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Lecture # 7 BJT Low Frequency Response Instructor: Dr. Ahmad El-Banna Benha University Faculty of Engineering at Shoubra November 2014 ECE-312 Electronic Circuits (A) © Ahmad El-Banna

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Agenda Low Frequency Analysis- Bode Plot Low Frequency Response – BJT Amplifier with RL Impact of RS on the BJT Low Frequency Response 2 © Ahmad El-Banna ECE-312 , Lec#7 , Nov 2014

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LOW FREQUENCY ANALYSIS- BODE PLOT 3 © Ahmad El-Banna ECE-312 , Lec#7 , Nov 2014

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Defining the Low Cutoff Frequency 4 ECE-312 , Lec#7 , Nov 2014 © Ahmad El-Banna • In the low-frequency region of the single-stage BJT or FET amplifier, it is the RC combinations formed by the network capacitors CC , CE , and Cs and the network resistive parameters that determine the cutoff frequencies • Voltage-Divider Bias Config.

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Defining The Low Cutoff Frequency .. 5 ECE-312 , Lec#7 , Nov 2014 © Ahmad El-Banna

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Bode Plot 6 ECE-312 , Lec#7 , Nov 2014 © Ahmad El-Banna • The piecewise linear plot of the asymptotes and associated breakpoints is called a Bode plot of the magnitude versus frequency.

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Bode Plot.. 7 ECE-312 , Lec#7 , Nov 2014 © Ahmad El-Banna • A change in frequency by a factor of two, equivalent to one octave, results in a 6-dB change in the ratio, as shown by the change in gain from fL /2 to fL . • For a 10:1 change in frequency, equivalent to one decade, there is a 20-dB change in the ratio, as demonstrated between the frequencies of fL /10 and fL . • Phase Angle:

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Example 8 ECE-312 , Lec#7 , Nov 2014 © Ahmad El-Banna

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LOW FREQUENCY RESPONSE – BJT AMPLIFIER WITH R L 9 © Ahmad El-Banna ECE-312 , Lec#7 , Nov 2014

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Loaded BJT Amplifier 10 ECE-312 , Lec#7 , Nov 2014 © Ahmad El-Banna Cs: In the voltage-divider ct.  the capacitors Cs, CC , and CE will determine the low-frequency response.  Cc:  CE : fL = min(fLs , fLc , fLE )

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IMPACT OF R S ON THE BJT LOW FREQUENCY RESPONSE 11 © Ahmad El-Banna ECE-312 , Lec#7 , Nov 2014

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Impact of R S 12 ECE-312 , Lec#7 , Nov 2014 © Ahmad El-Banna

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Example 13 ECE-312 , Lec#7 , Nov 2014 © Ahmad El-Banna

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• For more details, refer to: • Chapter 9 at R. Boylestad, Electronic Devices and Circuit Theory, 11th edition, Prentice Hall. • The lecture is available online at: • http://bu.edu.eg/staff/ahmad.elbanna-courses/11966 • For inquires, send to: • ahmad.elbanna@feng.bu.edu.eg 14 ECE-312 , Lec#6 , Nov 2014 © Ahmad El-Banna