E3 237 Integrated Circuits for Wireless Communication

Transcription

1 E3 237 Integrated Circuits for Wireless Communication Lecture 16: Power Amplifiers Gaurab Banerjee Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore

2 Outline Basic Performance Metrics of Power Amplifiers Classic Power Amplifiers Class A Class B and AB Class C Switching PAs Class D and E Class F Advanced Concepts Envelope Elimination and Restoration Envelope Tracking Outphasing PAs Doherty PAs

3 RF Power Amplifiers Final stage of amplification before modulated power is transmitted by the antenna. Both narrowband and broadband implementations are possible. Popular implementations: Linear (information resides on amplitude, e.g. AM, linear amplification required) -> harder to design Constant envelope (information resides in phase/frequency) -> switching PAs. Common design tradeoffs: Power Gain Linearity Efficiency Output Power Classic power amplifiers : Class A, B, AB and C are also linear power amplifiers distinguished by bias conditions.

4 Waveform Dependence QPSK/QAM -> require efficient utilization of scarce spectrum -> Shaped Data Pulses from square root raised cosine (SRRC) filters. Many modulation schemes require multiple carriers (MC) with low data rates versus single carrier with high data rate -> OFDM. High Peak to Average Ratio (PAR) -> 3-6 db for SRRC, 8-13 db for MC. Power Amplifier should be able to handle wide transitions in amplitude and phase.

5 Nonlinearities Nonlinearities arise due to variable gain or saturation in amplifiers. AM to PM conversion, typically due to voltage dependent capacitors. Source/Drain junction capacitance is a diode. Gate capacitance can also be non-linear at high swings. Also critical in VCOs -> determines phase noise. Memory Effects : Due to charge storage and thermal effects -> High power PAs.

6 Measures of Nonlinearity Carrier to Intermodulation (C/I) -> Two tone based. Noise Power ratio -> drive PA with Gaussian noise with a notch -> noise reappears in notch due to nonlinearities. ACPR (common in NADC and CDMA) -> Ratio of power in specified adjacent band to signal power. EVM : Distance between desired and actual signal vectors, normalized to a fraction of the signal amplitude. Example: WCDMA ACPR-> -33 db@ 5 MHz, -43 MHz, 25% peak EVM.

7 Efficiency Drain Efficiency -> Ratio of RF output power to DC input power -> η= P out /P in,dc. Power Added Efficiency (PAE) : Subtract drive power from output power -> (P out P DR )/P in. Overall Efficiency (Used because PAE can get negative at low amplitudes) -> P out /(P in + P DR ). For most PAs, the instantaneous efficiency at one power level is highest at Peak Output Power (PEP) and decreases as o/p power decreases. AM -> time varying amplitude -> time varying efficiency. Define average efficiency -> Avg. o/p power divided by Avg. DC input power.

8 Probability and Efficiency Envelope PDF p(e) provides relative time an envelope spends at an amplitude. CW, FM, GSM -> Constant Envelope -> Efficiency is always at peak output PAR values of SRRC (3-6 db) and MC (8-13 db) impose special requirements on PAs. Tradeoff with linearity determines efficiency. Class A and B : Theoretical efficiency : 50% and 78.5% Actual efficiency: 5 and 28% (average) Class B performance degrades much less than class A for non-constant envelopes.

10 Class A Bias : M1 is always on. V BIAS > V T Class-A PAs Big Fat Inductor : Provides constant DC current. Big Fat Capacitor : Prevents DC dissipation in load. Load (usually 50 Ohm antenna) Bias determines the class of operation General PA circuit model for linear PAs Tank circuit to filter output (high Q) Drive shuts off here Class A : operates linearly at all times -> good linearity, poor efficiency 360 o conduction angle -> always on!

11 Class-A PAs Let ω 0 = carrier frequency and tank resonant frequency => Note : frequency of power = 2x frequency of current or voltage (With max. swing, irf max results In transistor cutting off.)

12 Class-A PAs : Efficiency Maximum efficiency = 50%!! => Practical values = 30-35%. Another FOM : Normalized power output capability (P N ) -> Ratio of actual output power to maximum drive voltage or current. For class A => Actual current and voltage peaks don t occur simultaneously. Class A is not used widely in PAs. Used in audio applications which are linearity sensitive.

13 Class-B PAs 180 o conduction angle -> half cycle operation. V BIAS = V T for the MOS corresponds to zero I BIAS M1 is on for 50% of the cycle. Class AB operation is between class A and class B in terms of conduction angle. Both Class B and AB are commonly used in a push-pull configuration. Tank circuit used to filter the harmonics of i d, leaving a sinusoidal voltage across R L.

