INTRODUCTION TO ELECTRONICS EHB 222E

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)

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BJT parameters

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BJT Amplifier Configurations

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3-basic configurations

Characterizing Amplifiers

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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

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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

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Biasing BJT Amplifiers

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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

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VEE replaces VBB

Same constraints apply

Biasing using a Collector-to-Base Feedback Resistor

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Biasing using a constant-current source

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• 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

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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

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Discrete circuit BJT Amplifiers – CE with Re

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PreviouslyNow:

Replace:

-> multiply Gv by

Discrete circuit BJT Amplifiers – CB

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Same as previous,Formulas apply here

Discrete circuit BJT Amplifiers – CB

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Discrete circuit BJT Amplifiers – CC

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Previously:RB!

Discrete circuit BJT Amplifiers – CC

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Previously:

Now:Multiply by

replace