Transistor and stuff introduction for young enthusiast technician

Self-Sufficient Guide to Electronics Engineering by JASON AMPOLOQUIO

2-1

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DEFINITION. Voltage-controlled device: Devices utilizing a static voltage as the controlling signal are, not surprisingly, called voltage-controlled devices. DEFINITION. Current-controlled device: Devices working on the principle of one current controlling another current are known as current-controlled devices. DEFINITION. Transistor is a three or more element solid-state device that amplifies by controlling the flow of current carriers through its semiconductor materials. DEFINITION. Gain is a term used to describe the amplification capabilities of an amplifier. It is basically a ratio of output to input.

HISTORICAL PERSPECTIVE

1928 The first patents for the transistor principle were

registered in Germany by Julius Edgar Lilienfeld. 1934 German physicist Dr. Oskar Heil patented the field-

effect transistor. 1947 John Bardeen and Walter Brattain succeeded in

building the first practical point-contact transistor at Bell Labs.

1948 The term "transistor" was coined by John R. Pierce. 1958 Jack Kilby, an electrical engineer at Texas Instruments and Robert Noyce

of Fairchild Semiconductor independently invent the integrated circuit. 1962 The metal oxide semiconductor field effect transistor (MOSFET) is invented

by engineers Steven Hofstein and Frederic Heiman at RCA's research laboratory in Princeton, New Jersey.

A. .FUNDAMENTALS OF TRANSISTORS.

1. The Junction Transistor

A bipolar junction transistor consists of three regions of doped semiconductors. A small current in the center or base region can be used to control a larger current flowing between the end regions (emitter and collector).

TRANSISTOR CIRCUITS

FUNDAMENTALS

Section

3

Read it till it

Hertz!

i know that…

BOOK 2: transistor circuits fundamentals 2-2

Read it till it Hertz! -JMA, PECE

i. Basic Construction

Characteristics of the most common semiconductor materials used to make transistors are given in the table below:

Semiconductor material

Forward Voltage (25°C)

Electron mobility (25°C)

Hole mobility (25 °C)

Max. Junction temp in °C

Ge 0.3 V 0.39 m/s 0.19 m/s 70 to 100 Si 0.7 V 0.14 m/s 0.05 m/s 150 to 200

GaAs 1.03 V 0.85 m/s 0.05 m/s 150 to 200

2. Transistor Structure

a. The collector region is the largest and is connected to a heat sink

since it dissipates most of the heat in operation.

b. The base region is very thin, to facilitate passage through it.

c. The emitter region is smaller and more heavily doped to promote conduction. Heavier (n+) doping also helps overcome the trivalent aluminum atoms which might diffuse in from the aluminum contacts.

d. The base-collector diode is reverse-biased. Yet its current is very large compared to the base current because of the thin base region

Collector Emitter

Base

N P N

Collector Emitter

Base

P N P

E

C

B

E

C

B

Collector Emitter

Base

N P N

VCC VEE

Can I eat this?

Ahhh.. Ok….


2-3




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The Author

Engr. Jason M. Ampoloquio Youngest Professional Electronics Engineer (PECE) President, Powerful Review Center Design Consultant MSECE Major in DSP-De La Salle University (units earned) BSECE-Central Colleges of the Philippines, 2000 HR Reyes Scholar Coach, IECEP Quizzers Champion: 1. ECE Quiz Show (1999) 2. 1st Brain Encounter (1998) 3. Physics Quiz Show (1996) 4. Mathematics Wizard (1996) 5. Inter Engineering Quiz Show (1995) Battle of the Brain School Representative (RPN-9) Quizzer-19th and 20th IECEP Quiz Show Author: 1. Electronics Engineering SUPERBook 2. EST SUPERBook EST Review Director Resource Speaker, Various Topics in Communications In-house reviewer, Various Colleges and Universities Sought after reviewer in Communications Engineering



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2-7




and the high field of the collector-base voltage. 99% of the carriers injected into the base region are swept to the collector.

e. The base-emitter diode is forward-biased. The base current is strongly dependent on the base-emitter voltage since it is a forward-biased diode.

