Amplifier and TransmitterTechnologies.Feedback
Linearization techniques are incorporated into power amplifiers and transmiters for the dual purposes of improving linearity and for allowing operation with less back-off and therefore higher effi- ciency. This article provides a summary of the three main families of techniques have been developed: Feedback, feedforward, and predistortion.
RF Feedback
The basis of this technique is similar to its audio-frequency counterpart. A portion of the RF-output signal from the amplifier is fed back to, and subtracted from, the RF-input signal without detection or down- conversion. Considerable care must be taken when using feedback at RF as the delays involved must be small to ensure stability. In addition, the loss of gain at RF is generally a more significant sacrifice than it is at audio frequencies. For these reasons, the use of RF feedback in dis- crete circuits is usually restricted to HF and lower VHF frequencies [99]. It can be applied within MMIC devices, however, well into the microwave region.
In an active RF feedback system, the volt- age divider of a conventional passive-feedback system is replaced by an active (amplifier) stage. The gain in the feedback path reduces the power dissipated in the feedback compo- nents. While such systems demonstrate IMD reduction [105], they tend to work best at a specific signal level.
Envelope Feedback
The problem of delay in RF feedback is alleviated to a large extent by utilizing the signal envelope as the feedback parameter. This approach takes care of in-band distortion products associated with amplitude nonlinearity. Harmonic distortion products, which are corrected by RF feedback, are generally not an issue as they can easily be removed by filtering in most applications. Envelope feed- back is therefore a popular and simple technique.Envelope feedback can be applied to either a complete transmitter or a single power amplifier. The principles of operation are similar and both are described in detail in [100]. The RF input signal is sampled by a coupler and the envelope of the input sample is detected. The resulting envelope is then fed to one input of a differential amplifier, which subtracts it from a similarly obtained sample of the RF output. The difference signal, representing the error between the input and output envelopes, is used to drive a modulator in the main RF path. This modulator modifies the envelope of the RF signal which drives the RF PA. The envelope of the resulting output signal is therefore linearized to a degree determined by the loop gain of the feedback process. Examples of this type of system are reported in [101] and [102].
The degree of linearity improvement that can be obtained when using this technique depends upon the relative levels of the AM-AM and AM-PM conversion in the amplifier. For a VHF BJT amplifier, AM-AM distortion is dominant and two-tone IMD is typically reduced by 10 dB. Since AM-PM distortion is not corrected by envelope feedback, no linearity improvement is observed if phase distortion is the dominant form of nonlinearity. This is often the case in, for example, class-C and LDMOS PAs. The use of envelope feedback is therefore generally restricted to relatively linear class-A or AB amplifiers
The degree of linearity improvement that can be obtained when using this technique depends upon the relative levels of the AM-AM and AM-PM conversion in the amplifier. For a VHF BJT amplifier, AM-AM distortion is dominant and two-tone IMD is typically reduced by 10 dB. Since AM-PM distortion is not corrected by envelope feedback, no linearity improvement is observed if phase distortion is the dominant form of nonlinearity. This is often the case in, for example, class-C and LDMOS PAs. The use of envelope feedback is therefore generally restricted to relatively linear class-A or AB amplifiers
Polar-Loop Feedback
The polar-loop technique overcomes the fundamental inability of envelope feedback to correct for AM-PM distortion effects [103]. Essentially, a phase-locked loop is added to the envelope feedback system. For a narrowband VHF PA, the improvement in two-tone IMD is typically around 30 dB.The envelope and phase-feedback functions operate essentially independently. In this case, envelope detection occurs at the intermediate frequency (IF), as the input signal is assumed to be a modulated carrier at IF. Likewise, phase detection takes place at the IF, with limiting being used to minimize the effects of signal amplitude upon the detected phase. Alternatively, it is possible to supply the envelope and phase modulating signals separately at baseband and to undertake the comparisons there.
The key disadvantage of polar feedback lies in the generally different bandwidths required for the amplitude and phase feedback paths.Thus,differing levels of improvement of the AM-AM and AM-PM characteristics usually result, and this often leads to a poorer overall performance than that achievable from an equivalent Cartesian-loop transmitter. A good example of the difference occurs with a standard two-tone test, which causes the phase-feedback path to cope with a discontinuity at the envelope minima. In general, the phase bandwidth must be five to ten times the envelope bandwidth, which limits available loop gain for a given delay.
Cartesian Feedback
The key disadvantage of polar feedback lies in the generally different bandwidths required for the amplitude and phase feedback paths.Thus,differing levels of improvement of the AM-AM and AM-PM characteristics usually result, and this often leads to a poorer overall performance than that achievable from an equivalent Cartesian-loop transmitter. A good example of the difference occurs with a standard two-tone test, which causes the phase-feedback path to cope with a discontinuity at the envelope minima. In general, the phase bandwidth must be five to ten times the envelope bandwidth, which limits available loop gain for a given delay.
