of the amplifiers A1 and A2 shown in <FIG> may be implemented by the two-stage amplifier <NUM> shown in <FIG>. The two-stage amplifier <NUM> receives input voltages VINN and VINP, and generates an output voltage VOUTP to a transconductance boosting target (e.g., NMOS transistor MN1 or MN2 shown in <FIG>). As shown in <FIG>, the two-stage amplifier <NUM> includes PMOS transistors MP1, MP2, MP3, NMOS transistors MN1, MN2, MN3, a resistor R, a current source <NUM>, a Miller compensation capacitor CM, and a feed forward compensation capacitor CFF. The use of Miller compensation capacitor CM and/or feed forward compensation capacitor CFF can ensure that the transconductance gm of the NMOS transistor MN1/MN2 is still boosted under higher frequencies. In this embodiment, the Miller compensation capacitor CM and the feed forward compensation capacitor CFF are both implemented in the two-stage amplifier <NUM>. However, this is for illustrative purposes only, and is not meant to be a limitation of the present invention. In one alternative design, the two-stage amplifier <NUM> may be modified to omit the feed forward compensation capacitor CFF. In another alternative design, the two-stage amplifier <NUM> may be modified to omit the Miller compensation capacitor CM.

Claim 1:
An envelope tracking supply modulator (<NUM>) arranged to generate a modulated supply voltage (VPA) according to an envelope input (SENV) of a radio-frequency signal (SRF), and provide the modulated supply voltage (VPA) to a power amplifier (<NUM>) for amplifying the radio-frequency signal (SRF), wherein the envelope input is a differential signal consisting of a positive signal (VP) and a negative signal (VN) and wherein the envelope tracking supply modulator (<NUM>) comprises:
an amplifier circuit (<NUM>, <NUM>) comprising:
a voltage-to-current conversion circuit (<NUM>, <NUM>), arranged to generate a current signal according to the envelope input, wherein the voltage-to-current conversion circuit (<NUM>, <NUM>) comprises:
an operational transconductance amplifier, OTA (<NUM>), arranged to output the current signal at an output port of the OTA (<NUM>); and
a current-to-voltage conversion circuit (<NUM>, <NUM>), arranged to generate an output voltage signal according to the current signal, wherein the current-to-voltage conversion circuit (<NUM>, <NUM>) comprises a linear amplifier, LA (<NUM>), wherein an input port of the LA (<NUM>) is coupled to the output port of the OTA (<NUM>), and the output voltage signal is involved in setting the modulated supply voltage (VPA) of the power amplifier (<NUM>) derived from an output signal at an output port of the LA (<NUM>),
wherein the OTA (<NUM>) is a source degenerated amplifier (<NUM>), the source degenerated amplifier (<NUM>) comprises a first differential amplifier (A1), a second differential amplifier (A2) and a differential pair (MN1, MN2) with source degeneration comprising a first transistor (MN1) and a second transistor (MN2), wherein the first differential amplifier (A1) has a first input coupled to the negative signal (VN), a second input directly connected to the source of the first transistor (MN1) and an output directly connected to the gate of the first transistor (MN1), wherein the second differential amplifier (A2) has a first input coupled to the positive signal (VP), a second input directly connected to the source of the second transistor (MN2) and an output directly connected to the gate of the second transistor (MN2), wherein each of the first and second differential amplifiers (A1, A2) is a two-stage amplifier (<NUM>) with at least one compensation capacitor.