Transistor Action Electrons moving from the emitter to the base have three options i. Combine with holes in the base. ii. Diffuse through the base and out of the

base connection iii. Diffuse across the base region into the

depletion region of the collector-base junction where they are swept by the electric field into the collector.

3. Voltage-Current Characteristics

Forward Region

>BEV 0 <BCV 0

Reverse Region

<BEV 0 >BCV 0

IE ⇒ Electron injection from collector to base, collection into emitter IB ⇒ Hole injection from base to collector, recombination in collector

Cut-off Region

<BEV 0 <BCV 0

IE ⇒ Hole generation in emitter, extraction into base IC ⇒ Hole generation in collector, extraction into base

Saturation Region

>BEV 0 >BCV 0

IC, IE ⇒ Balance of electron injection from emitter/collector into base IB ⇒ Hole injection into emitter/collector, recombination in emitter/collector, respectively

hmmm…



Summary

Region of Operation Base-Emitter Diode

Base-Collector Diode

Forward/Active Region Forward Biased Reverse Biased

Reverse Region Reverse Biased Forward Biased

Cut-off region Reverse Biased Reverse Biased

Saturation region Forward Biased Forward Biased

4. Testing Transistors with Ohmmeter

This series of tests is based on the diode nature of the transistor junctions. If all these conditions are met, then at least the diode behavior is functional and the transistor is in good condition.

Rx1 Ohmmeter Readings as Tests for Transistors Type RBE REB RBC RCB RCE REC PNP High Low High Low High HighNPN Low High Low High High High

5. Transistor Maximum Values

Part of the manufacturer's data for transistors is a set of maximum values which must not be exceeded in its operation. These form some of the constraints on transistor operation which are a part of the design of any circuit. A typical set, for the silicon transistor 2N2222:

Parameter Symbol Value

Collector-Base Voltage VCB 60 V

Collector-Emitter Voltage VCE 30 V

Base-Emitter Voltage VBE 5 V

Power dissipation VCE x IC 500 mW

Temperature T 125°C

E

C

B

E

C

B NPN PNP


2-9




6. Transistor Characteristic Curves (Approximate for 2N2222)

7. Transistor Operation A transistor in a circuit will be in one of three conditions In the active region (some collector current, more than a few tenths of

a volt above the emitter), useful for amplifier applications

In saturation (collector a few tenths of a volt above emitter, VCE≈0) Both pn junctions are forward biased and the collector current is maximum (IC (max)) and the transistor ideally behaves like a closed switch between collector and emitter, useful for "switch on" applications.

In cut off (no collector current except for leakage current, ICBO) Both pn junctions are reversed biased and there is essentially no collector current and the transistor ideally behaves like an open switch between collector and emitter, useful for "switch off" applications.

VCE (V) 0 2 4 6 8 10 12 14 16 18

20

40

60

80

I B (μA

)

2

4

6

8

10

IC (mA)

Ic

ICQ

VCEQ

Vce

Ib IBQ

Q-point

I’m done!



The curve shows that the transistor cannot be operated in the blacker shaded portion of the graph. IC(max) is the limiting rating between points A and B, PD(max) is the limiting rating between point B and C, and VCE(max) is the limiting rating between points C and D.

8. Transistor “Rule of Thumb”

Some useful "rules of thumb" which help in understanding transistor action are:

A base emitter voltage VBE of about 0.7-V will "turn on" the base-

emitter diode and that voltage changes very little, < ± 0.1 V throughout the active range of the transistor which may change base current by a factor of 10 or more.

An increase in base-emitter voltage VBE by about 60 mV will increase the collector current IC by about a factor of 10.

The effective AC series resistance of the emitter is about 25/IC ohms.

The base-emitter voltage VBE is temperature dependent, decreasing about 2.1 mV/°C.

The base-emitter voltage VBE varies slightly with the collector-emitter voltage VCE at constant collector current IC: ΔVBE ≈ -0.001ΔVCE

9. Standard Transistor Marking

Joint Electron Device Engineering Council (JEDEC).

