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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