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Lecture # 7 BJT Low Frequency Response
Instructor: Dr. Ahmad El-Banna
Benha University Faculty of Engineering at Shoubra
No
vem
ber
2014
ECE-312 Electronic Circuits (A)
© A
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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
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LOW FREQUENCY ANALYSIS- BODE PLOT
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Defining the Low Cutoff Frequency
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• 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.
Defining The Low Cutoff Frequency ..
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Bode Plot
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• The piecewise linear plot of the asymptotes and associated breakpoints is called a Bode plot of the magnitude versus frequency.
Bode Plot..
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• 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:
Example
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LOW FREQUENCY RESPONSE – BJT AMPLIFIER WITH RL
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Loaded BJT Amplifier
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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)
IMPACT OF RS ON THE BJT LOW FREQUENCY RESPONSE
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Impact of RS
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Example
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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
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