UC Berkeley EECS 242 UC Berkeley EECS 242 PA system level specifications Power devices briefly and compact models EECS 231 Power amplifiers EECS 142 242 Linearization techniques Matching networks EECS 217 Power combining EECS 217 Copyright Prof Niknejad 2 Peak Output Power Efficiency Power Gain Amplifier Linearity Stability over VSWR Ability to transmit into an unknown varying load Power Control Step size range High efficiency at back off UC Berkeley EECS 242 Copyright Prof Niknejad 3 The peak output power determines the range for two way communications The peak power is often specified at the 1 dB compression point Need about 1W for cellular handsets 1 km distance Need about 100mW for W LAN 100 m Need about 10mW for W PAN Bluetooth 1 10 m Need about 1mW for UWB and sensor networks In practice the average power transmitted may be much lower than the peak output power due to power control slow time scale or the amplitude modulation fast time scale UC Berkeley EECS 242 Copyright Prof Niknejad 4 Power Added Efficiency PAE is a popular metric Pout is the output power Pin is the input power and Pdc is the DC power consumption of the PA For high power gain systems Gp the efficiency approaches the collector drain efficiency c or simply the efficiency of the last stage The efficiency of the PA is an important measure of the battery life of the wireless transceiver Since the PA power dwarfs the power consumption in the receiver it is usually the most important specifications For lower power systems below 10 mW the power of the entire transmitter chain is important and should be taken into consideration UC Berkeley EECS 242 Copyright Prof Niknejad 5 For a constant envelope signal phase frequency modulation the average efficiency is equal to the average efficiency at peak power Due to power control though we must take into account the statistics of the transmitted signal Modern systems use power control to minimize the impact of a transmitter on nearby systems interference and hence only use as much power as needed to achieve low data rate communication with the base station Thus the actual average efficiency depends on how the efficiency varies with output power UC Berkeley EECS 242 Copyright Prof Niknejad 6 g P P Given the distribution of power levels or the PDF g P we can calculate the expected value of the efficiency Unfortunately for most PAs the efficiency drops at low power UC Berkeley EECS 242 Copyright Prof Niknejad 7 Peak is much larger Average Power For signals with amplitude modulation the average efficiency depends not only on the desired power level but also on the statistics of the envelope The amount of power variation is usually captured by the PAR or the Peak to Average Radio The PAR is a strong function of the type of modulation Systems with the highest PAR are OFDM systems employing multiple carriers UC Berkeley EECS 242 Copyright Prof Niknejad 8 The traditional way to characterize narrowband system linearity is with IM3 Since the system may be driven into a strongly non linear regime all odd order harmonics should be carefully taken into account to ensure that excessive spectral leakage does not occur UC Berkeley EECS 242 Copyright Prof Niknejad 9 PAs exhibit nonlinear distortion in amplitude and phase For a mulated signal both sources of distortion are significant The dominant sources are AM to AM and AM to PM Amplitude distortion AM to AM conversion Phase distortion AM to PM conversion For input Corresponding output AM to AM conversion dominated by gm non linearity AM to PM conversion dominated by non linear capacitors UC Berkeley EECS 242 Copyright Prof Niknejad 10 AM AM Curve AM PM Curve For a narrowband signal we can partition the non linearity into an amplitude amplitude AM AM component and an amplitude phase AM PM component This behavioral model can be used to run system level simulations to see the effect of non linearity on a modulated waveform UC Berkeley EECS 242 Copyright Prof Niknejad 11 For modern communication systems the IM specifications leave a lot to be desired since they are only based on multi tone excitation Increasingly the actual modulation waveform needs to be tested with the PA To ensure proper etiquette the amount of power leaking into an adjacent channel is carefully specificed UC Berkeley EECS 242 Copyright Prof Niknejad 12 Figures from Microwave Journal April 2005 DESIGN OF LINEAR SBAND POWER AMPLIFIERS WITH HIGH POWER ADDED EFFICIENCY by Y W Yeap and T W Chua While the ACP is a good way to measure how much the PA signal will deteriorate a neighboring channel signal the EVM is a measure of how much the PA interferes with itself The EVM measures the systematic deviation of the constellation points from the ideal positions due to amplifier non linearity UC Berkeley EECS 242 Copyright Prof Niknejad 13 Both digital and analog modulation schemes involve amplitude and or phase modulation Linearity specs of PA determined by envelope variation Most spectrally efficient modulation schemes have large envelope variations Analog FM AMPS uses constant envelope can use efficient non linear power amplifiers 60 70 GMSK GSM uses constant envelope as well pi 4 DQPSK IS54 136 has 3dB peak to average envelope fluctuation PAEF QPSK CDMA base station has 10dB of PAEF OQPSK CDMA handset has 3dB of PAEF UC Berkeley EECS 242 Copyright Prof Niknejad 14 Key specifications are the peak to average radio the peak power and the power control range Constant modulation schemes much easier GSM AMPS OFDM very hard UC Berkeley EECS 242 Copyright Prof Niknejad 15 Two general classes of PA Linear and Non Linear Linear Pas preserve amplitude and phase information while Non linear PAs only preserve phase mod Linear PAs employ transistors as current sources high Z Non linear PAs employs transistors as switches low Z Linear PAs can drive both broadband and narrowband loads Non linear PA usually drive a tuned circuit load Amplitude information in a nonlinear PA can be recovered by Oversampling duty cycling or varying the supply voltage UC Berkeley EECS 242 Copyright Prof Niknejad 16 All amplifiers eventually clip that is the output cannot be some multiple of the power supply If we back off sufficiently from the peak so that the amplifier never clips then we compromise the efficiency We can generally make a compromise and choose sufficient back off to meet the EVM specs UC Berkeley EECS 242 Pout Limited Output Ideal Clipping Point Pin Copyright Prof Niknejad 17 In applications