Digit Letter Serial number [suffix]

The letter is always 'N', and the first digit is one less than the number of legs, (2 for transistors) except for 4N and 5N which are reserved for optocouplers. The serial numbers runs from 100 to 9999 and tell nothing about the transistor except its approximate time of introduction. The (optional) suffix indicates the gain (hfe) group of the device:

0 VCE (V)

Cutoff

A B

C

D

Active Region

VCE(max)

IC(max)

Breakdown

Saturation


2-11




I’m lost…

A = low gain B = medium gain C = high gain No suffix = ungrouped (any gain).

Examples- 2N3819, 2N2221A, 2N904.

10. Comparison between transistor α, β, and γ

For a typical transistor, a graph of IOUTPUT vs. IINPUT is nonlinear. At different points on the nonlinear curve, the ratio of ΔIOUTPUT/ΔIINPUT will be different, and it may also differ from IOUTPUT/IINPUT ratio at the Q-point

α (alpha) or Common Base Amplification Factor It is the ratio of the collector current change to the change in emitter current assuming that the collector base voltage is constant.

Δα = = ⇒ α = ∴ = α

ΔC C C

ac dc C EE E E

dI I II I

dI I I

*Typical values of α: 0.9 to 0.999

β (beta) or Common Emitter Forward Current Amplification Factor It is the ratio of change in collector current to the base current.

Δβ = = ⇒ β = ∴ = β

ΔC C C

ac dc C BB B B

dI I II I

dI I I

*Typical values of β: 20 to 600

γ (gamma) or Common Collector Forward Current Amplification Factor

It is the ratio of change in emitter current to the base current.

Δγ = = ⇒ γ = ∴ = γ

ΔE E E

ac ac E BB B B

dI I II I

dI I I

*Seldom used!

11. Relation between transistor α, β, and γ

β β

α = ∴α =+ β + β

ac dcac dc

ac dc1 1

α αβ = ∴β =

− α − αac dc

ac dcac dc1 1

γ = + β ∴ γ = + β

β γ ≈ βac ac dc dc1 1

since 10;



12. DC and AC gain Gain is a technical term for an amplifier's output/input magnitude ratio.

Gain DC AC

Current = outi

in

IA

I

Δ=

Δout

iin

IA

I

Voltage = outv

in

VA

V

Δ=

Δout

vin

VA

V

Power = =outp i v

in

PA A xA

P

Δ= =

Δout

p i vin

PA A xA

P

B. .TRANSISTOR AMPLIFIER CONFIGURATIONS.

1. Common Emitter The common emitter configuration lends itself to voltage amplification and is the most common configuration for transistor amplifiers.

Qualitative

Characteristics Typical Value

Current Gain High

Voltage Gain High

Power Gain High

Input Impedance Average

(500 to 1500 Ω)

Output Impedance Average

(30 to 50 kΩ)

NPN

IC

RC

VCCIE

VCB

VCE

VBB

RB

IB

VBE OUTPUT terminal:

Collector INPUT terminal:

Base


2-13




2. Common Base This configuration is used for high frequency applications because the base separates the input and output, minimizing oscillations at high frequency. It has a high voltage gain, relatively low input impedance and high output impedance compared to the common collector.

Qualitative Characteristics

Typical Value

Current Gain ≈1

Voltage Gain Highest

Power Gain ≈Av

Input Impedance Low

(30 to 60 Ω)

Output Impedance Highest

(250 to 550 kΩ)

3. Common Collector The common collector amplifier, often called an emitter follower since its output is taken from the emitter resistor, is useful as an impedance matching device since its input impedance is much higher than its output impedance. It is also termed a "buffer" for this reason and is used in digital circuits with basic gates.

IE

VBB

NPN RE

VEEIC

RB

IB

VBE

VCE

VCB

IC

RC

VCCIB

VBB

RB

IE

VCE

VBE VCB

NPN

INPUT terminal:

Collector

OUTPUT terminal:

Emitter

INPUT terminal:

Base OUTPUT terminal:

Emitter

Is that all!