14 Class-B PAs Use Fourier Integral to determine the amplitude of fundamental component.

15 Class-B PAs: Efficiency Normalized power output capability (P N ) -> Same as class A!

16 Class-C PAs Conduction angle < 180 o. Bias Condition -> V BIAS < V T -> Negative I BIAS. > 2Φ = Conduction angle > 100% Efficiency corresponds to very little output power!

18 Switching PAs Key Idea: Use a MOSFET as a switch rather than as a voltage controlled current source or transconductor, unlike linear PAs Observation for Ideal Switch ON V = 0 I > 0 -> Power = 0 OFF V >0 I = 0 -> Power = 0 No dissipation in switch -> 100 % Efficiency.

19 Only one transistor driven per half cycle : M1/M2 Class D PAs Output filter Drain Voltage/Current for M1 Secondary voltage/current of T2

20 Class D PAs Alternate cycles drive the primary of T2 to zero on each side (V DD, M1/M2) Transformer action ensures that one drain is at zero and the other at 2V DD. At series resonance of output RLC network, the current at fundamental flows through R L. No current components off resonance. Sinusoidal secondary current -> sinusoidal primary current (half sinusoids in M1/M2)

21 Class D PAs Fundamental components of V T2 Normalized power output efficiency: Much better than linear PAs Problems: Static dissipation due to non-zero V DS in a switch that is fully ON. Current flows through nonzero voltage during switching -> current and voltage waveforms overlap. Switching PAs typically work well when f << f T -> harmonics in square waves need to be handled properly.

22 Class E PAs Key Idea: Modify circuit so that the switch voltage becomes zero before current flows. Use a high order reactive network to shape the pulses -> Zero Value and Zero Slope at turn on. Absorb M1 drain cap into C1 Classic Paper : Sokal and Sokal, JSSC, Overlap problem solved on this edge -> turn on Overlap problem still exists at this edge -> turn off

23 Class E PAs Design Equations (From JSSC Paper) : Very demanding switches -> Do not scale well. Still popular in discrete implementations, but not ICs.

24 Class F PAs Key Idea: Based on the reciprocation property of transmission lines. Z in = Z 02 /Z L Transforms open to short and vice-versa short at all frequencies away from resonance Note that:

25 Class F PAs At ω 0, drain sees pure resistance R L = Z 0, since the tank is open. At 2ω 0, 4ω 0, 6ω 0...nω 0 (n even), tank is a short and transmission line is nλ/4 (n even) -> Drain will see a short circuit. At 3ω 0, 5ω 0, 7ω 0...mω 0 (m odd), tank is a short circuit and line is odd number of λ/4 segments -> Drain sees open circuit for odd harmonics. At the drain, odd harmonics are not loaded -> square wave is preserved as drain voltage. Even harmonics are shorted : square wave of voltage is not affected. Only current flows into the line at fundamental frequency.

26 Class F PAs Efficiency = 100% ideally, since the switch dissipates no power. Normalized power output capability (P N ) -> 1/(2 π) = > Better than class E. Good PA, but narrowband by nature. Quarter wave lines are typically implemented off chip -> on chip implementations possible based on lumped element equivalents. May be possible to implement at 60 GHz, where wavelengths are small.

27 In Summary Source : Atheros Communications, California, USA

29 Envelope Elimination and Restoration (Also known as Kahn s Technique) Key Idea => Deconstruct the signal in the beginning, reconstruct it during final amplification. Removing amplitude modulation allows the use of high efficiency, constant envelope PAs. Supply modulation is used to add the AM component back.

30 Envelope Tracking Architecture Key Idea => Use a linear PA at the output stage, but modulate the supply with a DC/DC converter -> Similar to Kahn s Technique (EER). Supply voltage varied dynamically to conserve power -> envelope detector provides information to DC/DC converter. Efficiency at high output power levels is determined by the converter efficiency as well. Works much better than a constant VDD system over a wide dynamic range.

31 Outphasing Architecture (Linear Amplification using Nonlinear Components LINC PA) Key Idea => Amplitude varying signal presented as the vector sum of two constant envelope signals, which are phase modulated. Each PA can be a switched amplifier -> high efficiency. Needs a low loss combiner. Additional shunt reactances at the input to the combiner can be used to maximize efficiency (Chireix s technique *) -> Additional tuning needed. Spectral re-growth of V 1 and V 2 is very large -> good matching required to cancel it in summation. * Chireix s pronounced as she wrecks

32 Doherty s Technique Use a Main (Carrier) PA in class B and an Auxiliary (Peaking) PA in class C (negative-bias). Key Idea => For low power levels, only use the carrier PA, for high power levels, use the peaking PA. Incorporate the presence of the off-state PA with transmission lines. Keeps the instantaneous efficiency high over a large dynamic range -> For some Doherty PAs, 70% efficiency achievable with 10 db PAR -> Active research area for multi-carrier PAs.