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/day50 kW RF @ 90% efficiency = $133/daySAVINGS = 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?

SGC Mini-Lini,

500W ‘linear’

4SQRP NS-40,

5W, 40M

CW Transmitter

Broadcast Electronics

1 – 5 KW

AM BC Transmitter

WA1QIX 400W 75M Class E Amp

www.classeradio.com

What devices are good for Class-E QRP transmitters?

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

MOSFETs can operate as near-perfect switching devices.

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 90-degree 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.jsp

TINA-TIhttp://focus.ti.com/docs/toolsw/folders/print/tina-ti.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 vs. 2:1 VSWR Angle

0123456789

10

0 100 200 300 400

VSWR Angle

Po

wer

Ou

tpu

t, W

atts

Class-E

Class-B

Transistor Dissipation vs. VSWR

Transistor Dissipation vs. 2:1 VSWR Phase Angle

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1

2

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5

6

0 100 200 300 400

VSWR Angle

Tra

nsi

sto

r D

issi

pat

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, W

atts

Class-E

Class-B

Efficiency vs. VSWR

Efficiency vs. 2:1 VSWR Phase Angle

0102030405060708090

100

0 100 200 300 400

VSWR Angle

Eff

icie

ncy

, %

Class-EClass-B

Peak Drain V vs. VSWR, Class-E

Class-E Peak Drain V vs. 2:1 VSWR Angle

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60

0 100 200 300 400

VSWR Angle

Pe

ak

Vo

lta

ge

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-and-Restoration 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

2 x IRF520, 95% efficient

2N3055, 70% 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 single-band 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?