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Low-Noise Amplifier

Low-Noise Amplifier - Low-Noise Amplifier 1 RF Receiver...

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1 Low-Noise Amplifier
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2 RF Receiver BPF1 BPF2 LNA LO Mixer BPF3 IF Amp Demodulator Antenna RF front end
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3 Low-Noise Amplifier First gain stage in receiver Amplify weak signal Significant impact on noise performance Dominate input-referred noise of front end Impedance matching Efficient power transfer Better noise performance Stable circuit LNA subsequent LNA frontend G NF NF NF 1 - + =
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4 LNA Design Consideration Noise performance Power transfer Impedance matching Power consumption Bandwidth Stability Linearity
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5 Noise Figure Definition As a function of device G: Power gain of the device out out in in out in N S N S SNR SNR NF = = source source device N G N G N NF + =
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6 NF of Cascaded Stages Overall NF dominated by NF 1 [1] F. Friis, “Noise Figure of Radio Receivers,” Proc. IRE , Vol. 32, pp.419-422, July 1944. S in /N in G 1 , N 1 , NF 1 G i , N i , NF i G K , N K , NF K S out /N out 1 2 1 2 1 3 1 2 1 1 1 1 1 1 - - + + - + - + - = - K K ...G G G NF ... G G NF G NF NF NF
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7 Simple Model of Noise in MOSFET f WLC k f V ox g = ) ( 2 Flicker noise Dominant at low frequency Thermal noise γ : empirical constant 2/3 for long channel much larger for short channel PMOS has less thermal noise Input-inferred noise m d g kT f I γ 4 ) ( 2 = V g I d V i f WLC k g kT f V ox m i + = γ 4 ) ( 2
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8 Noise Approximation Thermal noise 1/f noise Band of interest Frequency Noise spectral density Thermal noise dominant
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9 Power Transfer and Impedance Matching L L L s s s del R jX R jX R V P 2 + + + = s s s X X R R L R V V P P L s L s 4 * 0 , max = = = + = Power delivered to load Maxim available power R s V s jX s jX L R L I V Impedance matching Load and source impedances conjugate pair Real part matched to 50 ohm
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10 Available Power Equal power on load and source resistors
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11 Reflection Coefficient * * * max 4 ) )( ( 4 aa R IZ V IZ V R V V P s s s s s s = + + = = s s R IZ V a 2 + = * * * * max 4 ) )( ( bb R Z I V IZ V P P P s s s del ref = - - = - = R s V s jX s jX L R L I V s s R IZ V b 2 * - = s L s L Z Z Z Z a b + - = = Γ * 2 ) ( * * L L del Z Z I I P + = L IZ V =
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12 Reflection Coefficient No reflection Maximum power transfer
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13 S-Parameters Parameters for two-port system analysis Suitable for distributive elements Inputs and outputs expressed in powers Transmission coefficients Reflection coefficients
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14 S-Parameters 2 22 1 21 2 2 12 1 11 1 a S a S b a S a S b + = + = a 1 b 1 b 2 a 2 S 11 S 12 S 22 S 21
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15 S-Parameters S 11 – input reflection coefficient with the output matched S 21 – forward transmission gain or loss S 12 – reverse transmission or isolation S 22 – output reflection coefficient with the input matched 0 1 2 2 22 0 1 2 1 12 0 2 1 2 21 0 2 1 1 11 = = = = = = = = a a a a a b S a b S a b S a b S
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16 S-Parameters S Z 1 Z 2 V s1 V s2 I 1 V 1 I 2 V 2 0 2 2 2 * 2 2 2 22 0 1 2 2 2 2 * 1 1 1 12 0 2 1 1 1 1 * 2 2 2 21 0 1 1 1 * 1 1 1 11 1 1 2 2 ) Re( ) Re( ) Re( ) Re( = = = = + - = + - = + - = + - = s s s s V V V V Z I V Z I V S Z Z Z I V Z I V S Z Z Z I V Z I V S Z I V Z I V S
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17 Stability Condition Necessary condition where Stable iff where 1 | | 2 | | | | | | 1 21 12 2 2 11 2 22 + - - = S S S S K S 21 12 22 11 S S S S S - = 1 | | 2 + - L L S 2 | | | | | | 2 22 2 11 21 12 S S S S L + + =
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