This application relates generally to power amplifiers, and more particularly, to a system for efficient and highly linear amplitude modulation.
Modern communication systems increasingly employ modulation methods such as 8-PSK, 16-QAM, 64-QAM, and OFDM to increase data speeds and to improve spectrum efficiency. This added complexity invariably increases the amplitude (or envelope) fluctuations of the transmit signal—generally measured by the transmit signal's peak-to-average (pk/ave) ratio. As a result, the radio transmitter sees higher peaks and tends to generate more distortion. To minimize distortion, most linear circuits operate at a bias current proportional to the largest amplitude or peak of the transmit signal. This can be very inefficient in systems that produce signals with a large peak-to-average ratio.
A radio transmitter generally uses a power amplifier to close the link to the receiver. The power amplifier typically dissipates more power than any other circuit so its efficiency is critical. It would therefore be advantageous to have a system for operating a power amplifier at lower power levels while keeping distortion low.
In one or more embodiments, a system for linear amplitude modulation is provided. In one embodiment, the system comprises a very efficient amplifier that can be used with any type of modulated signal, including signals modulated with constant and envelope-varying techniques.
In one embodiment, apparatus is provided for linear amplitude modulation of an amplifier. The apparatus comprises a processing circuit that receives an amplitude modulation signal and produces one or more amplifier control signals that are coupled to the amplifier. The apparatus also comprises a feedback circuit that generates a feedback signal from an output of the amplifier that is input to the processing circuit, and a network that controls a bias of the amplifier in response to the feedback signal to linearize the amplifier's amplitude control.
In one embodiment, apparatus is provided for linear amplitude modulation of an amplifier. The apparatus comprises means for receiving an amplitude modulation signal and producing one or more amplifier control signals that are coupled to the amplifier. The apparatus also comprises means for generating a feedback signal from an output of the amplifier that is input to the means for receiving, and means for controlling a bias of the amplifier in response to the feedback signal to linearize the amplifier's amplitude control.
Other aspects of the embodiments will become apparent after review of the hereinafter set forth Brief Description of the Drawings, Detailed Description, and the Claims.
The foregoing aspects of the embodiments described herein will become more readily apparent by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:
a-d show a detailed diagram of one embodiment of a processing circuit for use in the modulation architecture of
a-d show graphs that illustrate one embodiment of a method to compensate for the effects of a narrow filter;
It's important that the power amplifier and each gain stage operate linearly at the peaks of the transmit signal. This situation places the greatest demands on the power amplifier and actually defines its operating parameters as described below.
where Imax is the maximum collector current of transistor Q1. The maximum current is important because, in practice, the active device (Q1) should operate at a nominal level that is about one-half the maximum level to ensure linear operation.
where Pout(pk) is the instantaneous peak output power.
At lower power levels and smaller signal peaks, it's possible to reduce the nominal operating current of the active device. However, it's important to keep the loadline resistance fixed, otherwise the output match and performance of the power amplifier suffers.
The efficiency of an amplifier is defined as an amplifier's ability to convert dc (or battery) power to radio energy;
where PRF is the RF output power and Pdc is the dc power used. The described linear amplifier—with continuous output current flow—achieves at best 50% efficiency. It's possible to lower the dc power used and thereby improve efficiency by limiting the time the output current flows.
The operating point of RF amplifiers can also be affected by the input signal. This effect, known as self-bias, occurs at large input signal amplitudes. The large signal peaks exponentially increase the transistor's output current. This phenomenon potentially compensates for typical gain compression effects and therefore actually helps class AB amplifiers operate more linearly than class A amplifiers.
Class B and C amplifiers are generally labeled switched amplifiers since the active device is turned on and off. As a result, this type of amplifier is inherently nonlinear and therefore unsuitable for use with most digitally modulated signals. It is however very efficient. Furthermore, its efficiency remains almost constant at different output power levels. This contrasts with linear amplifiers, where the efficiency falls off dramatically at low to moderate power levels.
