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  1. 射频工程基础Fundamentals of RF Engineering 学时:60/20 学分: 3.5 孙利国 中国科技大学信息学院电子工程与信息科学系

  2. 第八讲 射频收发系统中的有源器件 (Session 8 RF Active Device) 教材:以课堂讲义为主。 主要参考书: [1]“Microwave and RF Design: A System Approach”, Michael Steer, SciTech Publishing, 2010 其它参考书: [2] “射频电路设计-理论与应用”,Reinhold Ludwig等著,王子宇等译,电子工业出版社,2002 [3] “射频微电子学”,拉扎维著,余志平等译,清华大学出版社,2006 [4] RF and Microwave Circuit Design for Wireless Communications, Lawrence Larson, Artech House, 1997 [5]”无线网络RF工程:硬件、天线和传播“, Daniel M.Dobkin 著 ,科学出版社 ,2007 [6] G. Gonzalez, “ Microwave Transistor Amplifiers: Analysis and Design”, Prentice Hall, 1997

  3. Outline • Transistors • Amplifier • Mixer • RF Switch • Oscillator Reference “Microwave and RF Design: A System Approach”, Chapter 11-12

  4. Outline • Transistors • BJT and HBT • MOSFET • JFET, MESFET and HEMT

  5. Transistor Technology • Main Semiconductor Materials • Si silicon (IV) • Compound Semiconductors (III-V) • GaAs gallium arsenide • GaN gallium nitride • InP indium phosphide • Three main transistor families • BJT and HBT • BJT(Bipolar Junction Transistor) for Silicon • HBT (Hetero-junction Bipolar Transistor) for compound semiconductors • MOSFET • Metal Oxide Semiconductor Field Effect Transistor • Requires very thin insulating layer, Silicon Dioxide, GaN has a nitride which also works (there is no GaAs oxide) • JFET • Silicon Junction Field Effect Transistor • MESFET (Metal Epitaxy Semiconductor) if compound semiconductor • HEMT (High Electron Mobility Transistor ) only compound semiconductor. Uses a layer of semiconductor put into strain.

  6. Transistor Technology • Silicon • Preferred semiconductor • Incredibly well characterized • Best choice for digital circuits as digital circuits can be implemented with the highest density. • Analog benefits from the intense investment for digital • Compound Semiconductors • High mobility is possible (electrons and holes travel faster) • Some compound semiconductors (eg GaN) have very large breakdown voltages • Required for very high power. • In semiconductors a very small amount of impurity (or dopant) dramatically changes the number of free electrons and/or holes than can carry charge. • p-type surplus of holes • n-type surplus of electrons

  7. Bipolar Junction Transistor (BJT) I C C B + v g v r r B E B E m O _ E B C E C u r r e n t F l o w • Name used when junction transistor is built in silicon • Emitter and collector regions • npn indicates • Emitter (E) and collector (C) regions are n type semiconductor • Base is p type • Relies on a very thin base region • Changing the base-emitter voltage has a tremendous effect on number of carriers. • Simple model of effect • Hybrid Pi model • Small signal model

  8. BJT Characteristics I C O LL E C T O R C C + BAS E B V + I C E B V B E E E M I TT E R While the physical process is that of VBE controlling the collector current IC such that IC is related to the exponential of VBE, IB is also related to the exponential of VBE so that in the active region (where VC > VCE,min) IC is approximately linearly proportion to IB, IC = bIB.

  9. BJT Characteristics Operation described by Gummel-Poon Equations A good designer remembers the essential of these equations. Base current: Collector current: I C O LL E C T O R C C + BAS E B Emitter current: V + I C E B V B E E The above are modified by reverse and non-ideal base and collector currents and by the build up of charge in the base. E M I TT E R

  10. Gummel-Poon Model R C NC o ll ec t o r C V I V C C B C B X B C I C I CE N B a s e B B X B C I R C I N B E B E B B X = I R V C O B E g V C V Basic equations are augmented by circuit elements that capture capacitances and resistances. m J S B E C J S E N Sub s t r a t e HBT Hetero Junction Transistor is the type of BJT implemented with compound semiconductors. The Gummel-Poon equations apply and the same model can be used. There are however specific and more complicated models for HBTs. R E I N E m itt e r E

  11. Two Types of BJT PNP E E M I TT E R NPN BAS E B V + I C E + B V I B E C O LL E C T O R C I C + C C BAS E B V + I C E B V B E E E M I TT E R C O LL E C T O R

  12. MOSFET Metal-Oxide-Semiconductor Field Effect Transistor N type MOSFET Body • E field from Gate to Body induces electrons into region • under gate oxide • Electrons for a conducting channel that would not be • there otherwise. IGFET is general name (Insulator Gate FET)

