Prepared for the Engineers of Samsung Electronics RF transmitter & power amplifier

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1 Prepared for the Engineers of Samsung Electronics RF transmitter & power amplifier Changsik Yoo Dept. Electrical and Computer Engineering Hanyang University, Seoul, Korea 1

2 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. 2

3 Access methods How do wireless standards differ? 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 3

4 RF transceiver 4

5 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 5

6 Exemplar RF transmit chain * From RF MicroDevices for CDMA cellular system 6

7 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) 7

8 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. 8

9 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 9

10 Digital modulation 10

11 Rectangular pulse 11

12 Raised cosine filter α = (0 : blue, 0.5 : red, 1 : green) 12

13 Raised cosine pulse α = (0 : blue, 0.5 : red, 1 : green) 13

14 Effect of raised cosine filtering Raised cosine filter can be easily implemented digitally. Baseband filtering drastically improves modulation spectral efficiency. 14

15 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 15

16 Power added efficiency A measure of power amplifier s effective use of battery energy. Drain efficiency is somewhat meaningless. DE 16

17 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 17

18 Efficient signal amplification ; out-of-band energy Amplifier s non-linearities spread modulated signal spectrum if modulation format has AM content. 18

19 Measure of out-of-band energy 19

20 Modulation format vs. amplifier non-linearity 20

21 Power amplifier Efficiently provides signal amplification to the appropriate power level. Supply (battery) voltage Output power PAE ACPR 21

22 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 22

23 Linear power amplifier 23

24 Non-linear power amplifier Class-D Switch voltage is square wave which results in the fundamental component of amplitude Vcc*4/π. 24

25 Class-E Non-linear power amplifier Soft-switching power amplifier Relatively insensitive to variations Large voltage stress to active device 25

26 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). 26

27 Power amplifier design flow 27

Load pull method can also be applied to build contours for

28 Output impedance matching Input should be conjugately matched 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 28

29 Trade-off between power and efficiency 29

30 Design example : class-e power amplifier

31 Design example : class-e power amplifier

32 Design example : class-e power amplifier

33 Spectral re-growth 33

34 How to measure ACPR? 34

35 Power amplifier performance comparison 35

36 PAE vs. output power of linear power amplifier 36

37 Characteristics of WCDMA signal Average power = 0dBm 37

38 Overall efficiency of power amplifier 38

39 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 39

40 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 40

41 EER 41

42 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 42

43 AM-PM distortion in class-e power amplifier 43

44 ACPR vs. delay difference in EER system ACPR specification for 3-G WCDMA : 35dB Delay difference should be smaller than 10ns. 44

45 Envelope of WCDMA signal How to design highly linear and efficient modulating power supply with bandwidth larger than 5MHz? 45

46 Examples of EER system For minimum phase shift at envelope path, envelope feedback is employed. See pp , JSSC 98 Basically the same architecture as above. See pp , MTT 98 46

47 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) 47

48 Role of RF transmitter Summary 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 48

Efficiency Enhancement of CDMA Power Amplifiers in Mobile Handsets Using Dynamic Supplies. Georgia Tech Analog Consortium Presentation

Efficiency Enhancement of CDMA Power Amplifiers in Mobile Handsets Using Dynamic Supplies Biranchinath Sahu Advisor: Prof. Gabriel A. Rincón-Mora Analog Integrated Circuits Laboratory School of Electrical