Cartesian Feedback
The Cartesian-feedback technique overcomes the problems associated with the wide bandwidth of the signal phase by applying modulation feedback in I and Q(Cartesian) components [104]. Since the I and Q components are the natural outputs of a modern DSP, the Cartesian loop is widely used in PMR and SMR systems.
The basic Cartesian loop consists of two identical feedback processes operating independently on the I and Q channels. The inputs are applied to differential integrators (in the case of a first-order loop) with the resulting difference (error) signals being modulated onto I and Q subcarriers and up-converted to drive the PA. A sample of the output from the PA is attenuated and quadrature-down-converted (synchronously with the up- conversion process). The resulting quadrature feedback signals then form the second inputs to the input differential integrators, completing the two feedback loops. The phase shifter shown in the up-converter local-oscillator path is used to align the phases of the up- and down-conversion processes, thereby ensuring that a negative feed- back system is created and that the phase margin of the system is optimized.The effects of applying Cartesian feedback to a highly nonlinear (class-C) PA amplifying an IS-136 (DAMPS) signal are shown. The first ACPR is improved by 35 dB and the signal is produced within specifications with an efficiency of 60 percent [100].
The basic Cartesian loop consists of two identical feedback processes operating independently on the I and Q channels. The inputs are applied to differential integrators (in the case of a first-order loop) with the resulting difference (error) signals being modulated onto I and Q subcarriers and up-converted to drive the PA. A sample of the output from the PA is attenuated and quadrature-down-converted (synchronously with the up- conversion process). The resulting quadrature feedback signals then form the second inputs to the input differential integrators, completing the two feedback loops. The phase shifter shown in the up-converter local-oscillator path is used to align the phases of the up- and down-conversion processes, thereby ensuring that a negative feed- back system is created and that the phase margin of the system is optimized.The effects of applying Cartesian feedback to a highly nonlinear (class-C) PA amplifying an IS-136 (DAMPS) signal are shown. The first ACPR is improved by 35 dB and the signal is produced within specifications with an efficiency of 60 percent [100].
Neyker Stewart Zambrano
CRF
http://www.highfrequencyelectronics.com/Archives/Nov03/HFE1103_RaabPart4.pdf
Amplifier and TransmitterTechnologies.Feedback
Linearization techniques are incorporated into power amplifiers and transmiters for the dual purposes of improving linearity and for allowing operation with less back-off and therefore higher effi- ciency. This article provides a summary of the three main families of techniques have been developed: Feedback, feedforward, and predistortion.
RF Feedback
The basis of this technique is similar to its audio-frequency counterpart. A portion of the RF-output signal from the amplifier is fed back to, and subtracted from, the RF-input signal without detection or down- conversion. Considerable care must be taken when using feedback at RF as the delays involved must be small to ensure stability. In addition, the loss of gain at RF is generally a more significant sacrifice than it is at audio frequencies. For these reasons, the use of RF feedback in dis- crete circuits is usually restricted to HF and lower VHF frequencies [99]. It can be applied within MMIC devices, however, well into the microwave region.
In an active RF feedback system, the volt- age divider of a conventional passive-feedback system is replaced by an active (amplifier) stage. The gain in the feedback path reduces the power dissipated in the feedback compo- nents. While such systems demonstrate IMD reduction [105], they tend to work best at a specific signal level.
Envelope Feedback
The problem of delay in RF feedback is alleviated to a large extent by utilizing the signal envelope as the feedback parameter. This approach takes care of in-band distortion products associated with amplitude nonlinearity. Harmonic distortion products, which are corrected by RF feedback, are generally not an issue as they can easily be removed by filtering in most applications. Envelope feed- back is therefore a popular and simple technique.Envelope feedback can be applied to either a complete transmitter or a single power amplifier. The principles of operation are similar and both are described in detail in [100]. The RF input signal is sampled by a coupler and the envelope of the input sample is detected. The resulting envelope is then fed to one input of a differential amplifier, which subtracts it from a similarly obtained sample of the RF output. The difference signal, representing the error between the input and output envelopes, is used to drive a modulator in the main RF path. This modulator modifies the envelope of the RF signal which drives the RF PA. The envelope of the resulting output signal is therefore linearized to a degree determined by the loop gain of the feedback process. Examples of this type of system are reported in [101] and [102].
