Transcript NS-40

Class-E Power Amplifiers
for QRP to QRO
David Cripe NMØS
Mount Vernon IA
August 6, 2011
QRP is all about efficiency…
QRP rigs can be small and simple…
…transmitting the least amount of power
necessary for a QSO…
…so why waste power with an inefficient PA?
QRO operators can't ignore efficiency
either…
50 kW RF @ 50% efficiency = $240/day
50 kW RF @ 90% efficiency = $133/day
SAVINGS = 1 full-time staff salary.
Why is PA efficiency important?
 Typical CW ham transmitter has power
amplifier efficiency of 50%.


A transmitter delivering 5W RF, at 50%
efficiency consumes 10W battery power.
Its PA transistor must be capable of
dissipating 5W power
Why is PA efficiency important?
 What would happen if PA efficiency of a 5W
transmitter was increased to 90%?


Power consumption from battery is reduced
from 10 watts to 5.5 watts
Power dissipation in the transistor is reduced
to 0.5 watts, a 90% reduction.
Why is PA efficiency important?
 A higher efficiency PA will result in:
Smaller, cheaper transistor required
 Cooler operation of PA
 Higher reliability of PA
 Reduced battery consumption.

Class-E is a simple, rugged, highly-efficient
power amplifier circuit capable of operating at
90% efficiency.
What is Class-E?
 The Class-E Power Amplifier was invented by
Nat and Alan Sokal in the 1970s.
 It is uses the power device as a switch, and is
capable of DC-to-RF efficiency nearing 100%.
 It uses a low-Q tuned drain network to obtain
specially-shaped voltage and current
waveforms that minimize transistor losses.
Class-E Waveforms*
*US3919656
Definition of Class-E Waveforms
 The active device is operated as a switch with
180 degree conduction per cycle.
 The drain network is tuned so that during the
transistor ‘off’ period, its voltage returns to
zero just before the beginning of switch
conduction
 The slope of the voltage waveform is zero just
before the beginning of transistor conduction
Single-Ended Class-E Circuit
Where can Class-E transmitters be
found?
Broadcast Electronics
SGC Mini-Lini,
4SQRP NS-40,
1 – 5 KW
500W ‘linear’
5W, 40M
AM BC Transmitter
CW Transmitter
WA1QIX 400W 75M Class E
Amp
www.classeradio.com
What devices are good for Class-E
QRP transmitters?
MOSFETs can operate as nearperfect switching devices.
 2N7000, 60v, 2 ohm Rds, 20pF Coss
1 W output
 ZVN4210A, 100v, 1.8 ohm Rds, 40 pF Coss
 1 W output

 IRF510, 100v, 0.5 ohm Rds, 81pF Coss

>5 W output
How is a Class-E PA designed?
 Unlike the empirical, rule-of-thumb design
process used with other PA types, there is a
specific set of component values that must be
selected for a Class-E power amplifier to
operate properly.
 A ‘cookbook’ set of equations can be used to
determine the design of the Class-E PA for a
given power, voltage and frequency.
 Equations found at WAØITP.com
Designing a Class-E PA
A simple prototype circuit will suffice for
most QRP applications.
Class-E Design Procedure
 The frequency F, supply voltage B, and
output power P are selected.
 Based on the output power, a MOSFET is
chosen.
 The circuit load resistance is calculated:
R = 0.28 · B2 / P – 1.5 · Rds
Class-E Design Procedure
 The MOSFET shunt capacitor C1 is
calculated:
C1 = 0.18 / ( 2  · F · R ) - Coss
Class-E Design Procedure
 The series network L2-C2 is determined next
 The capacitor C2 is selected to have one to
two times the reactance of the load, R. A
common standard value is best.
 L2 is calculated:
L2 = [ 1.8 · R + 1 / ( 2  · F · C2 )] / ( 2  · F )
Class-E Design Procedure
 The load impedance of the PA must be
transformed to 50 ohms.
 A preferred way to achieve this is with a 90degree PI network.
 A second-harmonic notch is added to the
series inductor L3.
Class-E Design Procedure
PI Network Component Calculations:
C3 = C5 = 1 / (2  · F · √( R · 50 ) )
L3 = 0.75 × √( R · 50 ) / ( 2  · F )
C4 = C3 / 3
Class-E Design Procedure
 Finally the drain choke L1 is chosen. Its value
is not critical, except it must be much larger
than L2.
L1 ≈ 10 · L2
*Equations found at WAØITP.com
Circuit Simulation and Optimization
 Class-E PAs may be optimized using circuit
simulation software.
 CAD freeware is available from:

LTSPICE IV (SWCAD III)
http://www.linear.com/designtools/software/ltspice.j
sp

TINA-TI
http://focus.ti.com/docs/toolsw/folders/print/tinati.html
Analytic Tools – SWCAD III*
* www.linear.com
SWCAD III Time-Domain Analysis
Efficiency and Thermal Management
 The heat loss in the MOSFET will be
approximately 2  P  Rds / R.
 A good rule of thumb for MOSFET reliability is
to keep the junction temperature below 100
degrees C.
 We can estimate MOSFET junction
temperature from thermal resistance data in
manufacturers’ data sheets.
Thermal Impedance
 A TO-92 transistor (2N7000) has 312 degrees
C-per-watt thermal resistance.
 Allowable dissipation in a TO-92 part is about
¼ watt.
 A TO-220 transistor (IRF510) has 62 degrees
C-per-watt thermal resistance.
 Allowable dissipation in a TO-220 is >1W
 Adding a heat sink to a TO-220 can further
increase allowable dissipation.
How is the Class-E PA driven?
 A MOSFET is a voltage-controlled device.
 The gate of a MOSFET is a relatively large
capacitance.
 The MOSFET driver circuit must handle the
large currents required to charge and
discharge the gate capacitance at the carrier
frequency.
Practical MOSFET Drive Circuitry
 Many MOSFETs are designed to be driven
directly from TTL-level signals.
 TTL Drive requires NO transformer or
impedance matching.
 One 74HCxx gate can drive a 2N7000 up to
14 MHz, two, paralleled 74HCxx gates can
drive an IRF510 up to 7 MHz.
 74ACxx logic has 4x drive capability of
74HCxx.
Practical Drive Circuit
 Adding 1.5 volts of bias to the TTL drive
signal improves MOSFET switching and
efficiency.
How do the Class-B and –E PAs
compare?
 SWCAD III simulations of IRF510, 5W Class-
E and Class-B PAs were compared in normal
operation into a 1:1 VSWR.
 The Class-B PA operated at 71% efficiency,
while the Class-E PA operated at 92%
efficiency.
 The performance of the Class-B and –E
circuits were then compared over eight points
on a 2:1 VSWR circle.
What happens to Class-B and Class-E
power output at 2:1 VSWR?
Power Output, Watts
Power Output vs. 2:1 VSWR Angle
10
9
8
7
6
5
4
3
2
1
0
Class-E
Class-B
0
100
200
VSWR Angle
300
400
Transistor Dissipation vs. VSWR
Transistor Dissipation, Watts
Transistor Dissipation vs. 2:1 VSWR Phase Angle
6
5
4
Class-E
Class-B
3
2
1
0
0
100
200
VSWR Angle
300
400
Efficiency vs. VSWR
Efficiency, %
Efficiency vs. 2:1 VSWR Phase Angle
100
90
80
70
60
50
40
30
20
10
0
Class-E
Class-B
0
100
200
VSWR Angle
300
400
Peak Drain V vs. VSWR, Class-E
Class-E Peak Drain V vs. 2:1 VSWR Angle
60
Peak Voltage
50
40
30
20
10
0
0
100
200
VSWR Angle
300
400
Class-E Harmonic Performance
Harmonic content at drain of MOSFET
A second harmonic notch is usually required to
provide sufficient attenuation!
Class-E LINEAR Amplifier
 ARRL Homebrew
Challenge


50W 40M linear
amplifier
LOWEST cost design
goal!
Strategies for Low Cost Design:
 Highest cost components in PA are RF
power devices, heat sinks, enclosure.
 Solution: Envelope-Elimination-andRestoration Architecture
 Uses cheap, efficient MOSFETs in Class-E
CW amplifier, cheap, slow BJT in linear
envelope amplifier.
 Minimal heat sink required.
‘Linear’ Amplification by Envelope
Elimination and Restoration
 Subdivide the amplification between the RF
phase and envelope paths to allow most
efficient, cost effective component choices
Component Choices
2N3055, 70%
efficient
2 x IRF520, 95% efficient
 Higher Efficiency permits minimal heat sinking
Heat Sink Detail
 Copper wire soldered directly to transistor
tabs: almost FREE heat sinking.
 Total amplifier cost: $30.
Conclusions  Class-E Power Amplifiers offer a significant
improvement in transmitter efficiency over
other designs.
 This results in reduced heating of the PA
transistor, reduced battery consumption.
 The circuits are simple to design and
construct using a cookbook approach.
 They are an extremely good choice for singleband CW transmitters.
But…
 Class-E circuits do not easily lend themselves
to multi-band operation.
 Their output power is controlled by supply
voltage (not a linear amplifier).
 The low-Q output network requires attention
to the 2nd harmonic.
 Watch the VSWR, especially when using 60
volt MOSFETs!
Class-E Power Amplifiers for QRP
 Questions?