Electronics Lecture10 11Amplifiers · Email: [email protected] . Korea University of Technology...

Korea University of Technology & Education

Department of Electrical, Electronics and Communications Engineering

Department of EECE

BJT Amplifiers

Electronic Circuits

Manar Mohaisen Office: F208

Email: [email protected]



₪ Explain the DC Operating Point

₪ Explain the Voltage-divider Bias

₪ Other Bias Methods

₪ Discussions

Review of the Precedent Lecture



₪ Explain the Amplifier Operation

₪ Explain the BJT AC Models

₪ The Common-emitter Amplifier

₪ The common-base amplifier

₪ The common-collector amplifier

₪ Multi-stage amplifiers

Class Objectives



₪ Quantities DC Quantities

Uppercase italic letters

Uppercase subscripts

IB, VC

AC Quantities Uppercase italic letters


Ib, Ve

Instantaneous Quantities Uppercase italic letters


ib, vc

Resistances RC DC Resistance, Rc AC resistance, r’e internal ac resitance.

Amplifier Operation



₪ The Linear Amplifier Coupling capacitors block the DC quantities.

Prevents the change of the DC bias voltages at the base and collector.

Collector voltage is inversely proportional to Ic. It has a 180 phase difference with Ic.

Amplifier Operation – contd.



₪ Graphical Representation AC load line.

Difference bet. DC and AC load line will be revisited in Chapter 7.




₪ Example 6.1




₪ r Parameters

r’b (small) can be neglected (short)

r’c (high) can be neglected (open)

Transistor AC Models – contd. r Param. Description

αac ac alpha

βac ac beta

r‘e ac emitter resistance

r‘b ac base resistance

r‘c ac collector resistance



₪ Determining r’e by a Formula

Note that Emitter current changes with temperature.

Also, in the derivation, it was considered that there is an abrupt junction between the n and p regions.

However, this approximation is valid for analysis.

Transistor AC Models – contd.

' 25mVe

Er I=



₪ Comparison between the AC Beta and the DC Beta




₪ h (hybrid) Parameters Supplied by the manufacturer.


H Parameter Description Condition

hi Input impedance (resistance) Output shorted

hr Voltage feedback ratio Input open

hf Forward current gain Output shorted

ho Output admittance (conductance) Input open

Configuration Description

Common-emitter hie, hre, hfe, hoe

Common-base hib, hrb, hfb, hob

Common-collector hic, hrc, hfc, hoc

ac fbhα =

ac fehβ =

' reeoe

hr h=

' 1recoe

hr h+=

' (1 )reieb feoe

hr h hh= − +



₪ Amplifier DC Analysis & AC Analysis

Common-emitter refers to the AC operation.

The Common-emitter Amplifier



₪ DC Analysis Q-point

> 10 R2, so it is neglected.

The Common-emitter Amplifier – contd.

EDCN(BASE) (150)(560 ) 84 kΩIR Rβ= = Ω =

2B CC

1 2

6.8 k 12V 2.83V28.8kRV VR R

⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠

Ω≅ = =+ Ω

E B BEE

E E

2.83V 0.7V 3.80mA560V V VI R R

− −= = = =Ω

C CC C C 8.2VV V I R= − =

ECE C 6.07VV V V= − =



₪ AC Analysis C1, C2, and C3 are replaced by shorts.

Their values are selected so that their impedances are negligible.

The DC source is replaced with ground.




₪ AC Analysis The ac input resistance is seen in parallel with R1 & R2.


1 2( ) ( )|| ||in tot in baseR R R R=

( )

( )

in totsb sin tot

RV VR R

⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠

= +



₪ Input Resistance looking in at the Base

₪ Output Resistance looking in at the collector


( )

' '/

in bin base

in b

e e ac ee ac

VVR I II r rI ββ

= =

= =

'C C|| coutR R r R= ≅



₪ Example 6-3 Find Vb.

IE = 3.8 mA.


'

E

25mV 25mV 6.583.8mAer I= = = Ω

'( ) 160(6.58 ) 1.05kac ein baseR rβ= = Ω = Ω

1 2( ) ( )|| || 873in tot in baseR R R R= = Ω

( )

( )

873 10mV(rms) 7.44mV(rms)1173in tot

sb s in tot

RV VR R

⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠

= = =+



₪ AC Voltage Gain Ratio between Output Voltage

and Input Voltage.

