Lecture #7 BJT and JFET Frequency Response - Bu Shoubra/Electrical... · BJT and JFET Frequency...
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Lecture #7 BJT and JFET Frequency Response Instructor: Dr. Ahmad El-Banna November 2014 J-601-1448 Electronic Principals Integrated Technical Education Cluster At AlAmeeria © Ahmad El-Banna
Transcript of Lecture #7 BJT and JFET Frequency Response - Bu Shoubra/Electrical... · BJT and JFET Frequency...
Lecture #7 BJT and JFET Frequency Response Instructor: Dr. Ahmad El-Banna N
ov
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2014
J-601-1448 Electronic Principals
Integrated Technical Education Cluster At AlAmeeria
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Agenda
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Introduction
General Frequency Considerations
Low Frequency Analysis- Bode Plot
BJT & JFET Amplifiers Low Frequency Analysis
Miller Effect
BJT & JFET Amplifiers High Frequency Response
INTRODUCTION 3
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Introduction
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• We will now investigate the frequency effects introduced by the larger capacitive elements of the network at low frequencies and the smaller capacitive elements of the active device at high frequencies
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Decibels • Power Levels t • Cascaded Stages t
• Voltage gain versus dB levels t
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GENERAL FREQUENCY CONSIDERATIONS
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Low, High & Mid Frequency Range
• The larger capacitors of a system will have an important impact on the response of a system in the low-frequency range and can be ignored for the high-frequency region.
• The smaller capacitors of a system will have an important impact on the response of a system in the high-frequency range and can be ignored for the low-frequency region.
• The effect of the capacitive elements in an amplifier are ignored for the mid-frequency range when important quantities such as the gain and impedance levels are determined.
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Typical Frequency Response
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The band frequencies define a level where the gain or quantity of interest will be 70.7% of its maximum value.
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• Phase plot
• Decibel plot
• Normalized plot
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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.
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Bode Plot
• The piecewise linear plot of the asymptotes and associated breakpoints is called a Bode plot of the magnitude versus frequency.
• 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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BJT & JFET AMPLIFIERS LOW FREQUENCY ANALYSIS
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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= max(fLs , fLc , fLE)
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Impact of RS
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Example
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FET Amplifier
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MILLER EFFECT 18
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Miller input capacitance
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• In the high-frequency region, the capacitive elements of importance are the interelectrode (between-terminals) capacitances internal to the active device and the wiring capacitance between leads of the network.
• For any inverting amplifier, the input capacitance will be increased by a Miller effect capacitance sensitive to the gain of the amplifier and the interelectrode (parasitic) capacitance between the input and output terminals of the active device.
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Miller output capacitance
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• A positive value for Av would result in a negative capacitance (for Av > 1). • For noninverting amplifiers such as the common-base and emitter-follower configurations,
the Miller effect capacitance is not a contributing concern for high-frequency applications. • The Miller effect will also increase the level of output capacitance, which must also be
considered when the high-frequency cutoff is determined.
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BJT & JFET AMPLIFIERS HIGH FREQUENCY RESPONSE
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High Frequency Response
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• At the high-frequency end, there are two factors that define the 3-dB cutoff point:
1. the network capacitance (parasitic and introduced) 2. the frequency dependence of hfe (β).
• For RC circuit:
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1. Network Parameters :
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• At high frequencies, the various parasitic capacitances (Cbe, Cbc, Cce) of the transistor are included with the wiring capacitances (CWi, CWo).
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2. hfe (or β) Variation
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• The quantity, fβ, is determined by a set of parameters employed in the hybrid π model
• The variation of hfe (or β) with frequency approaches the following relationship:
• fβ is a function of the bias configuration.
• the small change in hfb for the chosen frequency range, revealing that the common-base configuration displays improved high-frequency characteristics over the common-emitter configuration.
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Example
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Gain-Bandwidth Product
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• There is a Figure of Merit applied to amplifiers called the Gain-Bandwidth Product (GBP) that is commonly used to initiate the design process of an amplifier.
• It provides important information about the relationship between the gain of the amplifier and the expected operating frequency range.
• at any level of gain the product of the two remains a constant. • the frequency fT is called the unity-gain frequency and is always equal to the product of the
midband gain of an amplifier and the bandwidth at any level of gain.
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FET Amplifier
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• For more details, refer to:
• Chapter 9, Electronic Devices and Circuits, Boylestad.
• The lecture is available online at:
• https://speakerdeck.com/ahmad_elbanna
• For inquires, send to:
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