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Analogue Electronic Circuits

Analogue Electronic Circuits - 2 2.1 FeedbackAmplifiers...

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2 Feedback Amplifiers 2.1 Feedback Concept S i S ε S o S f b L o a d S o u r c e Σ Α β + - o fb S S = β   feedback transfer function (feedback factor) ε S S A o =   amplification factor (open loop gain) KCL :  fb i S S S - = ε A A S S AS S S A fb i o f β ε ε + = + = = 1   closed loop transfer function Where  β A  is commonly known as the loop gain
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2.1.1 Gain Stability A A A f β + = 1 For A >> 1,   β β 1 = A A A f 2.1.2 Gain Sensitivity A A A f β + = 1 ( 29 ( 29 2 2 1 1 1 1 1 A A A A dA dA f β β β β + = + - + = ( 29 A dA A A dA A dA dA f f f β β + = + = 1 1 1 2
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2.1.3 Bandwidth Extension An amplifier with high frequency response characterised by  a single pole can be described by, + = H M s A s A ϖ 1 ) ( where A M  – mid-band gain ϖ H  – upper -3dB frequency with negative feedback, ( 29 M H M M M H M f A s A A A s A s A s A s A β ϖ β β ϖ β + + × + = + + = + = 1 1 1 1 1 ) ( 1 ) ( ) ( ( 29 M H H A β ϖ ϖ + = 1 '   the new upper -3dB frequency Similarly for the low frequency response it can be shown,
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( 29 M L L A β ϖ ϖ + = 1 '   the new lower -3dB frequency
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2.1.4 Non-linear Distortion β D D A D - β Σ Α β + - Open loop distortion  D Closed loop distortion  D’ = D – A β D’ β A D D + = 1 '
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2.1.5 Noise Sensitivity i i i N S
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