Chapter_7_Kim_F.pdf

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Chapter 7 Frequency Response 1
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Objective Discuss the general frequency response characteristics of amplifiers of amplifiers. Derive the system transfer functions Develop the Bode diagrams of the magnitude and phase of the transfer functions. Analyze the frequency response of transistor circuits with capacitors with capacitors. Determine the Miller effect and Miller capacitance. Determine the high-frequency response of basic Determine the high frequency response of basic transistor circuit configurations. 2
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7.1 Amplifier Frequency Response So far, it is assumed in AC circuit analysis that: analysis that: (1) The coupling and the bypass capacitor are short (2) The parasitic load and transistor capacitance are open. In reality, gain factor including voltage, current, transconductance are a function of frequency. We need to investigate the effects f it i ll f 3 of capacitors in all frequency range.
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7.1 Amplifier Frequency Response Band Width Why this kind of shape? (1) f<f L Coupling and bypass capacitor is not a perfect short. (2) f L <f<f H Coupling and bypass capacitor are short. Load and TR capacitors are open (3) f>f H All are getting short. Ex) Audio : 20Hz to 20Khz
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7.1.1. Equivalent Circuit Depending on frequency range, the equivalent circuit changes. (1) f < f L (Low frequency range) C li d b it t h t - Coupling and bypass capacitor are not short. - Load and TR capacitance is ignored. (2) f L < f < f H (Mid frequency range) Co pling and b pass capacitor are short - Coupling and bypass capacitor are short. - Load and TR capacitors are open (3) f > f H (High frequency range) Coupling and bypass capacitor are short - Coupling and bypass capacitor are short. - Load and TR capacitors are not open, getting short. Assumption: f L << f H 5
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7.2. System Transfer Function -Transfer Function- ) ( ) ( s Y H h(t) or H(s) x(t) y(t) X(s) Y(s) ) ( s X s = ) ( ) ( ) ( s X s H s Y = Name of function Expression Voltage TF C t TF ) ( / ) ( s Vi s Vo ) ( / ) ( s Ii s Io Current TF Transresistance TF T d TF ) ( / ) ( s Ii s Vo ) ( / ) ( 6 Transconductance TF s Vi s Io
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