High Frequency Response of Transistor Amplifiers

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High Frequency Response of Transistor Amplifiers

• CEC

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Contents

• Introduction.
• h-parameter model.
• Capacitive Effects.
• Base Spreading Resistance.
• Approximate hybrid-p model.
• CE Short Circuit Current Gain.
• Gain-Bandwidth Product.

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Transistor
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Approximate Hybrid Parameter Model
Common Emitter Configuration h parameter model
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Hybrid Parameter Model
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h-parameter Model

• Transistor modeled as a two port network.
• Will suffice as a low frequency model for
  transistor amplifiers.
• At high frequencies, junction capacitances will
  come into play.
• Forward biased emitter base junction exhibits
  capacitance.
• Reverse biased collector base junction exhibits
  capacitance.

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h-parameter Model

• Capacitive effects to be considered at high
  frequencies
• Transistor/Amplifier parameters may vary at high
  frequencies.
• h-parameter model may not be suitable at high
  frequencies.

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High Frequency Response

• Depends on transistor capabilities.
• To consider capacitive effects of PN Junctions.
• Hybrid p model incorporates junction
  capacitance effects.
• Hybrid - p model permits determine CE short
  circuit current gain and its dependence on
  frequency.
• Hybrid - p model also called Giacoletto Model.

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High Frequency Response

• To consider capacitive effects of the junctions
  to determine the upper 3 dB frequency.
• Forward biased PN junction exhibits diffusion
  capacitance.
• Reverse biased PN junction exhibits transition
  capacitance.
• Base Spreading Resistance bulk resistance of
  the base, portion of the base through which base
  current proper flows.

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Base Spreading Resistance
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Base Spreading Resistance
Resistance between base region and base terminal.
rbb Base Spreading Resistance
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Hybrid p Model of Common Emitter Transistor
Bulk Resistance of the base
Transition Capacitance due to reverse biased
collector-base junction
Virtual Base
Base Terminal
Vbe
Diffusion Capacitance due to forward biased
emitter junction
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Capacitive Effects

• Capacitive effects negligible at low frequencies.
• Input resistance from base to emitter with output
  shorted hie rbb rbe.
• rbc that shunts Cbc is normally large 4MO ,
  neglected.
• Output Resistance rce gtgt RL, ignored when RL
  connected across.

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Transconductance

• Mutual Conductance or Transconductance at
  constant VCE , gm since VBE
  cannot be measured.
• More accurately gm where VT .
• Output current generator has a value given by
  gmVbe.
• Since rbc is very large and rce gtgt RL, we can
  approximate the hybrid p model.

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Approximate hybrid p model
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CE Short Circuit Current Gain
Current Gain with output shorted
Ii
where
Ai (IL/Ii)
CE short circuit forward current transfer ratio
cutoff frequency.
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Beta Cut Off Frequency

• fß is the common emitter forward current transfer
  ratio cut off frequency.
• Frequency at which a transistors CE short
  circuit current gain drops 3 dB from its value at
  low frequencies.
• Maximum attainable bandwidth for the current gain
  of a CE amplifier with a given transistor.

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a and ß Cut Off Frequencies
fa gt fß
CE Configuration
CB Configuration
fa
fß
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Approximate High Frequency Model with Resistive
Load
Subject to Miller Transformation
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Approximate High Frequency Model with Resistive
Load
Additional Capacitance due to Miller Effect
C Cbe Cbc(1 gmRL)
fT 1/(2prbeC)
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Gain Bandwidth Product

• fT is the frequency at which common emitter
  short circuit current gain falls to unity.
• fT hfefß hfbfa
• fT is the product of low frequency gain hfe and
  the CE bandwidth fß .

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Gain Bandwidth Product
Gain
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Effect of Source Resistance
Upper Cut off frequency
fH
R (Rs rbb) rbe
Source Resistance
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References

• Allen Mottershead, Electronic Devices and
  Circuits, Prentice Hall of India Pvt. Ltd..
• Your prescribed references.

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Thank You