electronic circuits elct604 (spring 2020) chapter 2...

Electronic Circuits ELCT604

(Spring 2020)

Chapter 2 (Cont.)

BJT AmplifiersDr. Eman Azab

Assistant Professor

Office: C3.315

E-mail: [email protected]

Dr. Eman Azab

Electronics Dept., Faculty of IET

The German University in Cairo

1

Analog Voltage AmplifiersCircuit Design and Configurations

Dr. Eman Azab



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Objective

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Implementing a voltage amplifier Circuit on the

transistor level

What are the Specifications for an Ideal Voltage Amp.?

Infinite Input Resistance: Rin

Infinite Voltage Gain: Avo

Finite Output Resistance (Short Circuit): Rout

'out

vo

in

vA

v

outv

sig

vA

v

in Lv vo

sig in out L

R RA A

R R R R

Signals in Amplifier Circuits

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Information signals that we want to amplify must be AC

signal

However, we will have three types of signals in the circuit.

Thus; we need to distinguish between them by symbol:

Type of Signal Signal Symbol Example

DC SignalsCapital Letter and

SubscriptVBE, VCE, IC

AC Signals

(Small Signals)

Small Letter and

Subscriptvbe ,vce, ic

Instantaneous

(Large Signals)

Small Letter and

Capital SubscriptvBE, vCE, iC

BJT Modes of OperationElectrical Equations of BJT I-V characteristics

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Mode BEJ BCJ Equations Condition

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

Active

(Forward)

ForwardReverse

𝑉𝐵𝐸 = 0.7𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵𝐼𝐶 = 𝛽𝐹𝐼𝐵= 𝛼𝐹𝐼𝐸

𝛼𝐹 =𝛽𝐹

1 + 𝛽𝐹

𝑉𝐵𝐶 < 0.5Or

𝑉𝐶𝐸 > 0.2

Saturation

Forward

Forward

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

𝐼𝐶 < 𝛽𝐹𝐼𝐵

Reverse

Active

Reverse

Forward

𝑉𝐵𝐶 = 0.5𝐼𝐶 = 𝐼𝐸 + 𝐼𝐵𝐼𝐸 = 𝛽𝑅𝐼𝐵= 𝛼𝑅𝐼𝐶

𝛼𝑅 =𝛽𝑅

1 + 𝛽𝑅

𝑉𝐵𝐸 < 0.7

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Q

Q

BJT NPN Modes of Operation

IC versus VCE

The Early effect

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CEBEC S

T A

vVi I exp( ) 1

V V

C Ao

CE C

i Vr

v I

Q

BJT Active mode I-V CKHs

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

BJT Large Signal Model in

Active Mode

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Q


BJT Large Signal Analysis

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Voltage Amplifier using BJT

Assume that we have instantaneous input voltage signal ‘vI’

The instantaneous output voltage ‘vO’ is measured from collector

referring to the ground

Using KVLs:


𝑣O = VCC − 𝑖CRC

𝑣I = 𝑣BE

𝑣O = 𝑣CE

BJT Large Signal Analysis

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Voltage Amplifier using BJT

The transistor mode of operation depends on the value of ‘vI’

Cutoff Mode

Active Mode

Saturation Mode

0 ≤ 𝑣I ≤ 0.5

𝑣O = VCC

0.5 ≤ 𝑣I ≤ 𝑣BE,sat 𝑣O > 𝑣CE,sat

𝑣O = VCC − ISRCexp𝑣IVT

𝑣I ≥ 𝑣BE,sat

𝑣O = 0.2

𝑖C = 0

𝑖C = ISexp𝑣BEVT

BJT as an Amplifier

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We can easily separate DC and AC Signals (Superposition)

Under the assumption that the AC signal amplitude is very small, such

that the transistor’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 make sure BJT operates in Active

mode

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

𝑣BE = VBE + 𝑣𝑏𝑒 𝑖C = ISexpVBE + 𝑣𝑏𝑒

VT

BJT as an Amplifier

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Assume BJT is in active mode & vbe<<VT


𝑖C = ISexpVBE + 𝑣𝑏𝑒

VT

𝑖C = ISexpVBEVT

exp𝑣𝑏𝑒VT

𝑖C ≅ IC 1 +𝑣𝑏𝑒VT

𝑔m =𝜕𝑖C𝜕𝑣𝑏𝑒

=ICVT

𝑖C = 𝑔m𝑣𝑏𝑒 = 𝛽𝑖b

BJT Small Signal Model

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We can place a resistance between Base and Emitter to

have a path for the base current


𝑖C = 𝑔m𝑣𝑏𝑒 = 𝛽𝑖b 𝑟π =𝑣𝑏𝑒𝑖b

=β

gm=VTIB

BJT Small Signal Model

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Note that: Early effect can be taken into consideration


𝑟o =VAIC

Analysis of BJT Amplifiers

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Objective: Calculate the voltage gain, Input and Output

Resistances

1. Determine the DC operating Point (Deactivate AC signals)

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

3. Replace the BJT with its small signal model (DC sources

are deactivated)

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

and Output Resistances

BJT Amplifiers

Configurations

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Common Emitter Amplifier

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Resistances

Input terminal Base

Output Terminal Collector



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1. Calculate the DC Current

2. Calculate gm and rπ

IC =β

1 + βI ≅ I


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3. Draw the equivalent small signal model (Include ro if

given)


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3. Draw the equivalent small signal model (Include ro if

given)

4. Calculate the gain, input and output Resistance


Av =𝑣O𝑣sig

= −gm(ro ∕∕ RC ∕∕ RL)RB ∕∕ rπ

RB ∕∕ rπ + Rsig

Rin = RB ∕∕ rπ Rout = ro ∕∕ RC ∕∕ RL


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Notes on Common Emitter Configuration:

Inverting Amplifier

Gain is greater than unity

High Input Resistance

High Output Resistance

Common Base Amplifier

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Resistances

Input terminal Emitter

Output Terminal Collector


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Voltage gain, input and Output Resistance (ro is

neglected)

Av =𝑣O𝑣sig

=gm(RC ∕∕ RL)

1 +RS

(RE ∕∕rπ

1 + β)

Rin = RS + (RE ∕∕rπ

1 + β)

Rout = RC ∕∕ RL


Dr. Eman Azab



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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

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Resistances

Input terminal Base

Output Terminal Emitter



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Voltage gain, input and Output Resistance

Av =𝑣O𝑣sig

=1 + β (ro ∕∕ RL)

rπ + 1 + β (ro ∕∕ RL) 1 +RsigRB

+ Rsig


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Voltage gain, input and Output Resistance

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 RE

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Exercise:

Find the Voltage gain, input and Output Resistance


electronic circuits elct604 (spring 2020) chapter 2...

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