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Class e power amplifiers for qrp2 qro


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Class e power amplifiers for qrp2 qro

  1. 1. Class-E Power Amplifiers for QRP to QRO <ul><li>David Cripe NMØS </li></ul><ul><li>Mount Vernon IA </li></ul><ul><li>August 6, 2011 </li></ul>
  2. 2. QRP is all about efficiency… <ul><li>QRP rigs can be small and simple… </li></ul>… transmitting the least amount of power necessary for a QSO… … so why waste power with an inefficient PA?
  3. 3. 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.
  4. 4. Why is PA efficiency important? <ul><li>Typical CW ham transmitter has power amplifier efficiency of 50%. </li></ul><ul><ul><li>A transmitter delivering 5W RF, at 50% efficiency consumes 10W battery power. </li></ul></ul><ul><ul><li>Its PA transistor must be capable of dissipating 5W power </li></ul></ul>
  5. 5. Why is PA efficiency important? <ul><li>What would happen if PA efficiency of a 5W transmitter was increased to 90%? </li></ul><ul><ul><li>Power consumption from battery is reduced from 10 watts to 5.5 watts </li></ul></ul><ul><ul><li>Power dissipation in the transistor is reduced to 0.5 watts, a 90% reduction. </li></ul></ul>
  6. 6. Why is PA efficiency important? <ul><li>A higher efficiency PA will result in: </li></ul><ul><ul><li>Smaller, cheaper transistor required </li></ul></ul><ul><ul><li>Cooler operation of PA </li></ul></ul><ul><ul><li>Higher reliability of PA </li></ul></ul><ul><ul><li>Reduced battery consumption. </li></ul></ul><ul><ul><li>Class-E is a simple, rugged, highly-efficient power amplifier circuit capable of operating at 90% efficiency. </li></ul></ul>
  7. 7. What is Class-E? <ul><li>The Class-E Power Amplifier was invented by Nat and Alan Sokal in the 1970s. </li></ul><ul><li>It is uses the power device as a switch, and is capable of DC-to-RF efficiency nearing 100%. </li></ul><ul><li>It uses a low-Q tuned drain network to obtain specially-shaped voltage and current waveforms that minimize transistor losses. </li></ul>
  8. 8. Class-E Waveforms * * US3919656
  9. 9. Definition of Class-E Waveforms <ul><li>The active device is operated as a switch with 180 degree conduction per cycle. </li></ul><ul><li>The drain network is tuned so that during the transistor ‘off’ period, its voltage returns to zero just before the beginning of switch conduction </li></ul><ul><li>The slope of the voltage waveform is zero just before the beginning of transistor conduction </li></ul>
  10. 10. Single-Ended Class-E Circuit
  11. 11. 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
  12. 12. WA1QIX 400W 75M Class E Amp
  13. 13. What devices are good for Class-E QRP transmitters? <ul><li>2N7000, 60v, 2 ohm R ds , 20pF C oss </li></ul><ul><ul><li>1 W output </li></ul></ul><ul><li>ZVN4210A, 100v, 1.8 ohm R ds , 40 pF C oss </li></ul><ul><ul><li>1 W output </li></ul></ul><ul><li>IRF510, 100v, 0.5 ohm R ds , 81pF C oss </li></ul><ul><ul><li>>5 W output </li></ul></ul><ul><li>MOSFETs can operate as near-perfect switching devices. </li></ul>
  14. 14. How is a Class-E PA designed? <ul><li>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. </li></ul><ul><li>A ‘cookbook’ set of equations can be used to determine the design of the Class-E PA for a given power, voltage and frequency. </li></ul><ul><li>Equations found at WAØ </li></ul>
  15. 15. Designing a Class-E PA A simple prototype circuit will suffice for most QRP applications.
  16. 16. Class-E Design Procedure <ul><li>The frequency F, supply voltage B, and output power P are selected. </li></ul><ul><li>Based on the output power, a MOSFET is chosen. </li></ul><ul><li>The circuit load resistance is calculated: </li></ul><ul><li>R = 0.28 · B 2 / P – 1.5 · R ds </li></ul>
  17. 17. Class-E Design Procedure <ul><li>The MOSFET shunt capacitor C 1 is calculated: </li></ul><ul><li> C 1 = 0.18 / ( 2  · F · R ) - C oss </li></ul>
  18. 18. Class-E Design Procedure <ul><li>The series network L 2 -C 2 is determined next </li></ul><ul><li>The capacitor C 2 is selected to have one to two times the reactance of the load, R. A common standard value is best. </li></ul><ul><li>L 2 is calculated: </li></ul><ul><li> L 2 = [ 1.8 · R + 1 / ( 2  · F · C 2 )] / ( 2  · F ) </li></ul>
  19. 19. Class-E Design Procedure <ul><li>The load impedance of the PA must be transformed to 50 ohms. </li></ul><ul><li>A preferred way to achieve this is with a 90-degree PI network. </li></ul><ul><li>A second-harmonic notch is added to the series inductor L 3 . </li></ul>
  20. 20. Class-E Design Procedure <ul><li>PI Network Component Calculations: </li></ul><ul><li>C 3 = C 5 = 1 / (2  · F · √( R · 50 ) ) </li></ul><ul><li>L 3 = 0.75 × √( R · 50 ) / ( 2  · F ) </li></ul><ul><li>C 4 = C 3 / 3 </li></ul>
  21. 21. Class-E Design Procedure <ul><li>Finally the drain choke L 1 is chosen. Its value is not critical, except it must be much larger than L 2 . </li></ul><ul><li>L 1 ≈ 10 · L 2 </li></ul><ul><li>*Equations found at WAØ </li></ul>
