Chap7part1 - Frequency Response of Amplifiers * * Effects...

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Ch. 7 Frequency Response Pt. 1 1 ECES 352 Winter 2007 Frequency Response of Amplifiers * Effects of capacitances within transistors and in amplifiers * Fifth Edition 1.6, 4.8, 4.9, 5.8, 5.9; Fourth Edition 7.1-7.7 * Build on previous analysis of amplifiers from ECES 351 h Transistor DC biasing h Small signal amplification, i.e. voltage and current gain h Transistor small signal equivalent circuit * Use in Bipolar and FET Transistors Amplifiers and their analysis * Build on previous analysis of single time constant circuits h Review simple RC, LC and RLC circuits h Recall frequency dependent impedances for C and L h Review frequency dependence in transfer functions * Magnitude and phase * Examine origins of frequency dependence in amplifier gain h Identify capacitors and their origins; find the dominant C h Determine equivalent R and determine RC time constant h Use to describe approximately the amplifier’s frequency behavior h Examine effects of other capacitors * GOAL: Use results of analysis to modify circuit design to improve performance.
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Ch. 7 Frequency Response Pt. 1 2 ECES 352 Winter 2007 Analysis of Amplifier Performance DC bias or quiescent point i B i C v BE v CE * Previously analyzed h DC bias point h AC analysis (midband gain ) * Neglected all capacitances in the transistor and circuit * Gain at middle frequencies , i.e. not too high or too low in frequency
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Ch. 7 Frequency Response Pt. 1 3 ECES 352 Winter 2007 Frequency Response of Amplifiers * In reality, all amplifiers have a limited range of frequencies of operation h Called the bandwidth of the amplifier h Falloff at low frequencies * At ~ 100 Hz to a few kHz * Due to coupling capacitors at the input or output, e.g. C C1 or C C2 h Falloff at high frequencies * At ~ 100’s MHz or few GHz * Due to capacitances within the transistors themselves. Equivalent circuit for bipolar transistor 20 logT(ω) ω Midband Gain
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Ch. 7 Frequency Response Pt. 1 4 ECES 352 Winter 2007 Frequency Response of Amplifiers * First approximation describe the amplifier’s high and low frequency responses in terms of that of single time constant (STC) circuits O High frequency falloff – h Like that of a low pass filter * Simple RC equivalent circuit * Shunting capacitor shorts signal at the output at high frequencies h Low frequency falloff h Like that of a high pass filter * Simple RC equivalent circuit * Series capacitor blocks output signal at low frequencies (acts like open circuit) * Amplifier frequency analysis h Determine equivalent R for each C h Compare and find the most important (dominant RC) combination h Find the dominant one (RC) at high frequencies h Find the dominant one (RC) at low frequencies Low-Pass Network High-Pass Network ( 29 = = 0 (open) 0 1 1 ϖ as as short C j sC Z C V i V i
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Ch. 7 Frequency Response Pt. 1 5 ECES 352 Winter 2007 High Frequency Response of Low Pass RC Filter o C C s sRC R sC sC R Z Z s T ϖ / 1 1 1 1 / 1 / 1 ) ( + = + = + = + = RC 1 0 = o j T / 1 1 ) ( + = Magnitude Phase Shift Log[ω/ω o ] Transfer Function T(s) = V o /V i Log[ω/ω o ] ω = ω o ω = ω o V i
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Ch. 7 Frequency Response Pt. 1 6 ECES 352 Winter 2007 Review of
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This note was uploaded on 04/28/2010 for the course ECECS 352 taught by Professor Cahay during the Spring '10 term at University of Cincinnati.

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Chap7part1 - Frequency Response of Amplifiers * * Effects...

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