Power Amplifiers Anurag Nigam Non-Constant Envelope Signal Peak Power Average Power Peak Power Crest Factor Non-Constant Envelope Signal Anurag Nigam

Contents Duration: 1.5 hour Topics: Concept of Power Amplification Classes of Power Amplifiers (PA) and Comparison PA Performance Parameters We will discuss in detail the requirement of Power Amplification in a wireless communication system, examples of wireless communication systems, various classes of power amplifiers, their distinguishing features and applications, various performance parameters of a power amplifier and their significance in the design.

Power Amplification Examples Long haul Downlink Uplink Various examples of wireless communication systems are- Mobile Communication System- GSM, PCS, DCS, Cellular, WCDMA (UMTS) Wireless LAN- WiFi (IEEE 802.11a/b/g) Wireless MAN- WiMAX (IEEE 802.16 a /b /c /d /e), LMDS Personal Communications- Bluetooth, WUSB, Fire Wire Industrial Communications- ZigBee (IEEE 802.15.4) Satellite Communications Extra Terrestrial Communications

Power Amplification Full Duplex Communication FDD System TDD System Power Amplifier Low Noise Amplifier Circulator Antenna FDD System Power Amplifier Low Noise Amplifier Switch Antenna BPF TDD System

Power Amplification What is expected of a PA in a Frequency Division Duplex System? Power Amplifier Low Noise Amplifier Circulator Antenna PA is continuously on unless in sleep mode High junction temperatures for devices VSWR constraints not stringent Expected reverse isolation high Turn on and off times not important Performance droop not expected during performance Receive band emissions are expected to be low Point to point link requires intermediate frequency translation

Power Amplification What is expected of a PA in a Time Division Duplex System? Power Amplifier Low Noise Amplifier Switch Antenna BPF PA operates in pulsed mode Low junction temperatures for the devices VSWR constraints stringent Expected reverse isolation low Turn on and off times important Performance droop expected at turn on Point to point devices can be identical

Power Amplification What decides gain and peak output power of a PA? Power Amplifier Low Noise Amplifier Switch Antenna BPF Link Budget and Peak PA Power Transmission Medium Related Factors In a line of sight transmission, path length, attenuation, fading and noise are easier to characterize compared to non-line of sight transmission and non-stationary channels. Receiver Related Factors Receiver sensitivity depends on System Bandwidth Minimum Signal-to-Noise Ratio required at the detector Noise Figures of the stages between the receiver antenna and the detector Gains and losses of stages between the receiver antenna and the detector

Power Amplification What are Saturated Power Amplifiers and Linear Amplifiers? Figure shows output power Vs input power of a power amplifier. Linear Region At low power levels the output power varies linearly with the input power. Non-Linear Region Supply voltage and current limit the output power for a given output resistance as the input power increases. The gain starts dropping in the non-linear region. Saturated Region Output power becomes constant limited by the DC supply and the output resistance in the saturated region. “Saturated Amplifiers” operate in “Saturated Region” while “Linear Amplifiers operate in “Linear Region” Linear Non Saturated P_input (dBm) P_output (dBm) Gain (dB)

Power Amplification What are the various modulation schemes used in communication systems? Most of the communication systems are digital as digital data can be compressed, correction codes can be applied and multiple access can be performed in a number of ways. Various modulation schemes are- As visible from the figures, FSK and QPSK are constant envelope modulation schemes while QAM 16 and higher are non-constant envelope schemes if transmitted as it is. In case QAM is broken down to I and Q signals, the transmission is non constant envelope. Constant envelope signals are not bandwidth efficient but can be amplified using high efficiency saturated amplifiers. Non-constant envelope signals are bandwidth efficient but have to be amplified using linear amplifiers. Frequency Shit Keying 00 01 11 10 Quadrature Phase Shift Keying QAM 16

Power Amplification What do the terms “Power Added Efficiency” and “Back off” mean? Linear Non Saturated P_input (dBm) P_output (dBm) PAE (%) Linear, Non-Linear & Saturated regions of operation of a PA Figure on left shows that PAE goes up with the input power. PAE reaches its peak as the amplifier compresses. To operate amplifier at its highest efficiency it must be operated in non-linear or saturated region. For non-constant envelope signals amplifier has to be operated in linear region such that the peak power is just in non-linear region. Figure below shows non-constant envelope signal. For peak efficiency the amplifier is operated backed off in output power by the crest factor. Average Power Peak Power Crest Factor Non-Constant Envelope Signal

Classes of PA & Comparison Power Amplifier Classes Power Amplifier Class F Class A Class E Odd order peaking resonators are used at odd harmonics Ideal efficiency of 100% is possible DUF of 0.159 is possible Class B Push-pull Total conduction angle is 360° Ideal efficiency of 25% is possible DUF of 0.125 is possible Used for linear applications Optimum output power at 50% Duty cycle switching Ideal efficiency of 100% is possible DUF of 0.0981 is possible Class C Class D Total conduction angle is 360° Ideal efficiency of 78.4% is possible DUF of 0.125 is possible Used for linear applications Total conduction angle is less than 180° Ideal efficiency of 89.6% at 120° conduction DUF of 0.0981 is at 120° conduction Push-Pull arrangement of Class F Ideal efficiency of 100% is possible DUF of 0.318 is possible

