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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)
2Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
BJT parameters
3Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
BJT Amplifier Configurations
4Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
3-basic configurations
Characterizing Amplifiers
5Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Amplifier circuit Amplifier model
1) Input resistance:
2) Open-circuit voltage gain:
3) Output resistance:(input cancelled)
voltage gain
Overall gain:
The Common-Emitter (CE) Amplifier
6Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
-> Most widely used configuration
π-model
ro can often be neglected
Neglect vi & vπ to find output resistance
Voltage gain with load (RL)
Overall voltage gain
CE Amplifier with an Emitter Resistance
7Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
T-model
Voltage gain
If load resistance is connectedReplace RC with RC II RL
Open circuit voltage gain
Overall gain
Avo
Common Base (CB) Amplifier
8Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Similar to CE Amplifier, without emitter resistance
If RL is connected:
Avo
Common Collector (CC) Amplifier or Emitter Follower
9Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
(voltage divider)
(RL -> ∞)
Summary and Comparison of 3 Amplifiers
10Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Summary and Comparison of 3 Amplifiers
11Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
• 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.
Summary and Comparison of 3 Amplifiers
12Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Rin & Rout
may cause the input to drop significantly
High Rin and low Rout
is particularly useful as a buffer
Biasing BJT Amplifiers
13Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Biasing BJT Amplifiers
14Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
2 bad solutions for biasing!
Fixing VBE Fixing IBSharp 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
Variations in β will cause variations in IC
Classical Bias Arrangement
15Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
• Supplies fraction of supply voltage.• RE is connected to the emitter
To make IE insensitive to temperature and β variation:
Classical Bias Arrangement with 2 supply voltages
16Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
VEE replaces VBB
Same constraints apply
Biasing using a Collector-to-Base Feedback Resistor
17Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Biasing using a constant-current source
18Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
• 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
Discrete circuit BJT Amplifiers – CE
19Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Combine CE&CB&CC Amplifiers + biasing arrangement
CE – bypass capacitor is to establish signal (ac) ground. Emitter signal current passes through this capacitor (bypasses the current source)
CC1: coupling capacitor:Lets ac signal in, blocks DC
CC2: coupling capacitor:Lets ac signal oout, blocks DC
Discrete circuit BJT Amplifiers – CE
20Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Thevenin equivalent to the right of rπ
Change the formulas in the table:-> replace Rin with RBIIrπ
-> multiply Gv by-> replace Rsig with
Discrete circuit BJT Amplifiers – CE
21Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Thevenin equivalent to the right of rπ
Change the formulas in the table:-> replace Rin with RBIIrπ
-> multiply Gv by-> replace Rsig with
Discrete circuit BJT Amplifiers – CE with Re
22Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Discrete circuit BJT Amplifiers – CE with Re
23Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
PreviouslyNow:
Replace:
-> multiply Gv by
Discrete circuit BJT Amplifiers – CB
24Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Same as previous,Formulas apply here
Discrete circuit BJT Amplifiers – CB
25Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Discrete circuit BJT Amplifiers – CC
26Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Previously:RB!
Discrete circuit BJT Amplifiers – CC
27Asst. Prof. Onur Ferhanoğlu BJT/ INTRODUCTION TO ELECTRONICS
Previously:
Now:Multiply by
replace