Transcript BJT amplifier & small

BJT amplifier & small-signal
concept
Lecture 8
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Preview
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Basic characteristics of BJT amplifiers
DC and ac analysis & equivalent circuits
Small-signal hybrid-π equivalent circuits
Small-signal voltage gain, input resistance &
output resistance
• Common-emitter amplifiers
• dc & ac load lines
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Linear analog amplifier
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Notation
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Basic characteristics of an amplifier
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Basic characteristics of an amplifier
(cont’d)
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Basic BJT amplifier
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Graphical analysis of BJT amplifiers
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Graphical analysis of BJT amplifiers
(Cont’d)
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Analysis of BJT amplifiers
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dc analysis and equivalent circuit
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ac analysis and equivalent circuit
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Small-signal hybrid-π equivalent circuit
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Small-signal hybrid-π equivalent circuit
(Cont’d)
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Small-signal voltage gain
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Input and output resistances
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Example: dc circuit analysis
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Example: dc circuit analysis (Cont’d)
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Example: dc circuit analysis (Cont’d)
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Small-signal hybrid-π equivalent circuit
with Early effect
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Example: determine BJT amplifier
parameters
• Determine the small-signal voltage gain, input resistance, and
output resistance of the BJT amplifier circuit in previous
example with the early effect. Assume that the early voltage is
50V.
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Common-emitter amplifiers (with voltagedivider biasing & coupling capacitor)
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Common-emitter amplifiers (with voltagedivider biasing & coupling capacitor)- Cont’d
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Common-emitter amplifiers (with voltage-divider
biasing & coupling capacitor & emitter resistor)
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dc & ac load lines
• Dc load line is used to find Q-point
• Ac load line is used to determine graphically
the operation of a BJT amplifier
• Dc and ac load lines are essentially different
since capacitors appear as an open circuit for
a de operation but a short circuit for an ac
operation
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dc load line
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ac load line
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Maximum output symmetrical swing
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Saturation
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Cutoff
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Example: Calculate BJT circuit
parameters
• Determine the small-signal voltage gain, input
resistance, and output resistance of the circuit
shown in attached Figure. Assume the transistor
parameters are: β=100, VBE(on) =0.7V, and VA=100V
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END
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• TQ8.1
• In Figure 8.1, let β=150,VA=200V, VCC=7.5V,
VBE(on)=0.7V, RC=15KΩ, RB=100kΩ, and
VBB=0.92V.
• (a) Determine the small-signal hybridparameters rπ, gm, and r0
• (b) Find the small-signal voltage gain Av
Fig. 8.1
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