S I G N E T I CDSO L B Y - BN O I S EP R O C E S S OrR5 4 5
D E S I G NP H I L O S O P H Y
T h e r e q u i r e m e n t sd e s c r i b e da b o v ew e r e m e t b y t h e d i s c r e t e
c o m p o n e n t c i r c u i t a l r e a d y i n e x t e n s i v eu s e a t t h e t i m e o f
t h e b e g i n n i n g o f t h e i n t e g r a t e dc i r c u i t d e v e l o p m e n t p r o -
gramme.
However, it is obviousthat from the hardwaremanufac-
turer's viewpoint the introduction of a noise reduction
system of such sophisticationinvolves extra complexity
d u r i n g a s s e m b l ya n d p a r t i c u l a r l yd u r in g t e s t i n g ,w h e n c a r e
a n d a c c u r a c ya r e r e q u i r e d i n m a k i n g t w o p r e - s e ta l i g n m e n t
adjustments.
T h e r e f o r e p r i m a r y d e s i g n c o n s i d e r a t i o n sf o r t h e D o l b y
B-type lC were compatibility with the existing discrete
c o m p o n e n tp r o c e s s oar n d e l i m i n a t i o no f t h e i n t e r n a la l i g n -
ment procedures.
O n c et h e f e a s i b i l i t yo f a n l C p r o c e s s o rh a d b e e ne s t a b l i s h e d ,
i t s e e m e dr e a s o n a b l et o a i m f o r a d e s i g nw h o s e i n t e r f a c i n g
parameters (input and output levels and impedances,
s w i t c h r e q u i r e m e n t sc, l o s et o l e r a n c ec o m p o n e n t s ,e t c . ) w e r e
s i m i l a rt o t h o s eo f t h e e x i s t i n gd i s c r e t ec o m p o n e n tc i r c u i t ,
s o t h a t t h o s e l i c e n s e e sw h o w a n t e d t o u s e t h e n e w i n t e -
grated circuit could transfer to it with a minimum of
d e s i g nc h a n g e s .
C I R C U I TD E S C R I P T I O N
Figure 33 shows the block diagram of the lC with all
functional block lettering the same as on Figure 25. The
l C i n c l u d e s f o u r i d e n t i c a lo p e r a t i o n a la m p l i f i e r s" B , C , D ,
a n d E K " , t w o b u f f e r a m p l i fi e r s , a v a r i a b l e r e s i s t o r , a
rectifier system and a referencebias supply.
Choosing an operational amplifier design for the gain
b l o c k s h a s t h e a d v a n t a g eo f p r o v i d i n g a m i n i m u m o f o f f s e t
for DC coupling, a largeoutput swing and a low output
i m p e d a n c e . F i g u r e 2 6 s h o w s t h e b a s i c c i r c ui t o f t h e
a m p l i f i e r . T h e i n p u t c o n s i s t so f a d i f f e r e n t i a l P N P t r a n s i s -
tor pair Ot, OZ which drives a Darlington stage05, 06
followed by a classAB output stage07, Og. Frequency
compensation, necessary in all two-stage amplifiers for
stableclosed loop operation, is provided by capacitorC1.
ResistorsRq, RO, R7, set the quiescentcurrent in the
o u t p u t s t a g e ,w h i l e R g l i m i t s t h e s h o r t c i r c u i t c u r r e n t a n d
isolatesthe output from capacitiveloads,which otherwise
m i g h t c a u s eo s c i l l a t i o nd u e t o e x c e s s i v ep h a s es h i f t a t h i g h
f r e q u e n c i e s .T h e c l o s e d - l o o p g a i n o f e a c h a m p l i fi e r i s
determined by internal resistors.The output swing and
d i s t o r t i o n c h a r a c t e r i s t i c so f t h e a m o l i f i e r s a r e s h o w n i n
F i q u r e s6 a n d 1 0 .
F I G U R E2 6
B A S I CO P E R A T I O N A AL M P L IF I E R
I
I
I
REFERENCE
V O L T A GE
N O T E :A L L B E S I S T O B S T A N D A R DA N D A R E I v I E A S U B E lDN O H M S .