The degree of linearity improvement that can be obtained when using this technique depends upon the relative levels of the AM-AM and AM-PM conversion in the amplifier. For a VHF BJT amplifier, AM-AM distortion is dominant and two-tone IMD is typically reduced by 10 dB. Since AM-PM distortion is not corrected by envelope feedback, no linearity improvement is observed if phase distortion is the dominant form of nonlinearity. This is often the case in, for example, class-C and LDMOS PAs. The use of envelope feedback is therefore generally restricted to relatively linear class-A or AB amplifiers
The degree of linearity improvement that can be obtained when using this technique depends upon the relative levels of the AM-AM and AM-PM conversion in the amplifier. For a VHF BJT amplifier, AM-AM distortion is dominant and two-tone IMD is typically reduced by 10 dB. Since AM-PM distortion is not corrected by envelope feedback, no linearity improvement is observed if phase distortion is the dominant form of nonlinearity. This is often the case in, for example, class-C and LDMOS PAs. The use of envelope feedback is therefore generally restricted to relatively linear class-A or AB amplifiers
Polar-Loop Feedback
The polar-loop technique overcomes the fundamental inability of envelope feedback to correct for AM-PM distortion effects [103]. Essentially, a phase-locked loop is added to the envelope feedback system. For a narrowband VHF PA, the improvement in two-tone IMD is typically around 30 dB.The envelope and phase-feedback functions operate essentially independently. In this case, envelope detection occurs at the intermediate frequency (IF), as the input signal is assumed to be a modulated carrier at IF. Likewise, phase detection takes place at the IF, with limiting being used to minimize the effects of signal amplitude upon the detected phase. Alternatively, it is possible to supply the envelope and phase modulating signals separately at baseband and to undertake the comparisons there.
The key disadvantage of polar feedback lies in the generally different bandwidths required for the amplitude and phase feedback paths.Thus,differing levels of improvement of the AM-AM and AM-PM characteristics usually result, and this often leads to a poorer overall performance than that achievable from an equivalent Cartesian-loop transmitter. A good example of the difference occurs with a standard two-tone test, which causes the phase-feedback path to cope with a discontinuity at the envelope minima. In general, the phase bandwidth must be five to ten times the envelope bandwidth, which limits available loop gain for a given delay.
Cartesian Feedback
The key disadvantage of polar feedback lies in the generally different bandwidths required for the amplitude and phase feedback paths.Thus,differing levels of improvement of the AM-AM and AM-PM characteristics usually result, and this often leads to a poorer overall performance than that achievable from an equivalent Cartesian-loop transmitter. A good example of the difference occurs with a standard two-tone test, which causes the phase-feedback path to cope with a discontinuity at the envelope minima. In general, the phase bandwidth must be five to ten times the envelope bandwidth, which limits available loop gain for a given delay.
Cartesian Feedback
The Cartesian-feedback technique overcomes the problems associated with the wide bandwidth of the signal phase by applying modulation feedback in I and Q(Cartesian) components [104]. Since the I and Q components are the natural outputs of a modern DSP, the Cartesian loop is widely used in PMR and SMR systems.
The basic Cartesian loop consists of two identical feedback processes operating independently on the I and Q channels. The inputs are applied to differential integrators (in the case of a first-order loop) with the resulting difference (error) signals being modulated onto I and Q subcarriers and up-converted to drive the PA. A sample of the output from the PA is attenuated and quadrature-down-converted (synchronously with the up- conversion process). The resulting quadrature feedback signals then form the second inputs to the input differential integrators, completing the two feedback loops. The phase shifter shown in the up-converter local-oscillator path is used to align the phases of the up- and down-conversion processes, thereby ensuring that a negative feed- back system is created and that the phase margin of the system is optimized.The effects of applying Cartesian feedback to a highly nonlinear (class-C) PA amplifying an IS-136 (DAMPS) signal are shown. The first ACPR is improved by 35 dB and the signal is produced within specifications with an efficiency of 60 percent [100].
The basic Cartesian loop consists of two identical feedback processes operating independently on the I and Q channels. The inputs are applied to differential integrators (in the case of a first-order loop) with the resulting difference (error) signals being modulated onto I and Q subcarriers and up-converted to drive the PA. A sample of the output from the PA is attenuated and quadrature-down-converted (synchronously with the up- conversion process). The resulting quadrature feedback signals then form the second inputs to the input differential integrators, completing the two feedback loops. The phase shifter shown in the up-converter local-oscillator path is used to align the phases of the up- and down-conversion processes, thereby ensuring that a negative feed- back system is created and that the phase margin of the system is optimized.The effects of applying Cartesian feedback to a highly nonlinear (class-C) PA amplifying an IS-136 (DAMPS) signal are shown. The first ACPR is improved by 35 dB and the signal is produced within specifications with an efficiency of 60 percent [100].
Neyker Stewart Zambrano
CRF
http://www.highfrequencyelectronics.com/Archives/Nov03/HFE1103_RaabPart4.pdf
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