# BJT III ```INTRODUCTION TO ELECTRONICS
EHB 222E
Bipolar Junction Transistors III
(BJT)
Asst. Prof. Onur Ferhanoğlu
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO
ELECTRONICS
1
BJT models (π vs. T)
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
2
BJT parameters
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
3
BJT Amplifier Configurations
3-basic configurations
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
4
Characterizing Amplifiers
Amplifier circuit
Amplifier model
1) Input resistance:
2) Open-circuit voltage gain:
voltage gain
3) Output resistance:
(input cancelled)
Asst. Prof. Onur Ferhanoğlu
Overall gain:
BJT/ INTRODUCTION TO ELECTRONICS
5
The Common-Emitter (CE) Amplifier
-&gt; Most widely used configuration
ro can often be neglected
π-model
Neglect vi &amp; vπ to find output resistance
Overall voltage gain
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
Voltage gain
6
CE Amplifier with an Emitter Resistance
T-model
Voltage gain
Replace RC with RC II RL
Open circuit
voltage gain
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
Overall gain
Avo
7
Common Base (CB) Amplifier
Similar to CE Amplifier, without
emitter resistance
If RL is connected:
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
Avo
8
Common Collector (CC) Amplifier or Emitter Follower
(voltage divider)
(RL -&gt; ∞)
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
9
Summary and Comparison of 3 Amplifiers
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
10
Summary and Comparison of 3 Amplifiers
• CE configuration is best suited for high gain applications
• Considering Re, CE stage provides performance improvements at the expense of
gain reduction
• Low input impedance of CB amplifier makes it useful for specific applications.
Well suited as a high frequency amplifier
• The emitter follower (CC), having a unity gain, finds application as a voltage
buffer for connecting a high resistance source to a low resistance load.
Can be used as the last stage of a multistage amplifier.
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
11
Summary and Comparison of 3 Amplifiers
Rin &amp; Rout
may cause the input to
drop significantly
High Rin and low Rout
is particularly useful as a buffer
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
12
Biasing BJT Amplifiers
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
13
Biasing BJT Amplifiers
Fixing VBE
Sharp exponential relationship between iC and vBE
implies that any small difference in VBE from desired
value will result in large deviations in IC and VCE
Asst. Prof. Onur Ferhanoğlu
Fixing IB
Variations in β will cause variations in IC
BJT/ INTRODUCTION TO ELECTRONICS
14
Classical Bias Arrangement
• Supplies fraction of supply voltage.
• RE is connected to the emitter
To make IE insensitive to
temperature and β variation:
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
15
Classical Bias Arrangement with 2 supply voltages
VEE replaces VBB
Same constraints apply
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
16
Biasing using a Collector-to-Base Feedback Resistor
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
17
Biasing using a constant-current source
• Emitter current becomes independent of β and RB
Implementation of the current source
Neglect base currents (high β )
Since Q1 and Q2 share the same VBE:
Collector current will remain constant
as long as Q2 is active, neglecting the early effect
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
18
Discrete circuit BJT Amplifiers – CE
Combine CE&amp;CB&amp;CC Amplifiers + biasing arrangement
CC2: coupling capacitor:
Lets ac signal oout, blocks DC
CC1: coupling capacitor:
Lets ac signal in, blocks DC
CE – bypass capacitor is to establish signal
(ac) ground. Emitter signal current passes
through this capacitor (bypasses the
current source)
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
19
Discrete circuit BJT Amplifiers – CE
Thevenin equivalent
to the right of rπ
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
Change the formulas in the table:
-&gt; replace Rin with RBIIrπ
-&gt; multiply Gv by
-&gt; replace Rsig with
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Discrete circuit BJT Amplifiers – CE
Thevenin equivalent
to the right of rπ
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
Change the formulas in the table:
-&gt; replace Rin with RBIIrπ
-&gt; multiply Gv by
-&gt; replace Rsig with
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Discrete circuit BJT Amplifiers – CE with Re
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
22
Discrete circuit BJT Amplifiers – CE with Re
-&gt; multiply Gv by
Previously
Now:
Replace:
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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Discrete circuit BJT Amplifiers – CB
Same as previous,
Formulas apply here
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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Discrete circuit BJT Amplifiers – CB
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
25
Discrete circuit BJT Amplifiers – CC
RB!
Asst. Prof. Onur Ferhanoğlu
Previously:
BJT/ INTRODUCTION TO ELECTRONICS
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Discrete circuit BJT Amplifiers – CC
Previously:
Now:
Asst. Prof. Onur Ferhanoğlu
Multiply by
BJT/ INTRODUCTION TO ELECTRONICS
replace
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