UC Berkeley EECS 242 UC Berkeley EECS 242 Polar modulators are gaining popularity by require complex feedback and or chip to external PA interaction Modulation bandwidth is a limiting factor In a digital system the magnitude phase signal are generated directly and fed into an offset PLL and PA supply voltage UC Berkeley EECS 242 Copyright Prof Niknejad 3 Need loop gain stability a big concern modulation bandwidth Envelope feedback only works for AM AM nonlinearity Cartesian requires linear mixers and good amplitude phase matching Complexity UC Berkeley EECS 242 Copyright Prof Niknejad 4 In a modern system a dynamic predistortion circuit can compensate for process temp variations Can implement predistortion at baseband 100k gates 1 pad UC Berkeley EECS 242 Copyright Prof Niknejad 5 Basestations use feedforward linearization since calibariton is a possibility Use couplers UC Berkeley EECS 242 Copyright Prof Niknejad 6 Envelope tracking supply and dynamic class A Efficiency always close to peak efficiency of amplifier say 30 regardless of PAR Need a very fast DC DC converter UC Berkeley EECS 242 Copyright Prof Niknejad 7 UC Berkeley EECS 242 The amount of power that we can extract from a PA device is limited by the output imepdance of the device As the device is made larger to handle a higher DC current without compromising the fT the lower the output impedance For a current source style of PA eventually the device is so large that power is lost in the device rather than the load This is the attraction of a switching PA UC Berkeley EECS 242 Copyright Prof Niknejad 9 UC Berkeley EECS 242 Copyright Prof Niknejad 10 100 fingers Finger width MSG Idc 1 m finger 1 m 7 6dB 25mA 2 m 8 4dB 47mA 4 m 6 8dB 94mA 100 fingers 100 fingers 2 m finger 4 m finger UC Berkeley EECS 242 Copyright Prof Niknejad 11 But for a non switching PA we must perform some power combining to use more than one device This way we can transform the load into a higher impedance seen by each PA The power combining networks are lossy and large We ll come back to them later UC Berkeley EECS 242 Lossy Power Combiner Copyright Prof Niknejad 12 Note that we cannot simply wire PAs together since the impedance seen by each PA increases by N if we connect N in parallel This means that each PA delivers less power for a fixed swing There is also load pulling effects if the sub PAs are not perfectly in phase UC Berkeley EECS 242 Copyright Prof Niknejad 13 Decompose the AM PM signal into two PM signals The two PM signals can get amplified by two non linear PA s These can be saturated and efficient amplifiers By combining the two signals the amplitude modulation is restored at the antenna How to combine signals Simple current mode will present a timevarying load to each PA Coupler or isolator will waste power UC Berkeley EECS 242 Copyright Prof Niknejad 14 In theory all we need to do is to compute the inverse cosine of the AM waveform to generate our outphasing signals In practice we can use a DSP to calculate these signals since the envelope rate is at the modulation rate and digital techniques work well Power combining is the main difficulty UC Berkeley EECS 242 Copyright Prof Niknejad 15 Source Naratip Wongkomet and P Gray UCB Invented by W H Doherty in 1936 Main Good power efficiency over a wide range of output power Auxiliary Efficiency max 0 Main on Aux on Main on Aux off Main on Aux on max M A Output power Overall efficiency Main on Aux off 0 Output power Must efficiently combine power without increasing Vswing UC Berkeley EECS 242 Copyright Prof Niknejad 16 Quarter wave line used as an impedance inverter Can be realized with LC equivalent UC Berkeley EECS 242 Copyright Prof Niknejad 17 UC Berkeley EECS 242 Small signal region auxiliary amplifier is off When the input crosses the threshold the action of the auxiliary amplifier is to dynamically lower the load seen by the main PA Finally both amplifiers operate at the peak power point Copyright Prof Niknejad 18 Vm Im Source Naratip Wongkomet and P Gray UCB I1 Vo Zm Zo2 Zmo Impedance Inverter Zo 3RL Zm Zmo Ia RL Zmo RL 1 Ia I1 Doherty s efficiency Imax Vmax Vm Ia Imax 2 0 Vo Vmax 3 Im Vin max 3 max Vin max 0 Typical PA s efficiency Vin max 3 Vin max 0 Pout max 9 Pout max Auxiliary amplifier actively changes load impedance of the main amplifier UC Berkeley EECS 242 Copyright Prof Niknejad 19 Can use lumped elements to realize 90 phase shift The CLC line is an impedance inverter that also provides VDD for the aux amp The LCL line is embedded into the matching network and provides 90 phase shift Simulations show an improved efficiency Zhao M Iwamoto D Kimball L Larson P Asbeck University of California San Diego UC Berkeley EECS 242 Copyright Prof Niknejad 20 Matching networks are needed to drive output power to the load which has a fixed impedance Large output powers require a large transformation ratio and low voltage operation means high currents in the CMOS stage sensitive to series resistance 30 dBm of output power requires a matching ratio of 100 UC Berkeley EECS 242 Copyright Prof Niknejad 21 The power loss of integrated matching networks is important The insertion loss can be derived by making some simple approximations The final result implies that we should minimize our circuit Q factor and maximize the component Qc UC Berkeley EECS 242 Copyright Prof Niknejad 22 From two stage Q we generalize to multi stage design Since the Q of each stage is lowered the insertion can improve UC Berkeley EECS 242 Copyright Prof Niknejad 23 Suppose a power amplifier delivering 100 W of power has an optimal load resistance of 5 but needs to drive a 50 I antenna Design a matching networkassuming that the component Q s of 30 are available First note that a matching factor of m 50 5 100 is needed Table above shows that 3 stages is optimum UC Berkeley EECS 242 Copyright Prof Niknejad 24 Power Enhancement Ratio source Aoki IEEE MTT S UC Berkeley EECS 242 Copyright Prof Niknejad 25 source Aoki IEEE MTT S Example Class E 0 13 um VPEAK 3 V POUT 50 8 mW Required POUT 100 mW RIN 4 PER 50 4 12 5 Q of 5 Q of 10 For the matching network alone impedance matching is limited problem for low voltage operation source Reynaert UC Berkeley EECS 242 Copyright Prof Niknejad 26 Quarter wave line is a nice way to impedance match source and load T line comes for free since we can use the board trace at high frequency How does this vary with matching ratio UC Berkeley EECS 242 Copyright Prof Niknejad 27 For FR4