Qualitative Characteristics Typical Value

Current Gain Highest

Voltage Gain ≈1

Power Gain ≈Ai

Input Impedance Highest

(2 to 550 kΩ)

Output Impedance Low

(50 to 1500 Ω) Transistor Configuration Comparison Chart

Amplifier

Type Common

Base Common Emitter

Common Collector (Emitter Follower)

Input/Output Phase

Relationship 0° 180° 0°

Voltage Gain (Av)

High Average Low

C

s e

RR r 'α+

( )π

β+

C o

s

R r

R r

1≅ c

e

Rr '

≈ C

e

Rr '

≈

Current Gain (Ai)

Low Average High

1α ≅ o

C o

rR r

β+

( ) o

o L

r1

r Rβ +

+

Power Gain (Ap=Av x Ai)

Low (≈Av)

High (Ap =Av Ai)

Medium (≈Ai)

Input Impedance

Low Average

(500 to 1500 Ω) High

er ' ( ) er 1 r 'π = β + ( ) ( )e L o1 r ' R r⎡ ⎤β + +⎣ ⎦

Output Impedance

High Average Low

Rc C oR r ( )s

ER

R1β +


2-15




C. .TRANSISTOR DC ANALYSIS. 1. Fixed Bias

DC Transistor Current Relation

DC Transistor Current General Solution

Base Current (IB) CC BE

BB

V VI

R−

=

Collector Current (IC) C dc BI I= β

( )CC BE

B

V VC dc RI −= β

Emitter Current (IE)

E B CI I I= +

( )E B dcI I 1= β +

≅ β ≅ ∴ β 10E B dc CI I I

DC Transistor Terminal-to-Ground Voltage Relation

DC Transistor

Terminal Voltage to Ground

General Solution

Base Voltage (VB) B BEV V 0.7 V= ≅

= −B CC B BV V I R

Collector Voltage (VC) C CEV V=

= −C CC C CV V I R

Emitter Voltage (VE) EV 0 V=

NPN

IC

RC

VCCIE

VBC

VCE

VCC

RB

IB

VBE

Can I eat this?



DC Transistor Terminal-to-Terminal Voltage Relation

DC Voltages from one transistor terminal to

another General Solution

Base-Emitter Voltage (VBE)

BE B EV V V= −

BE BV V 0.7 V= ≅

Base-Collector Voltage (VBC)

BC B CV V V= −

BC BE CEV V V= −

( )BC B B C CV I R I R= − −

Collector-Emitter Voltage (VCE)

CE C EV V V= −

CE C EV V V 0= ∴ =

CE CC C CV V I R= −

Sample Questions: Determine how much the Q-point (IC, VCE) will change over a temperature range where βdc increases from 80 to 100 and VBE decreases from 0.7-V to 0.6-V. Use VCC = 15 V, RC = 550 Ω, RB = 100 kΩ Solution:

( ) ( )

dc BE

CC BEC dc

B

CE CC C C

For 80 and V 0.7 V

V V 15 0.7I 80 10.4mA

R 110k

V V I R 15 10.4mA 550 9.28V

β = =

⎛ ⎞ ⎛ ⎞− −= β = =⎜ ⎟ ⎜ ⎟⎜ ⎟ Ω⎝ ⎠⎝ ⎠= − = − =

( )

( ) ( ) ( )

dc BE

CC BEdcC new

B

CC C CCE new

For 100 and V 0.6 V

V V 15 0.6I 100 13.1mA

R 110k

V V I R 15 13.1mA 550 7.8V

β = =

⎛ ⎞ ⎛ ⎞− −= β = =⎜ ⎟ ⎜ ⎟⎜ ⎟ Ω⎝ ⎠⎝ ⎠= − = − =

( ) ( )

( )

( ) ( )

( )

C CE

C new C oldC

C old

CE new CE oldCE

CE old

Solving for %change in I and V

I I 13.1 10.4% I x100% 25.96%

I 10.4

V V 7.8 9.28% V x100% 15.95%

V 9.28

⎛ ⎞− −⎛ ⎞⎜ ⎟Δ = = =⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠⎛ ⎞− −⎛ ⎞⎜ ⎟Δ = = = −⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ ⎠

The Q-point will be shifted upward…

The Q-point of fixed or base bias circuit is very dependent on βdc and therefore makes this arrangement very unstable.


