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RF Module Design - [Chapter 5] Low Noise Amplifier


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Low Noise Amplifier

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RF Module Design - [Chapter 5] Low Noise Amplifier

  1. 1. RF Transceiver Module Design Chapter 5 Low Noise Amplifier 李健榮 助理教授 Department of Electronic Engineering National Taipei University of Technology
  2. 2. Outline • Basic Amplifier Configurations • Cascode Low Noise Amplifier (LNA) • Feedback Topologies • Classical Two-port Noise Theory • Input Matching for an LNA • Noise Figure and Bias Current • Effect of the Cascode on Noise Figure • Summary Department of Electronic Engineering, NTUT2/26
  3. 3. Simple Transistor Amplifier (I) • Common-emitter (CE) configuration • Common-base (CB) configuration • Common-collector (CC) configuration CE (driver) CCV inV outV EEV CB (cascode) CCV inV outV EEV Department of Electronic Engineering, NTUT3/26 CC (buffer) CCV inV outV EEV
  4. 4. • Bipolar Transistor Amplifier • MOSFET Transistor Amplifier Simple Transistor Amplifier (II) CE CB CC Current Gain High (β) Low (~1) High (1+β) Voltage Gain High High Low (~1) Power Gain High Medium High Zin Medium Low High Zout Medium High Low I/O Phasing 180o In-phase In-phase CS CG CD Voltage Gain High High Low (~1) Power Gain High Medium High Zin High Low High Zout High High Low I/O Phasing 180o In-phase In-phase Department of Electronic Engineering, NTUT4/26
  5. 5. Common-Emitter Configuration • Gain • Input Impedance o L vo m L i b e v r Z A g Z v r r r π π = = − + ≃ er : B-E diode resistance as seen from emitter er rπ β= 1m eg r= in bZ r rπ= + For low frequencies, the parasitic capacitances have been ignored and rb has been assume to be low compared to .rπ CE (driver) CCV inV outV EEV LZormg vπrπCπ br iv ov Cµ vπ + − + − + − and Department of Electronic Engineering, NTUT5/26
  6. 6. Miller Effect (I) • Impedance that connects from input to output fZ LZ inv outv inZ outZ vA ( ) 1 fin in in out f v Zv Z v v Z A = = − − ( ) ( ) for 1 1 1 fout out f v out in f v Zv Z Z A v v Z A = = >> − − ≃ fC LZ inV outV inZ outZ vA ( ) 1 1 1 1 f in v f v sC Z A sC A = = + + ( ) ( ) 1 1 1 1 1 1 f out v f v sC Z A sC A = = +  +  Like larger cap Slightly larger Department of Electronic Engineering, NTUT6/26
  7. 7. Miller Effect (II) • At radio frequencies: • Miller’s theorem Cπ : Low impedance Cµ : Provides feedback ( )1 1o A m L m L v C C C g Z C g Z v µ µ µ π   = − = +    ≃ 1 1 1B o m L v C C C C v g Z π µ µ µ     = − = +        ≃ The dominant pole is usually the one formed by andAC Cπ ( ) ( )1 1 2 || p b s A f r r R C Cπ ππ = + +   sR : source resistance Note that as ZL decreases, CA is reduced and the dominant pole frequency is increased. Cµ vπ ov vπ ov AC BC Department of Electronic Engineering, NTUT7/26
  8. 8. Simplified CE Small-signal Model • Simplified model for transistor above the dominant pole: Ignore and just use in transistor model with little error. • Knowing the pole frequency, we can estimate the gain at higher frequencies, assuming that there are no other poles present, with ( ) 1 1 vo v p A A f f j f = + 目前無法顯示此圖像。 rπ br iv vπ Cπ Cµ mg vπ or LZ ovsv sR + − + − + − + − Department of Electronic Engineering, NTUT8/26
  9. 9. Common-Base Configuration • CB amplifier is often combined with the CE amplifier to from an LNA but it can be used by itself as well. • Low Zin when driven from a current source, it can pass current through it with near unity gain up to very high frequency. Therefore, with an appropriate choice of impedance levels, it can also provide voltage gain. ini br vπ Cµ Cπ mg vπrπ LZ outi + − Ignoring output impedance Department of Electronic Engineering, NTUT9/26
  10. 10. Cascode LNA (I) • CB + CE to form a cascode LNA. • Since the CB amplifier has a current gain of approximately 1, then, ic1 ≈ ic2 = gm1vi . • The gain is the same as for the CE amplifier. However, the cascode transistor reduces the feedback of , resulting in increased high- frequency gain. 1Cµ CR CCV CbiasV outv inv EEV Driver Q1 Cascode Q2 2ci 1ci ( ) ( )1 1 1 1 2 || 2 p b s f r r R C Cπ π µπ = + +   21 mg≈ Department of Electronic Engineering, NTUT10/26