  22. 22. Circuit Simulation and Optimization <ul><li>Class-E PAs may be optimized using circuit simulation software. </li></ul><ul><li>CAD freeware is available from: </li></ul><ul><ul><li>LTSPICE IV (SWCAD III) </li></ul></ul><ul><ul><ul><li> </li></ul></ul></ul><ul><ul><li>TINA-TI </li></ul></ul><ul><ul><ul><li> </li></ul></ul></ul>
  23. 23. Analytic Tools – SWCAD III* *
  24. 24. SWCAD III Time-Domain Analysis
  25. 25. Efficiency and Thermal Management <ul><li>The heat loss in the MOSFET will be approximately 2  P  R ds / R. </li></ul><ul><li>A good rule of thumb for MOSFET reliability is to keep the junction temperature below 100 degrees C. </li></ul><ul><li>We can estimate MOSFET junction temperature from thermal resistance data in manufacturers’ data sheets. </li></ul>
  26. 26. Thermal Impedance <ul><li>A TO-92 transistor (2N7000) has 312 degrees C-per-watt thermal resistance. </li></ul><ul><li>Allowable dissipation in a TO-92 part is about ¼ watt. </li></ul><ul><li>A TO-220 transistor (IRF510) has 62 degrees C-per-watt thermal resistance. </li></ul><ul><li>Allowable dissipation in a TO-220 is >1W </li></ul><ul><li>Adding a heat sink to a TO-220 can further increase allowable dissipation. </li></ul>
  27. 27. How is the Class-E PA driven? <ul><li>A MOSFET is a voltage-controlled device. </li></ul><ul><li>The gate of a MOSFET is a relatively large capacitance. </li></ul><ul><li>The MOSFET driver circuit must handle the large currents required to charge and discharge the gate capacitance at the carrier frequency. </li></ul>
  28. 28. Practical MOSFET Drive Circuitry <ul><li>Many MOSFETs are designed to be driven directly from TTL-level signals. </li></ul><ul><li>TTL Drive requires NO transformer or impedance matching. </li></ul><ul><li>One 74HCxx gate can drive a 2N7000 up to 14 MHz, two, paralleled 74HCxx gates can drive an IRF510 up to 7 MHz. </li></ul><ul><li>74ACxx logic has 4x drive capability of 74HCxx. </li></ul>
  29. 29. Practical Drive Circuit <ul><li>Adding 1.5 volts of bias to the TTL drive signal improves MOSFET switching and efficiency. </li></ul>
  30. 30. How do the Class-B and –E PAs compare? <ul><li>SWCAD III simulations of IRF510, 5W Class-E and Class-B PAs were compared in normal operation into a 1:1 VSWR. </li></ul><ul><li>The Class-B PA operated at 71% efficiency, while the Class-E PA operated at 92% efficiency. </li></ul><ul><li>The performance of the Class-B and –E circuits were then compared over eight points on a 2:1 VSWR circle. </li></ul>
  31. 31. What happens to Class-B and Class-E power output at 2:1 VSWR?
  32. 32. Transistor Dissipation vs. VSWR
  33. 33. Efficiency vs. VSWR
  34. 34. Peak Drain V vs. VSWR, Class-E
  35. 35. Class-E Harmonic Performance <ul><li>Harmonic content at drain of MOSFET </li></ul>A second harmonic notch is usually required to provide sufficient attenuation!
  36. 36. Class-E LINEAR Amplifier <ul><li>ARRL Homebrew Challenge </li></ul><ul><ul><li>50W 40M linear amplifier </li></ul></ul><ul><ul><li>LOWEST cost design goal! </li></ul></ul>
  37. 37. Strategies for Low Cost Design: <ul><li>Highest cost components in PA are RF power devices, heat sinks, enclosure. </li></ul><ul><li>Solution: Envelope-Elimination-and-Restoration Architecture </li></ul><ul><li>Uses cheap, efficient MOSFETs in Class-E CW amplifier, cheap, slow BJT in linear envelope amplifier. </li></ul><ul><li>Minimal heat sink required. </li></ul>
  38. 38. ‘ Linear’ Amplification by Envelope Elimination and Restoration <ul><li>Subdivide the amplification between the RF phase and envelope paths to allow most efficient, cost effective component choices </li></ul>
  39. 39. Component Choices <ul><li>2 x IRF520, 95% efficient </li></ul>2N3055, 70% efficient <ul><li>Higher Efficiency permits minimal heat sinking </li></ul>
  40. 40. Heat Sink Detail <ul><li>Copper wire soldered directly to transistor tabs: almost FREE heat sinking. </li></ul><ul><li>Total amplifier cost: $30. </li></ul>
  41. 41. Conclusions - <ul><li>Class-E Power Amplifiers offer a significant improvement in transmitter efficiency over other designs. </li></ul><ul><li>This results in reduced heating of the PA transistor, reduced battery consumption. </li></ul><ul><li>The circuits are simple to design and construct using a cookbook approach. </li></ul><ul><li>They are an extremely good choice for single-band CW transmitters. </li></ul>
  42. 42. But… <ul><li>Class-E circuits do not easily lend themselves to multi-band operation. </li></ul><ul><li>Their output power is controlled by supply voltage (not a linear amplifier). </li></ul><ul><li>The low-Q output network requires attention to the 2 nd harmonic. </li></ul><ul><li>Watch the VSWR, especially when using 60 volt MOSFETs! </li></ul>
  43. 43. Class-E Power Amplifiers for QRP <ul><li>Questions? </li></ul>