  13. MOSFET Metal-Oxide-Semiconductor Field Effect Transistor The FET works by inducing a conducting channel under the gate when a gate voltage is applied. The field pulls charges into the region that would be intrinsic semiconductor. Normal operating region for amplifiers is the SATURATION REGION. The linear region is sometimes used to advantage, e.g. in a mixer. I D DR A I N D + G A T E G V D S + V G S S N type MOSFET S O URC E

  14. MOSFET Metal-Oxide-Semiconductor Field Effect Transistor Essential characteristic equations Transconductance

  15. MOSFET Enhancement & Depletion The enhancement mode is the mode of operation if the channel region is intrinsic semiconductor. If the semiconductor under the gate is doped a channel will exist even without a gate voltage, this leads to an depletion MOSFET.

  16. MOSFET Symbols • U is the body connection. The body voltage affects operation. • The MOSFET is a very difficult transistor to model and there are many different models. • BJTs are much better microwave transistors, so why are MOSFETs used for RFICs? U

  17. JFET Junction Field Effect Transistor N type JFET The JFET works by a reverse bias junction between the gate and the semi insulating substrate. Varying the voltage on the gate moves the reverse-biased region in which there are no carriers. Thus the cross-section of the conducting channel is varied.

  18. JFET Enhancement & Depletion

  19. JFET Symbols A JFET strictly refers to a silicon JFET. The compound semiconductor version is the MESFET. The HEMT is a more complicated compound semiconductor structure in which the channel is buried .

  20. JFET • The MESFET and HEMT are types of JFETs fabricated using compound semiconductors, with JFET most commonly referring to silicon devices only. • With compound semiconductors such as GaAs, the pn junction of a silicon JFET is replaced by a Schottky barrier junction and the transistor is called a metal-epitaxy-semiconductor FET (MESFET). • A device similar to the MESFET is the high electron mobility transistor (HEMT), where the field is established at the junction of two compound • semiconductor materials with different band gaps (i.e. a heterojunction) which forms the channel instead of a doped region. The HEMT is also called the heterostructure FET (HFET). • A MESFET with a graded junction is called a modulation-doped FET (MODFET). A pseudomorphic HEMT (pHEMT) has an extremely thin layer establishing the channel so that the crystal structure stretches and a very high bandgap is established. • Enhancement- mode and depletion-mode JFETs are contrasted.

  21. Outline • Amplifier Design Strategy • Amplifier Topology • Amplifier Efficiency • Amplifiers: Class A, AB, C • Stability • Gain (Figures of Merit) • Linear Amplifier Design • Amplifier: Noise and Distortion

  22. AmplifiersDesign Strategies

  23. Amplifier Design Strategies A M P L I F I E R M T R A N S I S T OR M 1 2 R F R F I N P U T O U T P U T I N P U T O U T P U T M A T CH I N G M A T CH I N G N E T W O R K N E T W O R K B I AS G A T E - - L O W PAS S L O W PAS S C O N T R O L F I L T E R F I L T E R O R BAS E CH I P N C O LL E C T O R O R DR A I • Basic topology shown above • Sometimes feedback across transistor • Use figures of merit (FOM) • Efficiency • Gain • Stability • Noise • Distortion • Single frequency design • Synthesis for broadband design

  24. Amplifier Design Strategies • Low pass filter • Simplest is an inductor • Matching network design • Single frequency (no problems) • Broadband, just like filter design • Stability • Often addressed through choice of matching network topology • Noise • Generally keep currents low • Efficiency • Addressed through biasing strategy • Distortion • Addressed through biasing strategy • Often bandwidth control

  25. Efficiency An amplifier with 1W RFoutput power KEY Point: Multiple FOMs are useful, different FOMs provided different insight. • Basically the power out divided by the power in. • But several measures • TOTAL efficiency • POWER ADDED EFFICIENCY (most comply used in RF and Microwave design). • Efficiency

  26. AmplifiersClass A, AB, C Amplifiers StabilityGain (Figures of Merit)

  27. Linear Amplifier Biasing V V CC D D I D I R R I I C L L C D R AI N D D C O L L E C T O R C V V BASE B G A T E G I N P U T C I N P U T D O U T P U T O U T P U T I B V E G S EM IT T ER S S O UR C E