  11. 11. Cascode LNA (II) • Advantages: Improves frequency response. Adding another transistor improves the isolation. • Disadvantages: Additional poles can become a problem for a large load resistance. An additional bias voltage is required, and if this cascode bias node is not properly decoupled, instability can occur. Reduce signal swing at a given supply voltage, compared to the simple CE amplifier. Department of Electronic Engineering, NTUT11/26
  12. 12. Common-Collector Configuration (I) • The CC amplifier (emitter follower) is a very useful general- purpose amplifier. • Voltage gain is close to 1 (buffer). • High input impedance and low output impedance (good buffer/output stage). ER CCV iv EEV ov iv B s bR R r= + vπ rπ Cπ ER mg vπ Cµ + − Department of Electronic Engineering, NTUT12/26
  13. 13. Common-Collector Configuration (II) • Miller effect is not a problem, since the collector is grounded. • Since is typically much less than , it can be left out of the analysis with little impact on the gain. • The input impedance: • The output impedance: CµCπ iv B s bR R r= + vπ rπ Cπ ER mg vπ Cµ + − ( )1A E mZ Z R g Zπ π= + + 1 1 B B out e m m r R sC r R Z r g r sC r g π π π π π π + + = ≈ ≈ + + Department of Electronic Engineering, NTUT13/26
  14. 14. CE with Series Feedback (I) • CE with Series Feedback (Emitter Degeneration) Cascode: Higher frequencies, superior reverse isolation, but suffers from reduced linearity. Most CE and cascode LNAs: Employing the degeneration transforms the impedance real part looking into the base to a higher impedance for matching. De-generation also trades gain for linearity. outRF CCV 1L 1C LR inRF 1Q eL CE tuned LNA CCV 1L 1C LR 2Q 1Q eL inRF biasV outRF Cascode tuned LNA Department of Electronic Engineering, NTUT14/26
  15. 15. CE with Series Feedback (II) • As the degeneration becomes larger, the gain becomes solely dependent on the ratio of the two impedances. • If ZE is inductive, then it will become a real resistance when reflected to base (raise Zin, useful for matching purposes). • Conversely, if ZE is capacitive, it will tend to reduce Zin and can even make it negative. 1 out m L L in EE m E v g R R v ZZ g Z Zπ − = ≈ −   + +    sR rπ Cπ mg vπ EL EREC xiinv vπ Zπ EZ + − ( )1in E mZ Z Z g Zπ π= + + Department of Electronic Engineering, NTUT15/26
  16. 16. CE with Shunt Feedback (I) • Matching over a broad bandwidth while having minimal impact on the noise figure. • Rf forms the feedback and Cf allow for independent biasing. • Can be modified to become a cascode amplifier. • Ignoring the Miller effect and assuming Cf is a short circuit (1/ωCf << Rf ), the gain is given by 1 1 L m L o m LF v L Li f f R g R v g RR A R Rv R R − − = = ≈ + + The gain without feedback (−gmRL) is reduced by the presence of feedback. sR fC fR LR ov sv inZ outZ Department of Electronic Engineering, NTUT16/26
  17. 17. CE with Shunt Feedback (II) • Input impedance The last term, which is usually dominant, shows that the input impedance is equal to Rf +RL divided by the open loop gain. Input impedance for the shunt feedback amplifier has less variation over frequency and process than open-loop amplifier. • Output impedance • Feedback results in the reduction of the role the transistor plays in determining the gain and therefore improves linearity, but the presence of Rf may degrade the noise depending on the choice of value for this resistor. ( ) ( ) || || 1 f L f L f L in f f L m L m L m L Z R R R R R R Z R Z R R Z g R g R g R π π π + + + = ≈ ≈ + + + ( ) ( )1 || ||1 1 || || f f out m s f s f m f R R Z g R R Z R R Z g R π π = ≈   + + −     Department of Electronic Engineering, NTUT17/26
  18. 18. Example 2.5 pF 2 V 3 V LR sR fR 12-GHz fT transistors currents about 5 mA ov sv Input matching Sample plots using shunt feedback 22 20 18 16 14 12 10 100 300 500 700 900 1100 1300 1500 Gain Noise figure OIP3 IIP3 2 0 2− 4− 6− 8− 10− 3 2.5 2 1.5 1.0 0.5 0 IIP3 (dBm) NF (dB) Rf Gain(dB),OIP3(dBm) Department of Electronic Engineering, NTUT18/26
  19. 19. CE w/ Shunt Feedback and CC Output Buffer • CE with an output tends to make for a better match. • With an output buffer, the voltage gain is no longer affected by the feedback, so it is approximately that of a CE amplifier given by [RL /(RE + 1/gm )] minus the loss in the buffer. fC fR LR CCV CCV biasI ER inV outVC Department of Electronic Engineering, NTUT19/26
