Crown Studio Reference I Service Manual download pdf (Page 20)

Figure

4.3

simplified

exampfe

the

grounded

bridgs output topology.

consists of

four

quadrants

three deep DarDington (composite) emitter-follsw~r

stages per channel: one

NPN

and

one

PNP

the

High Side of

the

bridge (driving the

Isad),

and

apse

NPN

and

one

PMP

the

Low

Side

of the bridge (con-

trolling

the ground reference for %Re rails).

The

output

stages

are biased

ta operate

class

AB+B

far ultra

low

distortion

the signal zero-crossing region and

high

efficiency.

4,5.%

High

Side

[NS)

Thhe

High

Side (HS)

af the bridge operates much like

conventional

bipolar push-pull output configuration,

the input drive vsitage becomes more po%itive, the

NPN

conducts

and

delivers positive voltage

the

10ad,

Eventually

the

NPN

devices reach

full

conduc-

tion

and

+Vcc

acrsss the

load,

this time the

PNP

biased

off.

When

the

drive

signal is negative

going,

"Be

PNP

canducts to deliver -Vcc to the load

and

the

NPN

stage

off.

The

output

the

+LVA

drjvm the base

$8.~

pradriver

device.

Sgsgethar, the predriver

and

driver farm

the

first

two parts

the three-deep Darlington and are

biased

class

AB,

They

provide output drive

through

the

bias

resistorx

bypassing

the

output devicas,

Bevels

be-

Isw

about

180mW,

RLC

network between

the

predriver and driver

provide

phase

shift

csmpensa-

tion

and

limit driver

base

current

safs Bevels. Output

devices

are biased class

Just

below

s%sculotf.

At about

1QOmW

blr~tpat

they

switch on

eonduct

high

current

the

load. Together with predriver

and

driver? the out-

put

device provides

averall cFass

AB+B

output,

The

negative half

the

almsst

identical

the

positive half, except that

the

devices are

PNP

One

difference

that the

PNP

bias

resistor is

slightly

greater

value

that

PNP

output

devices

run

closer

the

cutoM

level

under static

(no

signal)

conditions,

This

because

PNP

devices require greater drive current.

bias

rsgulated

Q18,

the

Bias

Seavs,

QlS

Vbe

multipiier which maintair-rs approximately

3,2V

Vee

under static conditions,

The

positive

and

negative

halves

the

output

are

parallel

with

this

3.2V

With

full base-emitter on voltage

drop

across

predrivers

and

drivers, tha balance of

voltage

rasuits

in approximately

.3V

drop across

the

bias

resistors in

the positive half,

and

about

.5V

acrsss

The

bias resis-

tor in the negative

half,

bn$

~~nd~clior~

{and

thus bias)

diode suing pr~vents

excessive

charge

build

within the

high

conduction output devices

whew

off,

Flyback

diodes

shunt

bask-EMF

pulses

from

reactive

loads

the

power supply to protect output devices

from

dangerous reverse voltage levels. An output

ter-

minating circuit blocks

output

lines

from

enter-

ing

the

amplifier

through its output connectors,

4,5,2

L@w

Side

(L8)

The

tow Side

(LS)

operates

quite

differently.

The

power

supply

bridge

rectifier

is not ground referenced, nor

the

secondary

the main transformer,

othsr

wards,

tha

high

vsltage power supply

floats

with respect

ground,

but

~Vcc remain canstant with respect

each

other,

This

allows

the pswer

supply

to deliver

+Vcc

and

-Vcc

from

the

same

bridge

rectifier

and

fiitsr

total difference

potential, regardless

their voltages

with

respect

ground.

The LS

uses

inverted feed-

back

from

the HS

output

control

the

ground refer*

ence

for

the

rails

(AVCC),

Both

quadrants are ar-

ranged

three-deep Darlington

and

are

biased

AB+B

the

same

manner as

thc

WS,

When

the

amplifisr output swings positive,

the

audio

fed to an

sp-amp

stage

where

inverted,

This

inverted signal is delivered direeIIy to

the

bases

the

positive

(NPN)

and

negative

(PNP)

predrrivers.

The

negative drive forces the

PMP

devices on

(NPN

off),

the

PNP

devices

coaduct, VCB

the

PNP

Darlington drops.

With

dsviics emitters tied

ground, -Vcc

pulled toward ground reference.

Since

the pswer supply

not ground referenced (and the

tatai

vsltage from

aVcc

-Vcc

constant)

+VGC

forced

higher

above

ground potential.

This

continues

until,

the

positive

amplifier output peak,

-Vcc

and

+VGC

eq~aIs the total

power

supply potential with

positive pslarily'

the

Reference

for

example,

%he

power

supply

produces

totat

af14488

from rail ts

rail

(k72VDC

me?asur@d

from

ground

with

signal),

therefore,

the

amplifier output can reach

positiva

peak

+"I

44v*

Cosrversely, during a negative swing

the

output

where

PNP

devices conduct,

the

op-amp

would

output

positive

voltage forcing

NPN

devices

canduct.

This

would

result

+Vcc

swinging

tsward

ground

potential

and

-Vcc

swinging

further from

groidnd

patentraf.

tl-$6

negative

amplifier sutpvt

peak,

SVCC

and

-Vcc

equals

the tstal power supply

potential with

negative pofariWy. Using

the

same

ex-

Circuit

Theory

4-4

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 ... 96 97

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