  28. Linear Amplifiers I N P U T O U T P U T C L ASS A C L ASS B C L ASS C C L ASS AB

  29. Class B Amplifiers Push-Pull Class B Single-Ended Class B Single-Ended Class B Push-Pull Class B

  30. Class A Amplifier

  31. Class A Amplifier Efficiency Determine the Maximum Efficiency of a Class A Amplifier ID Efficiency Strategy: Determine total DC power consumed, PDC = IDQ VDD IDQ = VDD/(2RL), PDC = V2DD /(2RL ) Determine PRF,out (AC power in RL) VD(peak) = VDD/2 PRF,out = V2D(peak)/(2RL ) = V2DD/(8RL ) h= PRF,out / PDC =1/4= 0.25=25%

  32. Class A Amplifier High Efficiency

  33. Amplifier Stability Z S Z A C T I V E V L D EV I C E S N O I S E N O I S E S I N L OU T S I N OU T L • For stable amplification • |GSGIN| must be less than one at all frequencies • |GLGOUT| must be less than one at all frequencies • For passive source and load |GS| < 1, |GL| < 1 • Thus for unconditional stability require • |GIN| < 1 • |GOUT| < 1

  34. ΓS ΓL Amplifier Stability b1 ′ a1 b2 a2 ′ Pinc PL ZS [S] ZL VS ′ a1 a2 b1 b2 ′ Γin Γout

  35. Γ <1, Γ <1 L S S11-Γ △ L Γ = <1 in 1-S22Γ L S22-Γ △ S Γ = <1 out 1-S11Γ S ΓS ΓL b1 ′ a1 b2 a2 ′ Pinc PL ZS [S] ZL VS ′ a1 a2 b1 b2 ′ Γin Γout Stable condition 其中△=S11S22-S12S21

  36. Γ =1 L output stability circle: From: The equation for circle: Γ =1 in rout Cout Cout Output Stability Circle

  37. Γ =1 out Γ =1 S Input stability circle: From: The equation for circle: rin Cin Cin

  38. Γ =1 Γ =1 L L Γ =1 Γ =1 in in rout rout Cout Cout Stable region Unstable region Cout Cout Unstable region Stable region Green region is stable region Common area is stable region

  39. Γ =1 Γ =1 out out Γ =1 Γ =1 S S rin rin Cin Cin Stable region Unstable region Cin Cin Unstable region Stable region Common area is stable region Green region is stable region

  40. Γ =1 Γ =1 out out Γ =1 Γ =1 S S Cin If radius of stability circle rin ( or rout) is larger than |Cin| (or |Cout|) rin Cin Unstable region Stable region | S22 |<1 Cin Cin |S22 |<1 rin rin< |Cin| Stable region rin> |Cin| ΓS=0 is in stable region Unstable region

  41. Γ =1 Γ =1 in out rin Cin |ΓS| =1 Cin Unconditional stability For unconditional stability, the stability circle should be beyond the unit circle |ΓS| =1 or |ΓL|=1. rout Cout |ΓL| =1 Cout Co

  42. Rollet’s Stability Criterion — k-factor: From: Get: Stability factor K is defined by: Criterion for unconditional stability is :

  43. Γ =1 Γ =1 S out If the unconditional stability is discussed in Γout plane, the area for |ΓS| ≤1 should be within the circle of|Γout|=1. In Γoutplane, |ΓS|=1 is a circle with the center and radius of a circle given by: center: radius: rS Unconditional stability: CS So that: IfΓLis discussed in Γin plane: Two are added: So that: Additional requirement is needed for unconditional stability. So, both |Δ| <1 andk>1 should be satisfied for unconditional stability.

  44. Amplifier Stability Z S Z A C T I V E V L D EV I C E S S I N OU T L Amplifier is unstable if Stable if (as long as source, load is passive) Note: magnitudes of complex numbers describe circles in the complex plane

  45. Amplifier Stability Z S Z A C T I V E V L D EV I C E S S I N OU T L Amplifier is stable if Output stability circles on the GL plane:

  46. Amplifier Stability Input stability circles on the GS plane:

  47. Rollet’s Stability Criterion, k factor • k > 1 for unconditional stability (but not enough to guarantee stability) • Must also satisfy any one of

  48. Amplifier Stability • If an amplifier is unconditionally stable design is considerably simplified. • Even if an amplifier is not “unconditionally stable” it could still be stable. This is tricky design and is only done in special circumstanes. • E.g. very high frequency operation. • Consider a cell phone power amplifier connected to an antenna • Must be stable • When antenna is covered by hand. • Phone is placed on a metal surface. • Severe price if amplifier goes unstable • In a communication system the whole EM spectrum could be polluted. • Amplifier could self-destruct (thermal runaway).

  49. Example: Previous pHEMT Transistor

  50. Example: pHEMT Transistor However: Larger k does not mean more stable!