2-17




2. Emitter Bias



Base Current (IB) CC BE E E

BB

V V I RI

R− −

=

Collector Current (IC)

C dc BI I= β

( )CC BE E E

B

V V I RC dc RI − −= β

B

dc

CC BEC R

E

V VI

R β

−≅

+


E B CI I I= +

( )E B dcI I 1= β +

≅ β ≅ ∴ β 10E B dc CI I I


DC Transistor Terminal Voltage to Ground

General Solution

Base Voltage (VB) = +B BE E EV V I R

B CC B BV V I R= −

Collector Voltage (VC) = +C CE E EV V I R

C CC C CV V I R= −

Emitter Voltage (VE) E E EV I R=

NPN

IC

RC

VCCIE

VBC

VCE

VCC

RB

IB

VBE

RE







BE B EV V V= −

BE CC B B E EV V I R I R= − −


BC B CV V V= −

BC BE CEV V V= −

( )BC B B C CV I R I R= − +


CE C EV V V= −

CE C E EV V I R= −

CE CC C C E EV V I R I R= − −

3. Voltage Divider Bias

NPNRC

VCC IE

VBC

VCE

VTH

RTH

IB

VBE

RE

IC

IE

VBC

VCE

R1

IB

VBE

RE

IC

R2

NPN

RC

+VCC

2TH

1 2

TH TH CC

where:R

RR +R

V R V

=

=


2-19






Base Current (IB) TH BE E E

BTH

V V I RI

R− −

=

Collector Current (IC)