If you would like to get the

overall amplifier voltage gain


' ' 'c ec C C Cve e e e eb

I R I R RVA V I r I r r= = ≅ =

( )'

( )

in totc b c vv s s sb in tot

RVV VA AV V V R R

⎛ ⎞⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠ ⎝ ⎠

= = = +



₪ Effect of the Emitter Bypass Capacitor On Voltage Gain To obtain a short circuit by the capacitor C2

₪ Example 6-4 Operation frequency range:

200 Hz ~ 10 kHz


2 10E

CRX ≤

25610

EC

RX = = Ω

22

1 14.2μF2 CC fXπ= =



₪ Voltage Gain without the Bypass Capacitor

₪ Example 6-5 r‘e = 6.58 Ω

Gain without Bypass Capacitor

Gain with Bypass Capacitor


C've E

RAr R

=+

C'

E

1000 1.76560 6.58ve

RAr R

= = =++

C'

1000 1526.58ve

RAr

= = =



₪ Effect of Load on the Voltage Gain In presence of a load

The load is connected in Parallel with RC.

Therefore,


C' '

|| Lcve e

R RRAr r

= =



₪ Effect of Load on the Voltage Gain – contd. Example 6-6

RL = 5 kΩ, r‘e = 6.58 Ω, RC = 1 k Ω

In Presence of Load

Without Load


C' '

|| 833 1276.58Lcv

e e

R RRAr r

Ω= = = =Ω

C' '

1000 1526.58cve e

RRAr r

Ω= = = =Ω



₪ Stability of the Voltage Gain Stability is a measure of how well an amplifier maintains its

design values for a change in temperature and β.

₪ Swamping r‘e Reduce its effect.

Both RE1 and RE2 affect the DC bias

Only RE1 affects the AC voltage gain.


1

11

'E

'

E, for 10

cve

c eE

RAr R

R R rR

=+

= >



₪ The Effect of Swamping on the Amplifier’s Input Resistance


'1( ) ( )ac e Ein baseR r Rβ= +



₪ Current Gain Current gain from base to collector

The overall current Gain

with

₪ Power Gain

with


cacb

IIβ =

ci s

IA I=( )

sss in tot

VI R R= +

'p v iA A A= ' cv

s

VA V=



₪ Example 6-8


DC analysis

AC analysis



₪ Example 6-8 – contd. DC Analysis

> 10 R2, so the amplifier is stiff.


DC E1 E2IN(BASE) ( ) 150(940 ) 141kR R Rβ= + = Ω = Ω

2B CC

2 11.75VRV VR R

⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠

≅ =+

E B BE 1.75 0.7 1.05VV V V= − = − =

EE

1 2

1.05V 1.12mA940E E

VI R R= = =+ Ω

C CC C C 10V (1.12mA)(4.7k ) 4.74VV V I R= − = − Ω =

ECE C 4.74 1.05 3.7 VV V V= − = − =



₪ Example 6-8 – contd. AC Analysis


'

E

25mV 25mV 221.12mAer I= = = Ω

1'

( ) )( 175(492 ) 86.1kEac ein base RR rβ += = Ω = Ω

1 2( ) ( )|| || 7.53 kin tot in baseR R R R= = Ω

( )

( )

600 7530attenuation 1.087530s in tots

b in tot

R RVV R

+ += = = = || 4.27kc LCR R R= = Ω

'119.09c cv

e EE

R RA Rr R= ≅ =

+' 1 9.09 8.451.08

bv vs

VA AV⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠

= = =

' (8.45)(10mV) 84.5mVc v sV A V= = =



₪ Example 6-8 – contd. DC & AC Analysis


C( )

C( )

Max 1.414 4.74 (84.5mV)(1.414) 4.86 VMin 1.414 4.74 (84.5mV)(1.414) 4.62 V

cc p

cc p

V V VV V V

= + = + =

= + = − =

C( ) ( ) 119 mVout p c pV V V= − =



₪ Emitter-follower Amplifier Common-collector amplifier

The Common-collector Amplifier



₪ Voltage Gain

₪ Note Since the output follows the input in both phase and amplitude,

The amplifier is referred to as “emitter-follower”.

The Common-collector Amplifier – contd.

'

'

( )

( )1

out e eve e ein

ee e

V I RA V I r RR

r R

= =+

=+

≅



₪ Input Resistance

Therefore,

₪ Output Resistance Without load.


