rf transmitter & power amplifiericlab.hanyang.ac.kr/research/data/rf_poweramp... ·...
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Prepared for the Engineers of Samsung Electronics
RF transmitter & power amplifier
Changsik YooDept. Electrical and Computer Engineering
Hanyang University, Seoul, Korea
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Wireless system market trends
• Flexibility through mobility
• Consumers are the main drivers of system features.– Talk time
– Standby time
– Size and weight
• Power amplifier is one of the key talk time drivers.
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How do wireless standards differ?
• Access methods– Frequency division multiple access (FDMA)
– Time division multiple access (TDMA)
– Combined FDMA and TDMA
– Code division multiple access (CDMA)
• Modulation schemes– Analog modulation
– Digital modulation
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RF transceiver
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Role of RF transmitter
• RF transmitter efficiently encode information on carrier and amplify for transmission.– Spectrally efficient modulation
– Spurious and noise filtering
– Analog signal processing precision
– Efficient signal amplification with power level control
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Exemplar RF transmit chain* From RF MicroDevices for CDMA cellular system
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Transmitter performance attributes
• How to quantify spectrally efficient modulation?– Modulation method– Baseband filtering
• How to quantify spurious and noise filtering?– IF and RF filtering– Quality of LO signal from the frequency synthesizer
• How to quantify analog signal processing precision?– High fidelity mixing– Linearity of variable gain amplifier
• How to quantify efficient signal amplification?– Power consumption– Out-of-band signal energy (i.e. multi-user interference)
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How to quantify spectrally efficient modulation?
• Digital modulation method occupy much more bandwidth than their bit rate.
• If unaltered, receiver bandwidth would have to be much wider than the bit rate.– Wide BW is required to avoid ISI caused by passing a spectrally
wide signal through too narrow a filter.
• A special class of filters, Nyquist filters, are used to reduce the effective modulation bandwidth.
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Principles of modulation
• Encode desired information on carrier frequency by varying ;– Amplitude
– Frequency
– Phase
• Analog modulation vs. digital modulation– Analog modulation ; poor frequency spectrum utilization
– Digital modulation ; more efficient frequency spectrum utilization
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Digital modulation
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Rectangular pulse
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Raised cosine filterα = (0 : blue, 0.5 : red, 1 : green)
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Raised cosine pulse
α = (0 : blue, 0.5 : red, 1 : green)
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Effect of raised cosine filtering
• Raised cosine filter can be easily implemented digitally.
• Baseband filtering drastically improves modulation spectral efficiency.
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How to quantify efficient signal amplification?
• Transmit power amplifier– Efficiently amplify signal while minimizing RF energy transmitted
outside channel.
• Minimum DC power consumption– Power added efficiency (PAE)
– Transmit signal power control
• Minimum multi-use interference– Signal energy transmitted in adjacent channels
– Broadband noise floor (particularly within receiver bandwidth)
– Spurious and inter-modulation energy
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Power added efficiency
• A measure of power amplifier’s effective use of battery energy.
• Drain efficiency is somewhat meaningless.
DE
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Power management for efficient amplification
• Wireless handset power consumption is reduced by controlling transmit power level.
• Basestation detects signal strength from mobile unit and encodes message to mobile indicating appropriate power adjustment.– Significant reduction of mobile unit’s average power consumption
– Minimized interference with other users
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Efficient signal amplification ; out-of-band energy
• Amplifier’s non-linearities spread modulated signal spectrum if modulation format has AM content.
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Measure of out-of-band energy
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Modulation format vs. amplifier non-linearity
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Power amplifier
• Efficiently provides signal amplification to the appropriate power level.– Supply (battery) voltage
– Output power
– PAE
– ACPR
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Classification of power amplifier
• Linear power amplifier ; class-A, AB, B, C
• Non-linear power amplifier ; class-D, E, F– Active device as a switching component
– Much higher PAE than linear power amplifier
– Spectral re-growth if used for modulation formats containing AM component such as QPSK
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Linear power amplifier
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Non-linear power amplifier
• Class-D– Switch voltage is square wave which results in the fundamental
component of amplitude Vcc*4/π.
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Non-linear power amplifier
• Class-E– Soft-switching power amplifier
– Relatively insensitive to variations
– Large voltage stress to active device
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Non-linear power amplifier
• Class-F– Adding third harmonics can reduce the peak voltage swing for the
same output power.
– Optimal case is V3/V1=1/9 (V3 ; 3rd harmonic, V1 ; fundamental).
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Power amplifier design flow
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Output impedance matching
• Input should be conjugatelymatched under all load impedance conditions.
• Load pull method can also be applied to build contours for constant efficiency and ACPR.
• Load impedances at harmonic frequencies are also important
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Trade-off between power and efficiency
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Design example : class-E power amplifier - 1
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Design example : class-E power amplifier - 2
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Design example : class-E power amplifier - 3
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Spectral re-growth
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How to measure ACPR?
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Power amplifier performance comparison
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PAE vs. output power of linear power amplifier
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Characteristics of WCDMA signal
Average power = 0dBm
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Overall efficiency of power amplifier
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Overall efficiency improvement
• Linear power amplifier with adaptive biasing
• Linearized non-linear power amplifier– EER (Envelope elimination and restoration)
– LINC (Linear amplification with non-linear components)
– Feedback
– Feedforward
• Complicated design– Path delay matching
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Linear power amplifier with adaptive biasing
• Changing bias point according to the power level
• Could be another source of distortion– Modulation effect
– AM-PM distortion
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EER
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EER with baseband pre-processing
• EER : complicated design– Delay matching between baseband envelope path and RF phase
path– AM-PM distortion : non-linear relationship between VDD and phase
delay of the RF power amplifier
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AM-PM distortion in class-E power amplifier
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ACPR vs. delay difference in EER system
• ACPR specification for 3-G WCDMA : 35dB– Delay difference should be smaller than 10ns.
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Envelope of WCDMA signal
• How to design highly linear and efficient modulating power supply with bandwidth larger than 5MHz?
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Examples of EER system
• For minimum phase shift at envelope path, envelope feedback is employed.
• See pp. 2252-, JSSC’98
• Basically the same architecture as above.
• See pp. 2220-, MTT’98
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LINC
• Advantage over EER– Supply voltage need not be modulated.
• Difficulties– Matching between two RF signal paths
– Power loss at power combining : Pavg/Pmax (only 33% for WCDMA)
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Summary
• Role of RF transmitter– Signal modulation with efficient spectrum utilization
– Signal amplification with efficient power utilization
• Power amplifier– Deep understanding of the wireless standard is required.
– No more small signal design methodology