C dc BI I= β

( )TH BE E E

TH

V V I RC dc RI − −= β

TH

dc

TH BEC R

E

V VI

R β

−≅

+


E B CI I I= +

( )E B dcI I 1= β +

≅ β ≅ ∴ β E B dc CI I I 10


DC Transistor Terminal

Voltage to Ground General Solution

Base Voltage (VB) = +B BE E EV V I R

B TH B THV V I R= −

Collector Voltage (VC) = +C CE E EV V I R

C CC C CV V I R= −

Emitter Voltage (VE) E E EV I R=





BE B EV V V= −

BE TH B TH E EV V I R I R= − −


BC B CV V V= −

BC BE CEV V V= −

( ) ( )BC TH CC B TH C CV V V I R I R= − − −


CE C EV V V= −

CE C E EV V I R= −

CE CC C C E EV V I R I R= − −



D. .TRANSISTOR SMALL SIGNAL AC ANALYSIS. 1. r-Parameter

r-Parameter Description

αac AC alpha= c

e

II

βac AC beta= c

b

II

er ' AC emitter resistance

br ' AC base resistance

cr ' AC collector resistance

βre Ic Vi ro

Ib

Vo

CE Configuration CE Configuration

CB Configuration CB Configuration

Emitter Follower Configuration

Voltage Follower Configuration

Fixed Bias

Emitter Bias

Voltage Divider

re model

Fixed Bias

Emitter Bias

Voltage Divider

BJT Small Signal Analysis

Hybrid model

b

e

c

e

re model for CE configuration


2-21




2. h-Parameter

= +i i i r oV h I h V = +o f i o oI h I h V

h-parameter subscript convention

Subscript Meaning

i input parameter

r reverse parameter

f forward parameter

o output parameter

h-Parameter Formula Description Condition

ih =

=o

ii

i V o

Vh

I Input Impedance

Output shorted

rh =

=i

ir

o I o

Vh

V Voltage feedback

ratio Input open

fh =

=o

of

i V o

Ih

I Forward current

gain Output shorted

oh =

=i

io

o I o

Ih

V Output admittance

(conductance) Input open

h-Parameter Common Emitter

Common Base

Common Collector

ih = bie

b

Vh

I = e

ibb

Vh

I = b

icb

Vh

I

rh = bre

c

Vh

V = e

rbc

Vh

V = b

rce

Vh

V

fh = cfe

b

Ih

I = c

fbb

Ih

I = e

fcb

Ih

I

oh = coe

c

Ih

V = c

obc

Ih

V = e

oce

Ih

V

Vi

hi

hrVo hfIi ho 1 Vo

Ii Io



Conversion of Hybrid Parameters

CE to CB CE to CC CB to CE CB to CC

=+1

ieib

fe

hh

h =ic ieh h =

+1ie

iefb

hh

h =

+1ib

icfb

hh

h

= −+1ie oe

rb refe

h hh h

h = − ≈1 1rc reh h = −

+1ib ob

re rbfe

h hh h

h −

= ++

11

ib obrc rb

fb

h hh h

h

−=

+1fe

fbfe

hh

h = − −1fc feh h

−=

+1fb

fefb

hh

h

−=

+1

1fcfb

hh

=+1

oeob

fe

hh

h =oc oeh h =

+1ob

oefb

hh

h =

+1ob

ocfb

hh

h

h-Parameter Common Emitter

Common Base

Common Collector

ih 1000 Ω 20 Ω 1000 Ω

rh 0.00025 0.0003 ≈1

fh 50 -0.98 -50

oh 25 μS 0.5 μS 25 μS

3. Relation between r-parameter and h-Parameter

r-Parameter h-Parameter

αac fbh

βac feh

er ' re

oe

hh

br ' ( )− +1reie fe

oe

hh h

h

cr ' + 1re

oe

hh


2-23




E. .POWER AMPLIFIERS.

1. In terms of Input Output Relation

Voltage-controlled Voltage source (Voltage Amplifier) The output is a voltage source that is a function of input voltage.

Current-controlled Current source (Current Amplifier) The output is a current source that is a function of input current.

Voltage-controlled Current source (Transconductance Amplifier) The output is a voltage source that is a function of input current.

+

Ro

RL R AvVi

Rs

vs V +

-

Ro RL R

gmVi

Rs

vs V -

Ro RL Ri

AiIi

Rs is

Ii



Current-controlled Voltage source (Transimpedance Amplifier) The output is a current source that is a function of input voltage

Parameter Voltage Amplifier

Current Amplifier

Transconductance Amplifier

Transimpedance Amplifier

Input Resistance ( )→ ∞ sR ( )→ s0 R ( )→ ∞ sR ( )→ s0 R

Output Resistance ( )→ L0 R ( )→ ∞ LR ( )→ ∞ LR ( )→ L0 R

Transfer Characteristics

ν=ν

ov

sA = o

is

iA

i =

νo

ms

ig

ν= o

ms

ri

2. Amplifier Class

Class A Class A amplifier operates entirely in the linear region of the transistor’s characteristic curves. The transistor conducts during the full 360° of the input cycle. Ideally, this class produces very little distortion, however consumes a lot of power and is also least preferred.

Ro

RL Ri

ZmIi

Rs is

Ii

VCE (V) 0 2 4 6 8 10 12 14 16 18

20

40

60

80

I B (μA

)

2

4

6

8

10

IC (mA)

Ic

ICQ

VCEQ

Vce

Ib IBQ

Q-point


2-25




η = = = ∴CQ CEQoutmax

dc CQ CEQ

0.5I VP0.25 25%

P 2I V

Class B The transistor is "on" for only half the cycle (exactly 180°) of a sine wave and is also very typically used in push-pull amplifier circuits. Ideally this class produces mostly odd order distortion. In audio applications it is believed that odd order distortion is not pleasing to hear. It is difficult to build a low distortion Class B amplifier and hence Class AB is almost universal.

The class B amplifier is biased at the cutoff point so that ICQ=0 and VCEQ=VCE(cutoff).

