4th Edition: Chapter 1

Lecture 10
Bipolar Junction Transistor
(BJT)
BJT
1-1
Outline
 Continue BJT
 Continue DC analysis
• More examples

Introduction to AC signal analysis
BJT
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Example (1)
 A bipolar transistor having IS = 5×10-16 A is
biased in the forward active region with VBE
=750 mV. If the current gain (or β) varies from
50 to 200 due to manufacturing variations,
calculate the minimum and maximum terminal
currents of the device.
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Solution
 For a given VBE, the collector current remains
independent of β
 The base current varies from IC/200 to IC/50:
 the emitter current experiences only a small
variation because (β+ 1)/β is near unity for large
β:
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Example (2)
 Determine the dc level of IB and
BJT circuit
VC for the
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Solution
 For the dc mode,
the capacitor assumes
the open-circuit equivalence
and RB =R1+R2
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Example (3)
 Determine the dc bias voltage
VCE and the current
IC for the voltage-divider configuration shown in
the figure. (use exact and approximate methods)
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Solution
 Using Exact Analysis Method
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Solution (cont’d)
 Return to the example:
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Solution (cont’d)
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Solution (cont’d)
 Using Approximate Analysis Method
The condition that will define
whether the approximate approach
can be applied
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Solution (cont’d)
 Return to the example:
 Testing:
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Solution (cont’d)
Now, compare between obtained results by the exact
and approximate methods
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Example (4)
 Determine the voltage
VCB and the current IB
for the common-base configuration as shown in
figure
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Solution
 Applying Kirchhoff’s voltage law to the input
circuit yields
 Applying Kirchhoff’s voltage law to the output
circuit gives
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Example (5)
 Given the device characteristics as shown
in figure, determine VCC, RB, and RC for the
fixed bias configuration
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Solution
 From the load line
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BJT Transistor Modeling
 A model is an equivalent circuit that represents
the AC characteristics of the transistor
 A model uses circuit elements that
approximate the behavior of the transistor
 There are two models commonly used in small
signal AC analysis of a transistor:
re model
 Hybrid equivalent model

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BJT AC Analysis
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The re Transistor Model
 BJTs are basically current-controlled devices;
therefore the re model uses a diode and a
current source to duplicate the behavior of the
transistor

Recall (from lecture 5): the ac resistance of a diode
can be determined by the equation r = 26 mV/ID,
where ID is the dc current through the diode at the
Q (quiescent) point
ac
 One disadvantage to this model is its sensitivity
to the DC level. This model is designed for
specific circuit conditions
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Common-Base Configuration
I c  I e
re 
26 mV
Ie
Input impedance:
Zi  re
Output impedance:
Zo  
Voltage gain:
AV 
Forwardbiased
junction
I e R L R L

I e re
re
Current gain:
RL
Ai    1
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Example
 For a common-base configuration, as shown in
figure, with IE = 4 mA, α = 0.98, and an ac signal
of 2 mV applied between the base and emitter
terminals:
(a) Determine the input impedance.
 (b) Calculate the voltage gain if a load of 0.56 kΩ is
connected to the output terminals.
 (c) Find the output impedance and current gain.

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Solution
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Lecture Summary
Covered material
 Continue BJT

DC analysis
• More examples

Introduction to AC signal analysis
Material to be covered next lecture
 Continue BJT analysis with AC signal
BJT
1-24

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