PA Performance Parameters Power Amplifier Performance Parameters Power Amplifier Gain Bandwidth Linearity Efficiency Cost/Size Modulation Scheme Quiescent Current Harmonic Distortion Yield Standby Current Ruggedness MTBF/MTTF Supply Voltage Range Switch-off Isolation Noise/Receive Band Noise Number of components Power Amplifier performance parameters decide amplifier class and circuit topology. Specifications for various performance parameters have to be met across operating condition extremes like supply voltage and ambient temperature range and across process corners. The specifications have to be met on volume with a certain yield.

PA Performance Parameters What do the terms “Small Signal” and “Large Signal” mean? Active devices like transistors or diodes may be biased to some DC Operating Point for optimum performance. As the signal level goes up there is clipping of the signal at saturation or cut-off regions. This changes the DC operating conditions. At large signal levels a linearized model cannot be used beyond certain threshold. For BJT and FET devices, threshold input signal voltage is as follows For BJT is large signal and otherwise is small signal For FET From the equations it is visible that FET can be operated in wider input range as small signal by choosing high bias current and low W/L ratio.

PA Gain and Return Losses Gain of Power Amplifier is defined as the ratio of the power at the output ( ) to power at the input ( ). If both the input and output matches are conjugate matches for maximum power transfer, the gain is referred to as Available Gain. Power delivered to the load under conjugate match to power delivered by the source is referred to as Transducer Gain. Ratio of power delivered to the load to the power delivered by the source is called Power Gain. Power Gain includes both input and output mismatch losses. Expressed in decibel (dB) Gain Power Amplifier Reflection Coefficients Return Losses Ratio of power reflected back into the source to the power incident at the input of a PA is called Input Return Loss. The definition holds true for small as well as large signals. PA is not bilateral and not reciprocal. Ratio of power reflected from the PA output to the power incident at the output is referred to as Output Return Loss. Output return loss is always measured small signal for a PA.

Nature of Gain & PA Transfer Characteristics Small Signal Gain Commonly used Gain is Power Gain. Expressed in terms of S- Parameters, Power Gain is given by For input and output match to characteristic impedance Power Gain and Available Gain are the same Large Signal Gain Large Signal Gain can be evaluated in same manner as small signal gain, by replacing small signal reflection coefficient by large signal reflection coefficients at the input. As the input power increases, DC Supply Current increases. Output power increases linearly with input power. Limit on DC Current causes Output power to saturate. The Power Gain drops and is referred to as Gain Compression. Power Gain and Output Power Vs Input Power Characteristics of PA

Power Amplifier Losses Source of losses in active device Active Device Losses PA Losses Ids Vds t Losses in PA Passive Losses Various Losses in a PA are- Device Loss due to voltage across the device and current through the device Loss due to finite voltage across the device during saturation (BJT) Finite resistance of the channel during saturation (on-time) (FET) Loss of energy stored in the reactive components Resistive losses in reactive components due to finite Quality Factor Conductor Losses in Inter-connects Dielectric Losses in the substrate Radiation Losses from traces

Efficiency Efficiency of Power Amplifier is the ratio of RF power produced at the output to power input to the Power Amplifier. In case only the DC Supply Power is accounted for as input power, the efficiency is called the drain or collector efficiency In case the DC Supply Power as well as the input RF Power is accounted for as input power, the efficiency is called Power Added Efficiency (PAE) In case Net RF Power is considered as output power and DC Supply Power is considered as input power, the efficiency is still called PAE but refers to conversion efficiency of the PA Drain/Collector Efficiency Power Added Efficiency Conversion Efficiency

Linearity and Harmonic Distortion Power Amplifier output to small signal input in linear region can be represented by Power Amplifier output to small signal input in non-linear region can be represented by For Small Signal Input Large Signal Input has fundamental and harmonics For Sense of Completion the non linear output of a PA when large signal input is applied to non linear transfer function of a PA, M tone input has 1 to N order mix components. For N-order transfer function, the response to M tone large signal input is given by For Modulated Signal with modulation component around fundamental and harmonics, the mixing is even more complicated.

Linearity and Harmonic Distortion What is the impact of PA non-linearity on non-linear input signal? Even order mixing causes out-band components displaced away from the fundamental and can easily be filtered using Band-Pass Filter at the output Odd order mixing causes in-band and adjacent band components- Inter-modulation and in-modulation components. For simplicity let us consider two tones and third order mixing as shown in figure. -f2 -f1 f1 f2 1 2 +-(2f2-f1) +-(2f1-f2) +-(2f2-f2) +-(2f1-f1) Cross Modulation (Inter-modulation) Cross Modulation (Inter-modulation) Co-modulation (In-modulation or AM-AM/AM-PM) AM-AM and AM-PM Non-Linearity