Communication Microelectronics ELCT508 (W17)eee.guc.edu.eg/Courses/Electronics/ELCT508...

Communication Microelectronics ELCT508 (W17)

Lecture 5: Analysis of BJT AmplifiersDr. Eman Azab

Assistant Professor

Office: C3.315

E-mail: [email protected]

Dr. Eman Azab Electronics Dept., Faculty of IETThe German University in Cairo

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BJT Modes of OperationElectrical Equations of BJT

Dr. Eman AzabElectronics Dept., Faculty of IETThe German University in Cairo

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BJT NPN Modes of Operation


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Mode BEJ BCJ Equations Condition

Cutoff Reverse Reverse 𝐼𝐶 = 𝐼𝐸 = 𝐼𝐵=0𝑉𝐵𝐸 < 0.7𝑉𝐵𝐶 < 0.5

Active (Forward)

Forward

Reverse

𝑉𝐵𝐸 = 0.7𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵

𝐼𝐶 = 𝛽𝐹𝐼𝐵 = 𝛼𝐹𝐼𝐸

𝛼𝐹 =𝛽𝐹

1 + 𝛽𝐹

𝑉𝐵𝐶 < 0.5Or

𝑉𝐶𝐸 > 0.2

SaturationForward

Forward

𝑉𝐵𝐸 = 0.7𝑉𝐵𝐶 = 0.5𝑉𝐶𝐸 = 0.2𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵

𝐼𝐶 < 𝛽𝐹𝐼𝐵

Reverse Active

Reverse

Forward

𝑉𝐵𝐶 = 0.5𝐼𝐶 = 𝐼𝐸 + 𝐼𝐵

𝐼𝐸 = 𝛽𝑅𝐼𝐵 = 𝛼𝑅𝐼𝐶

𝛼𝑅 =𝛽𝑅

1 + 𝛽𝑅

𝑉𝐵𝐸 < 0.7Q

Q

BJT Large Signal Model in Active Mode


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Q

BJT Voltage AmplifiersDesign and Analysis of BJT Amplifiers


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Objective

We want to implement a voltage amplifier using BJT

What are the Specifications of an Ideal Voltage Amp.?

Infinite Input Resistance: Rin (Thevinen at the Input Ports)

Infinite Voltage Gain: Avo

Finite Output Resistance (Short Circuit): Rout (Thevinen at theOutput Ports)


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'out

vo

in

vA

v

outv

sig

vA

v

in Lv vo

sig in out L

R RA A

R R R R

Important Notes

BJT Amplifiers deals with DC and AC signals, Thus weneed to have different symbols to distinguish betweenthem


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Type of Signal Signal Symbol Example

DC SignalsCapital Letter and

Subscript VBE, VCE, IC

AC Signals (Small Signals)

Small Letter and Subscript vbe ,vce, ic

Instantaneous (Large Signals)

Small Letter and Capital Subscript vBE, vCE, iC

BJT Amplifier Large Signal Analysis

Voltage Amplifier using BJT

Assume that we have instantaneous input voltage signal ‘vI’


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Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.

For Active Mode

ONLY

𝑣O = VCC − 𝑖CRC

𝑖C = ISexp𝑣IVT

𝑣BE = 𝑣I

BJT Large Signal Analysis

Voltage Amplifier using BJT

1. Cutoff Mode

2. Active Mode

3. Saturation Mode


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0 ≤ 𝑣I ≤ 0.5 𝑣O = VCC

0.5 ≤ 𝑣I ≤ 𝑣BE,sat 𝑣O > 𝑣CE,sat

𝑣O = VCC − ISRCexp𝑣IVT

𝑣I ≥ 𝑣BE,sat 𝑣O = 𝑉CE,sat

𝑣O = VCC − 𝑖CRC

BJT as an Amplifier

We can easily separate DC and AC Signals (Superposition)

Under the assumption that the AC signal amplitude is very small, such that thetransistor’s mode will remain the same for the complete cycle

