CHAPTER 6
Applying the BJT in
Amplifier Design
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Obtaining a Voltage Amplifier
vC  VCC  iC RC
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Obtaining a Voltage Amplifier
Voltage Transfer Characteristics
iC  I S e vBE / VT
vCE  VCC  RC I S e vBE / VT
vC  VCC  iC RC
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Biasing the BJT to Obtain Linear Operation
VCE  VCC  RC I S e vBE / VT
I C  I S eVBE / VT
v BE (t )  VBE  vbe (t )
Figure 6.32 Biasing the BJT amplifier at a point Q
located on the active-mode segment of the VTC.
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Av  
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I V RC
V
  RC
VT
VT
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Figure 6.34 Graphical construction for determining the VTC of the
amplifier circuit of Fig. 6.33(a).
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Locating the bias point Q
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Small-signal operation and models
I C  I S e VBE / VT
I E  IC / 
I B  IC / 
VCE  VCC  I C RC
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The Collector Current and the Transconductance
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The Emitter current
and the Input
Resistance at the
Emitter
The Base current and
the Input Resistance at
the base
Figure 6.38 Illustrating the definition of rπ and re.
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Separating the Signal and the DC Quantities
Figure 6.39 The amplifier circuit of Fig. 6.36(a) with the dc sources
(VBE and VCC) eliminated (short-circuited). Thus only the signal
components are present. Note that this is a representation of the
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signal operation of the BJT and
not an actual
amplifier circuit.
The Hybrid- Model (npn and pnp)
Figure 6.40 Two slightly different versions of the hybrid- model for the smallsignal operation of the BJT. The equivalent circuit in
(a) represents the BJT as a voltage-controlled current source (a
transconductance amplifier), and that in (b) represents the BJT as a currentSedra/Smith
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Microelectronic Circuits, Sixth Edition
controlled current source
(a current
amplifier).
The T Model (npn and pnp)
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Application of the Small-Signal Equivalent Circuits
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Example 6.14 (p. 417)
Input and output waveforms
for the circuit of Fig. 6.44.
Observe that this amplifier is
non-inverting, a property of
the grounded base
configuration
Figure 6.44 (a) circuit; (b) dc analysis; (c) circuit with the dc sources
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eliminated; (d) small-signal analysis
using the
T model for the BJT.
Figure 6.46 Performing signal
analysis directly on the circuit
diagram with the BJT small-signal
model implicitly employed:
(a) Circuit for Example 6.14; (b)
Circuit for Example 6.16.
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Figure 6.47 The hybrid- small-signal model, in its two versions, with
the resistance ro included (taking into account the early effect).
VA
r0 
IC
v 0   g m vbe ( RC || r0 )
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Exercise 6.41
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Table 6.4 Small-Signal
Models of the BJT
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