η = = π = ∴c(sat) CCoutmax

dc c(sat) CC

0.25I VPx 0.79 79%

P I V

VCE (V) 0 2 4 6 8 10 12 14 16 18

20

40

60

80

I B (μA

)

2

4

6

8

10

IC (mA)

Ic

ICQ

VCEQ

Vce

Ib IBQ

Q-point



Comparison between Class A and Class B Large Signal Amplifiers

Quantity of Interest Class A Class B

Output voltage swing −max minV V −max minV V

Output current swing −max minI I −max minI I

Maximum output power = CC Qmax max V IV I2 2

= CC Qmax max V IV I2 2

Average supply power CC Q2V I πCC Q2V I

Maximum conversion efficiency

25% 79%

Class AB The transistor is "on" for slightly more than half the cycle (>180°) of a sine wave and is the most common configuration used in push-pull audio power amplifiers. In push-pull amplifiers, Class AB produces mostly odd order distortion, however it is far more power efficient than Class A.

VCE (V) 0 2 4 6 8 10 12 14 16 18

20

40

60

80I B

(μA

)

2

4

6

8

10

IC (mA)

Ic

ICQ

VCEQ

Vce

Ib IBQ

Q-point


2-27




Class C The transistor is "on" less than half the cycle of a sine wave. We say <180° of conduction. This class produces both even and odd order distortion however is very efficient.

η = ≈ ∴+

outmax

out d(avg)

P1 100%

P P

Sample Questions: Determine the efficiency of a class C amplifier driven by a 500 kHz signal if the transistor is on for 1.5 μs and the amplifier is operating over 100% of its loadline. Use Ic(sat) =100 mA and Vce(sat) = 0.25 V, VCC = 25 V, and Rc =100 Ω. Solution:

out

out d(avg)

P 3.1250.994

P P 3.125 0.01875

99.4% answer

η = = =+ +

= ⇒

( ) ( ) ( ) ( ) ( ) ( )( )

on

22CC

c

1500 kHz

t 1.5 sd(avg) c sat ce satT 2 s

0.5 250.5Vout R 100

Note :

T 2 s

P I V 100 mA 0.25 V 18.75mW

P 3.125W

μμ

∴ = = μ

∴ = = =

∴ = = =

VCE (V) 0 2 4 6 8 10 12 14 16 18

20

40

60

80

I B (μA

)

2

4

6

8

10

IC (mA)

Ic

ICQ

VCEQ

Vce

Ib IBQ

Q-point



Class D The transistor is either fully on or fully off. This class is the most efficient, because either the transistor is off or the current is zero (so the amount of power wasted heating up the transistor is zero), or the transistor is fully on and the voltage across it is very close to zero (so the amount of power wasted heating up the transistor is very close to zero).

This class has the most distortion.

SUMMARY

Amplifier Class

Q-point Location

Current Conduction

Angle

Maximum Efficiency

Class A Linear region 360° 25%*

Class B Cutoff region 180° 78.5%

Class AB Above cutoff Between

180 and 360° Between

25% to 78.5%

Class C Below cutoff Less than 180° ≈100%

*50% (transformer coupled) F. .MULTISTAGE AMPLIFIER CONFIGURATIONS.

Two or more amplifier can be connected in a cascaded arrangement with the output of one amplifier driving the input of the next. The basic purpose of multistage arrangement is to increase the overall voltage gain.

1. Overall Voltage Gain

= ∗v(T) v1 v2 vnA A A A

= + +v(T)dB v1(dB) v2(dB) vn(dB)A A A A

2. Methods of Coupling

Direct Coupling The method of coupling that uses the least number of circuit elements and that is, perhaps, the easiest to understand is direct coupling. In direct coupling the output of one stage is connected directly to the input of the following stage.

Av1 Av2 Avn Input Output


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Direct coupling provides a good frequency response since no frequency sensitive components (inductors and capacitors) are used. The frequency response of a circuit using direct coupling is affected only by the amplifying device itself.

RC Coupling

The most commonly used coupling in amplifiers is RC coupling. This arrangement allows the coupling of the signal while it isolates the biasing of each stage. This solves many of the problems associated with direct coupling.

RC coupling does have a few disadvantages. The resistors use dc power and so the amplifier has low efficiency. The capacitor tends to limit the low frequency response of the amplifier and the amplifying device itself limits the high-frequency response.