BJT must work in Active Mode to avoid signal distortion

DC Sources are used to set the DC Operating point to Activemode

Input terminals of the amplifier are Base/Emitter

Output terminals of the amplifier are Collector/Emitter

Equivalent Circuit for Small Signal Analysis can be derived


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𝑣BE = VBE + 𝑣𝑏𝑒 𝑖C = ISexpVBE+𝑣𝑏𝑒

VT

BJT Small Signal Model Derivation

Assume BJT is in active mode & vbe<<VT


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𝑖C = ISexpVBE + 𝑣𝑏𝑒

VT

𝑖C = ISexpVBEVT

exp𝑣𝑏𝑒VT

𝑖C ≅ IC 1 +𝑣𝑏𝑒VT

𝑔m =𝜕𝑖C𝜕𝑣𝑏𝑒

=ICVT

𝑖C = 𝑔m𝑣𝑏𝑒 = 𝛽𝑖b

BJT Small Signal Model

We can place a resistance between Base and Emitter tohave a path for the base current


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𝑖C = 𝑔m𝑣𝑏𝑒 = 𝛽𝑖b 𝑟π =𝑣𝑏𝑒𝑖b

=β

gm=VTIB

BJT Small Signal Model

Note: Early effect can be taken into consideration (if VA

is given)


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𝑟o =VAIC

Analysis of BJT Amplifiers

Objective: Calculate the Voltage gain, Input Resistance andOutput Resistance

Solution Steps:

1. Determine the DC operating Point (Deactivate AC signals & All ExternalCapacitors impedances are considered open Circuit)

2. Calculate the small signal model parameters: gm, rπ

3. Replace the BJT with its small signal model (DC sources are deactivated&All External Capacitors impedances are considered Short Circuit)

4. Analyze the circuit to calculate the voltage gain, Input and OutputResistances


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BJT Amplifier ConfigurationsBasic Configurations and their Characteristics


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Common Emitter Amplifier

Objective: Calculate the voltage gain, Input and OutputResistances

Input terminal Base

Output Terminal Collector


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1. Calculate the DC Current

2. Calculate gm and rπ


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IC =β

1 + βI ≅ I


3. Draw the equivalent small signal model (Include ro ifgiven)


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3. Draw the equivalent small signal model (Include ro ifgiven)

4. Calculate the gain, input and output Resistance


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Av =𝑣O𝑣sig

= −gm(ro ∕∕ RC ∕∕ RL)RB ∕∕ rπ

RB ∕∕ rπ + Rsig

Rin = RB ∕∕ rπ Rout = ro ∕∕ RC ∕∕ RL


Notes on Common Emitter Configuration:

Inverting Amplifier

Gain is greater than unity

High Input Resistance

High Output Resistance


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Common Base Amplifier


Input terminal Emitter

Output Terminal Collector


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Voltage gain, input and Output Resistance (ro isneglected)


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Av =𝑣O𝑣sig

=gm(RC ∕∕ RL)

1 +RS

(RE ∕∕rπ

1 + β)

Rin = RS + (RE ∕∕rπ

1 + β)

Rout = RC ∕∕ RL



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Notes on Common Base Configuration:

Non-Inverting Amplifier

Gain is greater than unity

Low Input Resistance

High Output Resistance

Common Collector Amplifier


Input terminal Base

Output Terminal Emitter


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Voltage gain, input and Output Resistance


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Av =𝑣O𝑣sig

=1 + β (ro ∕∕ RL)

rπ + 1 + β (ro ∕∕ RL) 1 +RsigRB

+ Rsig


Voltage gain, input and Output Resistance


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Rin = RB ∕∕ rπ + 1 + β ro ∕∕ RL

Rout = ro ∕∕rπ + RB ∕∕ Rsig

1 + β



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Notes on Common Collector Configuration:

Non-Inverting Amplifier

Gain is less than unity

Emitter Follower (Buffer)

High Input Resistance

Low Output Resistance

Common Emitter with emitter Resistance


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Exercise:

Find the Voltage gain, input and Output Resistance


Communication Microelectronics ELCT508 (W17)eee.guc.edu.eg/Courses/Electronics/ELCT508...

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