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NE545B View Datasheet(PDF) - Unspecified

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NE545B
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NE545B Datasheet PDF : 20 Pages
First Prev 11 12 13 14 15 16 17 18 19 20
S I G N E T I C SD O L B Y - BN O I S EP R O C E S S O RI 5 4 5
F I G U R E2 7
I M P E D A N C EC O N V E R T E R
BEFERFNCL
VOLTAGF
N O T E :A L L R E S I S T O RS T A N D A R DA N D A R E f u l E A S U R E IDN O H M S
C I R C U I TD E S C R I P T I O N( c o n ' t . )
Amplifier'A'(Figure 27) is a unity gain impedance
converter.providing high input and a given output impe-
d a n c e .A D a r l i n g t o nc o n fi g u r a t i o ni s u s e df o r m i n i m u m b i a s
current, keeping the output offset with respect to the
r e f e r e n c ev o l t a g et o a m i n i m u m . R 2 1 g i v e st h e a p p r o p r i a t e
s o u r c e i m p e d a n c ef o r t h e e x t e r n a l l o w p a s sf i l t e r r e q u i r e d
t o e l i m i n a t e s u p e r s o n i cs i g n a l so r i g i n a t i n gf r o m F M t u n e r s
or from the bias and erase oscillators of tape recorders.
One of the most challengingaspectsof the lC design
w a s t h e d e v e l o p m e n to f a v a r i a b l er e s i s t o r( b l o c k F ) w i t h
a w i d e d y n a m i c r a n g e a n d a r e s i s t a n c ev e r s u s c o n t r o l
voltage law similar to that of the field effect transistor
e m p l o y e d i n t h e d i s c r e t ec o m p o n e n t c i r c u i t .
The drain source conductance Gp5 of a field effect
transistor operating as a variable resistor is given by
G
o
s=
2
.
I
i
ns
ix
s
Vo'
[
(
v
c
s
-
Vns
---:
2
:
)
-vYn-l
where IDSS is drain saturationcurrent, Vp is gate pinch-
o f f v o l t a g e , V 6 5 i s g a t e - s o u r c eb i a s , V p 5 i s d r a i n - s o u r c e
voltage.
Provided the signal level applied to the device is small,
G p g i st h e r e f o r ea l i n e a rf u n c t i o n o f t h e g a t e - s o u r c veo l t a g e .
F o r a f o r w a r d b i a s e dj u n c t i o n , t h e s l o p e r e s i s t a n c eR J i s
givenby
KT
R l=-
ol
w h e r e k i s B o l t z m a n n ' sc o n s t a n t ,T i s a b s o l u t et e m p e r a t u r e ,
q is electronic charge,and I is forward current through
junction.
H e n c e t h e s l o p e c o n d u c t a n c eo f t h e j u n c t i o n G 1 i s a l i n e a r
function of the forward current, and it is possibleto match
the FET.
T h e c h o s e nv a r i a b l er e s i s t o rc i r c u i t i s s h o w n i n i t s b a s i cf o r m
in Figure28. BVvaryingthe currentthrough the differential
pair 01, 02 with the current sources1 and 2, the input
r e s i s t a n c ea t p o i n t 3 c h a n g e sa n d i t s v a l u e i s a p p r o x i m a t e l y
26 X 10'3
2 X_ohms
at room temoerature.
 

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