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

2. 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.

3. 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 a/b/g)
Wireless MAN- WiMAX (IEEE a /b /c /d /e), LMDS
Personal Communications- Bluetooth, WUSB, Fire Wire
Industrial Communications- ZigBee (IEEE )
Satellite Communications
Extra Terrestrial Communications

4. 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

5. 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

6. 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

7. 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

8. 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)

9. 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

10. 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

11. 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 is possible
Class B
Push-pull
Total conduction angle is 360°
Ideal efficiency of 25% is possible
DUF of is possible
Used for linear applications
Optimum output power at 50% Duty cycle switching
Ideal efficiency of 100% is possible
DUF of is possible
Class C
Class D
Total conduction angle is 360°
Ideal efficiency of 78.4% is possible
DUF of is possible
Used for linear applications
Total conduction angle is less than 180°
Ideal efficiency of 89.6% at 120° conduction
DUF of is at 120° conduction
Push-Pull arrangement of Class F
Ideal efficiency of 100% is possible
DUF of is possible

12. 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.

13. 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.

14. 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.

15. 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

16. 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

17. 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

18. 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.

19. 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