  20. 20. Classical Two-port Noise Theory (I) • Use these equivalences, the expression for noise factor can be written purely in terms of impedances and admittances: Noisy Two-portsYsi sYsi ne ni Noiseless Two-port 22 2 s n s n s i i Y e F i + + = n c ui i i= + c c ni Y e= ( ) 2 22 2 2 2 2 1 s u c s n u c s n s s i i Y Y e i Y Y e F i i + + + + + = = + where 2 4 n n e R kTB ≡ 2 4 u u i G kTB ≡ 2 4 s s i G kTB ≡ ( ) ( ) 2 2 2 1 1 u c s c s nu c s n s s G G G B B RG Y Y R F G G  + + + ++ +  = + = + , ,and Department of Electronic Engineering, NTUT20/26
  21. 21. Classical Two-port Noise Theory (II) • Optimum source admittance: s c optB B B= − = 2u s c opt n G G G G R = + =and 2 min 1 2 1 2 u n opt c n c c n G F R G G R G G R    = + + = + + +     ( ) ( ) 2 2 min n s opt s opt s R F F G G B B G  = + − + −   GA circles NF circles Input matching Output matching Amplifier sΓ LΓ 0Z 0Z inΓ outΓ outZinZ Department of Electronic Engineering, NTUT Min. noise figure, min ,, s optNF Γ Max. available power gain, s in ∗ Γ = Γ 21/26
  22. 22. Input Matching of LNAs for Low Noise • Many methods for matching the input using passive circuit elements are with varying bandwidth and complexity. • Use two inductors to provide the power and noise match for the LNA, the input impedance is (assume Miller effect is not important and that rπ is not significant at the frequency of interest) • To be matched: , therefore If Miller effect is considered, the capacitance will be larger than Cπ , and a larger inductor will be required to perform the match. Also, the imaginary part of the input impedance must equal zero. Therefore, inRF bL 1Q eLC m e s g L R Cπ = s e m R C L g π = 2 1 s b m R C L C g π πω = − m e in e b g Lj Z j L j L C Cπ π ω ω ω − = + + + Department of Electronic Engineering, NTUT22/26
  23. 23. NF and Bias Current (I) • Noise due to the base resistance is in series with the input voltage, so it sees the full amplifier gain. The output noise due to base resistance is given by Note that this noise voltage is proportional to the collector current, as is the signal, so the SNR is independent of bias current. • Collector shot noise is in parallel with collector signal current and is directly sent to the output load resistor: Note that this output voltage is proportional to the square root of the collector current, and therefore, to improve the noise figure due to collector shot noise, we increase the current. , 14bno r b m Lv kTr g R≈ ⋅ , 2Cno I C Lv qI R≈ ⋅ Department of Electronic Engineering, NTUT23/26
  24. 24. NF and Bias Current (II) • Base shot noise can be converted to input voltage. If Zeq is the impedance on the base (formed by a combination of matching, base resistance, source resistance, and transistor input impedance), then Note that this output voltage is proportional to the collector current raised to the power of 3/2. Therefore, to improve the noise figure due to base shot noise, we decrease the current. • At low currents, collector shot noise will dominate and noise figure will improve with increasing current. However, the effect of base shot noise also increases and will eventually dominate. Thus, there will be some optimum level to which the collector current can be increased, beyond which the noise figure will start to degrade again. , 2 B C no I eq m L qI v Z g R β ≈ ⋅ Department of Electronic Engineering, NTUT24/26
  25. 25. Effect of the Cascode on NF • The cascode transistor is a CB amplifier with current gain close to 1. The cascode transistor is forced to pass the current of the driver on to the output. This includes signal and noise current. Thus, to a first order, the cascode can have no effect on the noise figure of the amplifier. In reality it will add some noise, the cascode LNA can never be as low noise as a CE amplifier. CCV EEV 1br iv 1cv 2ei 2ci outv 2br CR cbiasv Department of Electronic Engineering, NTUT25/26
  26. 26. Summary • For three transistor amplifier configurations, the CE amplifier has higher gain but poor frequency response than CB and CC amplifiers due to miller effects. • Cascode configuration of CE and CB has the advantages of improving frequency response and a little impact on noise figure. • Feedback topologies are usually used to improve linearity with sacrificing some power gain and noise performance. • Using two inductors (one at emitter and the other at base) to provide the power and noise match is a common and convenient matching strategy for the LNA design. Department of Electronic Engineering, NTUT26/26