18 Power Amplifiers

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Power Amplifiers

Prof. Bhaskar Banerjee

EERF 6330- RF IC Design

Bhaskar Banerjee, EERF 6330, Sp2013, UTD 2

Power Amplifier

What is a Power Amplifier (PA)? The final stage of amplification in a transmitter Needs to drive a large power into an antenna.


Basic Amplifier

Matching N/W RL

RFC

vin M1

Voltage-Swing at X: 2Vdd (Inductor allows for that swing) dc Stress for M1

Large Signal Impedance Matching Conjugate Matching?

Even if it worked ~ 50% efficiency!

X


Figure-Of-Merits Efficiency

Drain efficiency ()

Power Added Efficiency (PAE)

PRFin PRFout

IDCVDC

PA


Figure-Of-Merits Linearity

AM-AM distortion (Gain compression) AM-PM distortion

PA



AM-AM distortion (Gain compression) IP1dB: Input P1dB OP1dB: Output P1dB Psat: Saturation power

Pout(dB)

Pin(dB)

1dBOP1dB

IP1dB

Psat



Spectral regrowth Adjacent Channel Power or Leakage Ratio (ACPR/ACLR)

ACPR



Spectral regrowth in CDMA Adjacent Channel Power Ratio (ACPR)



Constellation distortion Error Vector Magnitude (EVM)

Used to determine errors and their cause Measured Vector Reference Vector = Error Vector

Measured = Actual signal magnitude and phase Reference = Ideal signal based on knowledge of data

(ie. bit rate, number of symbols, filtering, etc.)

Sideal

Serror


Trade-off: Efficiency and Linearity Efficiency improves with drive level, highest when the gain is

several dB compressed Gain compression, phase distortion, and inter-modulation

cause the bandwidth limited signal to distort and spill over into adjacent channels, resulting in spectral regrowth and increase in EVM


Power Performance Measurement Load-pull and Source-pull system configuration

Tuner for Source-pull Tuner for load-pull


Power Performance Measurement Load-pull and Source-pull measurement

Finding optimum load and source impedances for.. maximum power gain maximum output power maximum PAE minimum ACPR

Varying the impedances seen by the Device Under Test (DUT) with tuners and measures its performance


Power Performance Measurement Contours with constant gain, output power, PAE, or ACPR

with specified step values Contours can be overlapped to find the optimum impedance

region to meet the multiple specifications

Constant gain contours Constant PAE contours Overlap of constant gain and PAE contours

Optimum impedance region


Power Performance Measurement Measurement example

Load-pull measurement Constant output power contours to find maximum output power

impedance region

S22* = Small Signal Conjugate Output Impedance


Amplifier Classification

Class of operation The manner in which transistors are operated or biased Output current waveform when the input is applied

Non-switch mode amplifiers (Conduction Angle based ) Power transistors behave as current sources Linearity and efficiency are trade-off Class A, B, AB, C Conduction angle = (fraction of a cycle the transistor is turned

on) 360

Switch mode amplifiers (Harmonic Termination based) Power transistors behave as switches 100% efficiency can be achieved in theory Nonlinear Class D (switch mode), E (switching), F (harmonically

terminated).


Amplifier Classification Non-switch mode amplifiers (Conduction angle based)

Class A : The device conducts current during the entire cycle of the

input waveform (conduction angle is 360 degrees) Class B :

The device conducts current during the half of the cycle of the input waveform (conduction angle is 180 degrees)

Class AB : The device conducts current more than half but less than the

entire cycle of the input waveform (conduction angle is 180-360 degrees)

Class C : The device conducts current less than half of the cycle of the

input waveform (conduction angle is 0-180 degrees)

*Conduction angle: the portion of the input cycle for which the transistor conducts and an output current flows.


Class operation Class A

Vk VDSmax

Im

VDS

IDS

VDD

IDS

VGS VGS=VP

VGS=0

VP2VPVGS

Q

P

2P

Q

VDS

IDS

Q

Im

P 2P


Class operation Class B

Vk VDSmax

Im

VDS

IDS

VDD

IDS

VGS VGS=VP

VGS=0

VP2VPVGS

Q

P

2P

Q

VDS

IDS

Q

Im

P 2P


Class operation Class AB

Vk VDSmax

Im

VDS

IDS

VDD

IDS

VGS VGS=VP

VGS=0

VP2VPVGS

Q

P

2P

Q

VDS

IDS

Q

Im

P 2P


Class operation Class C

Vk VDSmax

Im

VDS

IDSIDS

VGS VGS=VP

VGS=0

VP2VPVGS

Q

P

2P

Q

VDS

IDS

Q

Im

P 2PVDD


Amplifier Classification Non-switch mode amplifiers

Voltage and Current at the output

t

t

VD

ID

Class A

t

t

VD

ID

Class A

VD

t

t

ID

Class B

t

t

ID

Class B

t

t

VD

ID

Class C


Class A Operation

similar to small-signal but on steroids!

23

iD = IDC + irf sin0t

v0 = irfR sin0t

Prf =i2rfR

2

IDC = irf =) PDC = IDCVDD = irfVDD

Drain eciency, =PrfPDC

=i2rf/2R

irfVDD=

irfR

2VDD

Maximum value of irfR is VDD =) max = 12= 50%


Class A Operation

similar to small-signal but on steroids!