The output of the synthesizer 600 at the VCO is a constant-envelop phase-modulated signal. Ideally, this signal is buffered using compressed amplifiers and then connected to the power amplifier, whereat amplitude modulation is applied. For example, the phase-modulated signal may be applied to the power amplifier 504 shown in
where Vam is the amplitude modulation (AM) signal. This relationship is extremely linear—even more so than the gain relationship of class A/AB amplifiers. Unfortunately, as the amplitude modulation voltage Vam is reduced, the gain stage and active device eventually saturate. This proves problematic for bipolar transistors (because it forward biases the base-collector junction which may harm the device) and for field effect transistors (since the device pushes into its linear region where its gain drops). Therefore the collector voltage for bipolar transistors and the drain voltage of field affect transistors must be limited. Severe phase shifts to the signal also occur near saturation and must be avoided because these produce spectral re-growth. As a result, the useful range for this type of amplitude modulation is approximately 20 dB.
Some modern communication systems such as CDMA and OFDM bundle multiple carriers together to deliver high data rates. This tends to increase the fluctuations in the transmit signal and generally produces very large peaks. As a result, the dynamic range of the amplitude modulation signal can reach 40 dB.
It's clear that amplitude modulation by a single stage amplifier—either through the supply or bias network—falls short of the requirements set by modern communication systems. This problem becomes even more acute when adaptive power control needs are considered. That's because the ideal point for amplitude modulation changes. At high output power levels, the amplitude modulation is best applied at the last stages of the power amplifier. In this way, the preceding gain stages can operate nonlinearly and thus very efficiently. As the output power of the radio transmitter decreases, the ability to apply amplitude modulation through the last power amplifier stage disappears. This is because an amplifier possesses an absolute lower limit to its modulation range (due to saturation and phase linearity) as well as it generally offers very little control at low power levels. Consequently, the amplitude modulation must shift to earlier gain stages and this further complicates the situation.
The amplitude modulation shift impacts both switched (
In one embodiment, the feedback circuit operates to provide a feedback loop that operates around a group comprising the high-power amplifier stages, since these stages are difficult to predict (sensitive to input signal amplitude, supply voltage, and bias current). This requires the response of the variable gain amplifier(s) to be linear or at least very predictable, which is practical with variable attenuators and current-steered variable gain amplifiers.
In one embodiment, the signal processing circuit actually slides the amplitude modulation, depending on the transmit output power level set by the TX control signal. The processing circuit also considers two important power levels that describe the power amplifier. The first level (P1) indicates the power level where the control through the supply or bias network becomes practical. It's related to the amplifier's design and varies with the peak-to-average ratio of the transmit waveform, which is readily available from the digital modulator using the radio configuration parameters or by means of some straightforward analysis. (Note that P1 is the maximum output power level for the switched amplifier, but somewhat less for the linear amplifier due to self-bias effects.) The second level (P2) marks the absolute lower limit of the power amplifier's modulation range;
P2=PMAX−ΔP2
where PMAX identifies the maximum output power level and ΔP2 corresponds to the detector range.
a shows one embodiment of a processing circuit 1200 suitable for use in the amplitude modulation architecture shown in
which essentially describes the AM signal in the range of P1 to P2.
The min and max functions also scale the AM signal to produce the variable gain amplifier control signal (VGA*). The VGA* control signal primarily covers the range outside of P1 to P2, but it also affects the range from P1 to P2. In this region, it varies the input signal to the power amplifier by using a scaled version of the PA* signal, resulting in;
where μ is the scaling factor. It's possible that the scaling factor better fits a logarithmic function whereby the VGA* signal becomes;
μ(PA*)→(PA*)μ
In either case, the scaling factor μ is always less than one.
It should be noted that the processing circuit 1200 use the same limits for splitting the AM signal to the VGA* and PA* control signals. As such, it reduces any discontinuities in the transfer functions for these control signals as illustrated in the graph of
The amplitude feedback loop also includes a low pass filter to ensure stability as well as to further attenuate the RF carrier signal that leaks through the peak detector. Its design is important because the phase and amplitude signals in a polar modulator must align, otherwise the modulated signal becomes distorted and spectral re-growth occurs as shown in the graph provided in
Referring to the feedback loop shown in
where k is the power amplifier control gain, α is the detector gain, and p1-p2 are the pole frequencies of the low pass filter 1104 and detector filter, respectively. The power amplifier gain is set by the control current (iPA) and is defined by;
where vrf(pk) is the peak transmit signal, less any coupling loss. Likewise, the detector is characterized by;
which is a linear relationship. Note that the pole frequency p2 of the detector (and any other circuit) is intentionally set much higher than the pole frequency p1 of the low pass filter. As a result, the transfer function for the feedback loop simplifies to;
where β is the dc gain equal to;
and p1(1+αk) is the effective or dominant pole frequency of the system. With these conditions, the low pass filter not only stabilizes the feedback loop but also removes the carrier signal extracted by the detector. Furthermore, this effective pole frequency sets the delay of the amplitude modulation signal.
