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Unformatted text preview: 1 LowNoise Amplifier 2 RF Receiver BPF1 BPF2 LNA LO Mixer BPF3 IF Amp Demodulator Antenna RF front end 3 LowNoise Amplifier First gain stage in receiver Amplify weak signal Significant impact on noise performance Dominate inputreferred noise of front end Impedance matching Efficient power transfer Better noise performance Stable circuit LNA subsequent LNA frontend G NF NF NF 1 + = 4 LNA Design Consideration Noise performance Power transfer Impedance matching Power consumption Bandwidth Stability Linearity 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 + = 6 NF of Cascaded Stages Overall NF dominated by NF 1 [1] F. Friis, Noise Figure of Radio Receivers, Proc. IRE , Vol. 32, pp.419422, 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 7 Simple Model of Noise in MOSFET f WLC k f V ox g = ) ( 2 Flicker noise Dominant at low frequency Thermal noise : empirical constant 2/3 for long channel much larger for short channel PMOS has less thermal noise Inputinferred noise m d g kT f I 4 ) ( 2 = V g I d V i f WLC k g kT f V ox m i + = 4 ) ( 2 8 Noise Approximation Thermal noise 1/f noise Band of interest Frequency Noise spectral density Thermal noise dominant 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 * , 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 10 Available Power Equal power on load and source resistors 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 + = = * 2 ) ( * * L L del Z Z I I P + = L IZ V = 12 Reflection Coefficient No reflection Maximum power transfer 13 SParameters Parameters for twoport system analysis Suitable for distributive elements Inputs and outputs expressed in powers Transmission coefficients Reflection coefficients 14 SParameters 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 15 SParameters 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 1 2 2 22 1 2 1 12 2 1 2 21 2 1 1 11 = = = = = = = = a a a a a b S a b S a b S a b S 16 SParameters S Z 1 Z 2 V s1 V s2 I 1 V 1 I 2 V 2 2 2 2 * 2 2 2 22 1 2 2 2 2 * 1 1 1 12 2 1 1 1 1 * 2 2 2 21 1 1 1...
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This document was uploaded on 02/01/2012.
 Spring '09
 Amplifier

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