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Lecture 13

# Lecture 13 - Lecture 13 Frequency Response General...

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1 Lecture 13 – Slide 1 EE105, Fall 2013 Prof. Ming C. Wu Lecture 13 § Frequency Response General considerations High-frequency BJT model Miller s Theorem Frequency response of CE stage § Reading: Chapter 11.1-11.3 Lecture 13 – Slide 2 EE105, Fall 2013 Prof. Ming C. Wu Review: Sinusoidal Analysis § Any voltage or current in a linear circuit with a sinusoidal source is a sinusoid of the same frequency We only need to keep track of the amplitude and phase, when determining the response of a linear circuit to a sinusoidal source. § Any time-varying signal can be expressed as a sum of sinusoids of various frequencies (and phases) § à Applying the principle of superposition: The current or voltage response in a linear circuit due to a time- varying input signal can be calculated as the sum of the sinusoidal responses for each sinusoidal component of the input signal.

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2 Lecture 13 – Slide 3 EE105, Fall 2013 Prof. Ming C. Wu High Frequency Roll-­‐Off in A v § Typically, an amplifier is designed to work over a limited range of frequencies. At high frequencies, the gain of an amplifier decreases. Lecture 13 – Slide 4 EE105, Fall 2013 Prof. Ming C. Wu A v Roll-­‐Off due to C L § A capaciDve load ( C L ) causes the gain to decrease at high frequencies. The impedance of C L decreases at high frequencies, so that it shunts some of the output current to ground. Ο Ο Π Ξ Μ Μ Ν Λ = L C m v C j R g A ω 1 ||
3 Lecture 13 – Slide 5 EE105, Fall 2013 Prof. Ming C. Wu Frequency Response of the CE Stage § At low frequency, the capacitor is effecDvely an open circuit, and A v vs. ω is ﬂat. At high frequencies, the impedance of the capacitor decreases and hence the gain decreases. The

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