24


Class B Operation

Bias is arranged to shut off the output device for half of every cycle. (duty cycle = 50%).

25

iD = irf sin0t for iD > 0.

ifund =2

T

T/2R0

irf (sin0t)(sin0t) dt =irf2

vout irf2

R sin0t

vout,max = VDD =) irf,max = 2VDDR

Pout =v2o2R

=) Pout,max = V2DD

2R

iD =1

T

R T/20

2VDDR

sin0tdt =2VDDR

=) PDC = 2V2DD

R

) = Pout,maxPDC

=

4 0.785 = 78.5%


Class B Operation

non linearity (not in terms of input-output waveforms, but in terms of input-output power proportionality).

26


Class C Operation

Bias is arranged to shut off the output device for more than half of every cycle. (duty cycle < 50%).

Maximum efficiency ~ 100% (when it is always off!)

27


Amplifier Classification Non-switch mode amplifiers

Trade-off between Efficiency and Linearity

Conduction angle = (fraction of a cycle the transistor is turned on) 360

output RF powerDC powerEfficiency =


Amplifier Classification Influence of conduction angel

Idc =Imax2

2sin(/2) cos(/2)1 cos(/2)

In =Imax

(1 cos(/2))

1n 1sin(n 1)

2 2

ncos(/2)sin(n/2) +

1n+ 1

sin(n+ 1)

2

I1 =Imax2

sin1 cos(/2)


Harmonics

Conduction Angle

Ampli

tude

Fundamental

DC

2nd 3rd 4th2 0

Imax


Amplifier Classification Influence of conduction angle

Imax/


Amplifier Classification Summary

Pmax =V 2DD2R

=(3.3)2

2 50 0.1 W

Rmax =V 2DD2Pmax

=(3.3)2

2 1 5.4

IDC =VDDR

= 825 mA Ipeak = 2 IDC = 1.65 A

=PoutPDC

=1 W

0.825 A 3.3 V 37%Bhaskar Banerjee, EERF 6330, Sp2013, UTD 33

Class A PA Example

Frequency = 1 GHz, POUT = 1 W into a 50 load, Vdd = 3.3 V

Hence for 1 W power we need impedance transformation

In practice, R would be less than this (~ 4 ), and hence, bias current:


Class A PA Example

When such a large current flows through a transistor, Ron becomes critical. Can lose say 200 mV => Ron < 200 m Transistor width ~ several mm Also, = 37%, hence 1 W output power => 1.7 W dissipated Packaging and Heat Sink becomes very critical! Class A - if input drive = 0 ; Power dissipated = 2.7 W! Design for worst case Use adaptive bias1

1 A. Saleh, and D. Cox, IEEE Trans. Microw. Theory Tech., v. 31, no. 1, Jan 1983, pp. 51-56.


Class A Example

vin

Id RFC

DC BlockRL = 50

vOUTvin

CL

VDD

XC = 5 => C =1

5 2 1 GHz = 31.8 pF


Class A Example

Output Filter is a simple parallel LC BW = 100 MHz, f0 = 1 GHz => Q = 10 One solution is XL = XC = 5

XL = 5 => L =5

2 1 GHz = 0.8 nH

RFC should be at least 10-15 times larger than R (4 ) RFC > 6.4 nH Also need matching network to transform 50 to 4 L-matching network

L1 =RL0Q

502109 3.5 2.3 nH

C1 =1

0QRS 1

2109 3.5 4 11.7 pF


Class A Example

Transformation ratio (50/4) sets the Q = 3.5 L-matching n/w:

Combine these values with the filter L and C values


Class A Example

vin

825 mA >6.4 nH

11.7 pFRL = 50

vOUTvin

32 pF0.6 nH

3.3 V


Class A Example

825 mA >6.4 nH

11.7 pFRL = 50

vOUT

vin

32 pF0.6 nH

3.3 VIbias/n

A/n A


3 Stage PA Configurations

MatchingNetwork

MatchingNetwork

MatchingNetwork

On-ChipOff-Chip3 dBm

Self-BiasIndep. Bias

Class of Operation

Device Size:40/3212 dB Gain15 dBm

On-ChipOff-Chip30 dBm


Class of Operation

Device Size: 160

Push-PullSingle Device6 dB Gain

MatchingNetwork


Class of Operation

Device Size: 409 dB Gain24 dBm


Addressing Vds(Vce), Vdg(Vcb) Breakdown

Methods to overcome voltage breakdown

Decrease effective output resistance by increasing current

Distribution of the power output over multiple devices.

Cascode design

Stacked power amplifier

Balanced power amplifier

Power Combining


CMOS Power Amplifier

Power Amplifiers in RF transmitters Consume Most of Power in Transmission Mode Linearity Cost

Cost Reduction Strategy Silicon-Based Technology High levels of Integration


Issues in CMOS PA Design

Transistor Characteristics Low Breakdown Voltage High Knee Voltage

Passive Component Lossy Si substrate Thin Metal Layers


CMOS PA Example

44

Power combining with transformers

ref: Aoki, et. al., JSSC 2002, vol. 37, no. 3.


CMOS PA Example

45

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18 Power Amplifiers