'( )

''

( )

( ) ( )

e e ein bin base

in b b

ac e eb ac e eb

VV I r RR I I II r R r RI

β β

+= = =

+= = +

1 2( ) ( )|| ||in tot in baseR R R R=

||sout Eac

RR Rβ⎛ ⎞⎜ ⎟⎜ ⎟⎝ ⎠

≅



₪ Current Gain

₪ Power Gain


( )/e e

iin in in tot

I IA I V R= =

p v i iA A A A= ≅



₪ The Darlington Pair

Overall current gain


1 1 1e ac bI Iβ=

2 2 1 22 1e ac ac acb bI I Iβ β β= =

1 2ac acβ β β=

E1 2in ac acR Rβ β=



₪ An Application Emitter-follower is used as interface bet. A circuit with high

output resistance and low-resistance load.

₪ Example Consider a common-emitter amplifier with RC = 1 kΩ, RL = 8 Ω

The loss in the voltage gain due to the load is high.

W.o. load

w. load

The solution is by using the Darlington pair.


C'1000 2005v

e

RAr

= = =

C' '

|| 7.94 1.595Lcv

e e

R RRAr r

= = = =



₪ An Application Interface the amplifier and the speaker.




₪ Example 6-10 Common-emitter:

r‘e = 5 , VCC = 12 V, RC = 1 k

Darlington emitter-follower: R1 = 10 kΩ, R2 = 22 kΩ, RE = 22Ω, RL = 8Ω, VCC = 12V, βac =βDC =100.


2E2 DC

B CC2E1 2 DC

|| 20 12V = 8.0V30||R RV V

R R Rββ

⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠

= =+

E B BEE

E E

2 6.6V 300mA22V V VI R R

−= = = =Ω

'

E

25mV 25mV 83m300mAer I= = = Ω



₪ Example 6-10 – contd. The load of the CE amplifier is Rin(tot) of the EF.

The voltage gain of

the common-emitter amplifier

The voltage gain of the Darlington EF

The overall voltage gain


1 2( ) ( )2 '

1 22

|| ||

|| || ( )10k || 22k ||100 (83m 5.87 ) 860

in tot in base

ac e e

R R R R

R R r Rβ

=

= += Ω Ω Ω + Ω = Ω

'860 1725

cve

RAr

= = =

'5.87 0.9983m 5.87

eve e

RAr R

= = =Ω ++

'(EF) (CE) (0.99)(172) 170v v vA A A= = =



₪ The Sziklai Pair




₪ Voltage Gain Notice that there is no phase inversion bet. output and input.

The Common-base Amplifier

' '( || ) ( || )out c c cv

e e ein E E

V I R RA V I r R r R= = ≅



₪ Input Resistance The resistance, looking in at the emitter

₪ Output Resistance The resistance, looking in at the collector

₪ Current Gain

The Common-base Amplifier –contd.

''

( )( || ) ( || )e ein e E e Ein emitter e ein

I r RV VR r RI I I= = = =

'C C||coutR r R R= ≅

1out ci ein

I IA I I= = ≅



₪ Multistage Voltage Gain

Multistage Amplifiers

'1 2 3v vnv v vA A A A A= L



₪ Capacitively-coupled Multistage Amplifier

Multistage Amplifiers – contd.

51 3 6 ( 2)|| || || 1.63kc in baseR R R R R= = Ω

' 'E 1( 2)

E

25mV1.05mA, 23.8 , and 3.57ke eacin baseI r R rI β= = = Ω = = Ω



₪ Capacitively-coupled Multistage Amplifier

The overall Voltage Gain


11 ' 68.5c

ve

RAr

= =

72 '

4.7k 19723.8ve

RAr

Ω= = =Ω

'1 2 (68.5)(197) 13,495v v vA A A= = =



₪ Direct-coupled Multistage Amplifier Advantages

No coupling capacitors

Can amplify low frequencies

Down to dc (0 Hz).

Disadvantage Affected by the changes due

to temperature.




₪ Explained the Amplifier Operation

₪ Explained the BJT AC Models

₪ Introduced the Common-emitter Amplifier

₪ Introduced the common-base amplifier

₪ Introduced the common-collector amplifier

₪ Discussed the operation of multi-stage amplifiers

Class Summary



Discussion & Notes

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Electronics Lecture10 11Amplifiers · Email: [email protected] . Korea University of Technology...

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Transcript of Electronics Lecture10 11Amplifiers · Email: [email protected] . Korea University of Technology...