The feedback loop is active only between power levels P1 to P2. This is important because the detector is linear only in this range. Outside this range, the detector, the feedback loop, and the power amplifier become very nonlinear. Therefore, in one embodiment, the limit functions restrict and actually prevent the operation of the feedback loop outside the range of P1 to P2.
where the 3 dB bandwidth (f3 dB) is necessarily greater than the carrier frequency. This results in an insignificant delay compared to the rate of the amplitude modulation signal. Simultaneously, the amplitude modulation signal (in analog form) is also applied to the analog signal processing circuit and power amplifier control loop.
Now, to align the modulation signals, the phase modulation signal must be delayed by the same amount as the amplitude modulation signal. In one embodiment, a duplicate low pass filter 1402 is inserted in the phase modulation path prior to the voltage-controlled oscillator. Its transfer function is simply;
where p3 represents the pole frequency and equals p1 (1+αfl ). This also corresponds to the pole frequency for the low pass filter 1404 connecting to the variable gain amplifier since it operates outside the feedback loop.
Some communication systems use modulation schemes with exceptionally wide frequency modulation as shown in the graphs illustrated in
a-d show graphs that illustrate the operation of one embodiment of a method to compensate for the effects of a narrow filter. For example, systems with narrow filters that attenuate the modulation signals generally need compensation or pre-distortion. That's because these filters severely limit the nulls in the modulation signal as shown in the graph of
The feedback loop and processing circuit shown in
Vbe1=ITxR1+Vbe2
where ITx is the power control signal (and is proportional to the required gain). This equation can be rewritten as;
where VT is the thermal voltage.
Referring again to
By design, the exponential current Iexp maps to the same scale as the amplitude modulation signal and above-defined power levels P1 and P2. This is important because it allows the signals to be easily compared by the min and max circuits described above.
Referring again to
where iC1 and IS1 are the collector and saturation currents of the transistor, respectively, vrf is the input signal with amplitude A, VB is the base bias voltage, and vdet is the output voltage developed across capacitor C1. The peaks of the input signal are held by capacitor C1, although some droop Δv occurs between these peaks, with;
I1 is the bias current, and Δt is approximately one-half of the carrier (or radio) frequency. Some droop is acceptable, since the envelope of the radio frequency signal changes slowly compared to the carrier signal. The circuit strives for equilibrium where the average current flowing through transistor Q1 is simply I1. This requires the voltage held by capacitor C1 to track the positive peaks (and effectively the envelope) of the radio signal as shown in the graph provided by
as shown in the graph provided by
The detector output voltage may be translated to an output current so that it readily interfaces to the processing circuits described herein.
i3=I3+idet i4=I4−idet
where i3 and i4 are the collector currents of transistors Q3 and Q4, respectively. The current idet is described by;
for vdet less than or equal to I4R3.
Transistors Q3 and Q4 with current source I5 form a simple differential pair where
which is cross-coupled to the previous outputs (i3 and i4). Since current source I5 is significantly smaller than current sources I3 and I4, the cross-coupled currents actually reduce the transconductance amplifier's gain at low input levels. This in turn effectively reduces the offset produced by the detector and extends its range beyond 30 dB to approximately 35 dB as illustrated by the graph shown in
In one or more embodiments, a system for linear amplitude modulation is provided that improves the efficiency of power amplifiers that can be used for various types of modulated signals, including constant and envelope-varying schemes. The system utilizes a feedback loop to linearize the response of these power amplifiers as well as an advanced gain control system to scale the amplitude modulation. The result is a robust and very efficient amplitude modulation system that may be used in radio transmitters or any other system or application that requires linear amplitude modulation.
Accordingly, while one or more embodiments of a system for linear amplitude modulation have been illustrated and described herein, it will be appreciated that various changes can be made to the embodiments without departing from their spirit or essential characteristics. Therefore, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.
This Application claims the benefit of priority from a co-pending U.S. Provisional Application entitled, “Linear Amplitude Modulation” having application No. 60/583,431 and filed on Jun. 26, 2004, the disclosure of which is incorporated herein by reference for all purposes.
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