BJT Frequency Response

LF Response, Miller Theorem & HF Response

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FR; Outline

- Introduction - Decibels - Bode Plot (Frequency Response) - Low Frequency Analysis - Miller Theorem - High Frequency Analysis

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Introduction

- Previously, effects of capacitive elements are ignored

- Capacitor affect the LF & HF of amplifiers - Swept Frequencies are used to test - Logarithmic scales are both used in gain & frequency axes

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Logarithm (I)

- Logarithm

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Logarithm (II)

- Some useful logarithm

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Logarithm (III)

- Semilog scale

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Important points in log scale

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Decibels

- Bel is the surname of Alexander Graham Bell

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Advantage of Logarithm

Gain calculation made easier

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Example of gain calculation

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General Frequency Considerations

RC coupling

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Cut-Off or Half-Power Frequency

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Bandwidth, BW

Linear

Log

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Phase plot

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Low Frequency Analysis

@ High Frequency

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Voltage Divider

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Gain Equation

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Bode Plot of Low Frequency Region

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Example of LF Bode Plot

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Phase response of LF region

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Miller Effect Capacitance (I)

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Miller Effect Capacitance (II)

CMo ~ Cf ; AV >>1

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High Frequency Analysis

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Parasitic Capacitances

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HF Model of BJT Amplifier

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Parameter Variation

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Full Frequency Response

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Square-wave Testing

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Results of Square-wave Tesing

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Rise Time & Tilt

or

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Where fHi is high cut off & fLo is low cut off frequencies. The fs is the frequency of square wave signal

Rise Time & Tilt; example

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Memo

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Free Logarithmic Graph Paper from http://incompetech.com/graphpaper/logarithmic/

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Free Logarithmic Graph Paper from http://incompetech.com/graphpaper/logarithmic/

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