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MIT6_012S09_lec21 - Lecture 21 Frequency Response of...

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Lecture 21 Frequency Response of Amplifiers (I) Common-Emitter Amplifier Outline Review frequency domain analysis BJT and MOSFET models for frequency response Frequency Response of Intrinsic Common-Emitter Amplifier Effect of transistor parameters on f T 6.012 Spring 2009 1 Reading Assignment: Howe and Sodini, Chapter 10, Sections 10.1-10.4
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I. Frequency Response Review Phasor Analysis of the Low-Pass Filter Example: C R V in V out + + Replacing the capacitor by its impedance, 1 / (j ω C), we can solve for the ratio of the phasors Phasor notation V out V in = 1 1 + j ω RC V out V in = 1/ j ω C R + 1/ j ω C V out V out V in 6.012 Spring 2009 2
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Magnitude Plot of LPF V out / V in --> 1 for “low” frequencies --> 0 for “high” frequencies V out / V in V out V in Break point log scale 0 1 3dB 0.1 20 1 = 0.707 2 40 0.01 1 dB 20 decade 0.001 60 V out V in dB± scale The “break point” is when the frequency is equal to ω o = 1 / RC The break frequency defines “low” and “high” frequencies.
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