Impedance Coupling

Impedance coupling is very similar to RC coupling. The difference is the use of an impedance device (a coil) to replace the load resistor of the first stage.

Transformer Coupling

Transformer coupling is very often used for the final output (between the final amplifier stage and the output device) because of the impedance-matching qualities of the transformer. The frequency response of transformer-coupled amplifiers is limited by the inductive reactance of the transformer just as it was limited in impedance coupling.

3. System Bandwidth

=−

1n

nBW

BW2 1

2 1BW f f= −

1n

11(n)

ff

2 1=

−

1n

2(n) 2f f 2 1= −

4. Gain-Bandwidth Product

The gain-bandwidth product is a transistor parameter that is constant and equal to the unity-gain frequency. Also known as the amplifiers figure of merit.

= v(mid)A xBW A xBW



G. .OTHER CLASSIFICATIONS.

1. According to frequency response

Audio Amplifier An audio amplifier is designed to amplify frequencies between 15 Hz and 20 kHz. Any amplifier that is designed for this entire band of frequencies or any band of frequencies contained in the audio range is considered to be an audio amplifier.

RF Amplifier

These amplifiers are designed to amplify frequencies between 10 kHz and 100,000 MHz. A single amplifier will not amplify the entire RF range, but any amplifier whose frequency band is included in the RF range is considered an RF amplifier.

Video Amplifier

A video amplifier is an amplifier designed to amplify a band of frequencies from 10 Hz to 6 MHz. Because this is such a wide band of frequencies, these amplifiers are sometimes called wide-band amplifiers.

2. According to function Other amps may be classified by their function or output characteristics. These functional descriptions usually apply to complete amplifier systems or sub systems and rarely to individual stages.

Servo Amplifier

A servo amp indicates an integrated feedback loop to actively control the output at some desired level. These are often used in mechanical actuators, or devices such as DC motors that must maintain a constant speed or torque. An AC servo amp can do this for some ac motors.

Linear Amplifier A linear amp denotes that it has a precise amplification factor over a wide range of frequencies, and is often used to boost signals for relay in communications systems.

Op Amp A special type of low power amp with almost ideal characteristics is used in instruments and for signal processing, among many other varied uses. These are known as operational amplifiers, or op-amps. This is because this type of amplifier is used in circuits that perform mathematical algorithmic functions, or "operations" on input signals to obtain specific types of output signals.


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3. According to Configuration

Darlington Configuration (Superbeta) This emitter follower has a pair of transistors in the Darlington configuration. In this arrangement, the emitter current of one transistor becomes the base current of the second. The Darlington configuration acts like one transistor with a beta which is the product of the betas of the two transistors. They are used where high output currents are needed. The input impedance of the Darlington configuration is quite high.

Cascode Configuration

4. Specialty classes

Class D A class D amplifier is a power amplifier where all power devices are operated in on/off mode. Output stages such as those used in pulse generators are examples of class D amplifiers.

Class E/F The class E/F amplifier is a highly efficient switching power amplifier, typically used at radio frequencies. The main concept used in this amplifier is to model the active switching device, such as a transistor or MOSFET, as a linear combination of two parts:

A (theoretical) "perfect" switching element, and A complex network of parasitic elements attached to it

(capacitors, inductors and resistors).

E

C

B

E C

B

E

C

B

E

C

B

A cascade connection has one transistor on top of (in series with) another. This arrangement is designed to provide high input impedance with low voltage gain to ensure that the input Miller capacitance is at a minimum.

β = β βD 1 2



Class G Class G amplifiers are a more efficient version of class AB amplifiers, which use "rail switching" to decrease power consumption and increase efficiency. The amplifier has several power rails at different voltages, and switches between rails as the signal output approaches each. Thus the amp increases efficiency by reducing the "wasted" power at the output transistors.

Class H

Class H amplifiers are similar to Class G, except that the power supply voltage "tracks", or is modulated by, the signal. The power supply is always kept slightly higher than the actual power required. Often it has two power supplies, like the class G, and only the higher is modulated.

Transistor and stuff introduction for young enthusiast technician

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