A constant-current generator in the audio-frequency range
by William D Sherwood
A THESIS Submitted to the Graduate Faculty In partial fulfillment of the requirements for the degree
of Master of Science In Electrical Engineering
Montana State University
© Copyright by William D Sherwood (1952)
Abstract:
The purpose of this thesis is to describe the design and operation characteristics of a constant-current
generator. The generator was designed to have a constant load current as the load resistance changes.
The circuit was designed to have a signal voltage gain that varies in a linear manner as the load
resistance is changed. The output signal voltage changes in a direct relationship with the load
resistance, maintaining a constant proportionality between the two. This ratio of proportionality is the
desired constant current.
The design method used was to take an amplifier circuit with desirable characteristics and improve
upon its gain characteristic. Good gain, large power capacity, and being capable of handling large
signal input without distortion are the desired characteristics. A 6L6 beam-power tube with an
unby-passed cathode resistor was selected for the amplifier circuit, A second 6L6 beam tube was
placed in the plate circuit of the amplifier to act as a constant d-c current device. This improves the gain
characteristic of the amplifier at the higher values of load resistance by making the amplifier plate
voltage more-or-less independent of its plate current. The gain characteristic of the amplifier was then
made linear by providing a positive feedback to the grid. It was necessary to provide a nonlinear
feedback voltage which increased exponentially with the load resistance. This was accomplished by
using the inverse current characteristics of two matched germanium diodes. A signal voltage from this
diode circuit was amplified by a 6J5 triode tube. The feedback signal was obtained from the plate
circuit of this amplifier with the proper phasing for positive feed back.
The constant-current generator works very well for a large range of load resistance variation for small
values of load current, but the range is somewhat limited for load currents over 4 ma. The frequency
response is very good in the range from 500 to 5,000 cps. The 3-db. loss levels are outside the desired
frequency range of 100 to 10,000 cps except on the low-frequency side at high values of load
resistance. The low-frequency response could have been improved by proper design of the R-C
circuits, Two examples of uses that may be made of this device are discussed at the end of the paper. A CONSTANT-CURRENT GENERATOR IN THE
AUDIO-FREQUENCY RANGE
Iy
WILLIAM D. SHERWOOD
A THESIS
S u b m itted to th e Q raduate F a c u lty
In
p a r t i a l f u l f i l l m e n t o f th e re q u ire m e n ts
f o r th e d eg ree o f
M aster o f S c ie n c e In E l e c t r i c a l E n g in eerin g
at
Montana S ta te C o lleg e
Approvedi
a d , M ajor D epartm ent
Chairm an, Sxami
Dean, G raduate D iv is io n
Bozeman, Montana
J u n e , 1952
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j i n
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S h S '- f t
TABLE OF CONTENTS
Acknowledgment
........................................................................................................
3
A b s t r a c t ............................................ .......................................................................................
I n tr o d u c tio n
................................
5
Problem A n a l y e l a .................................................................
C i r c u i t A nalyela
.
,
.
&
. . , . . 1 0
D lseu aeio n o f R e s u l t s ................................
. , 2 0
Summary and C o n c l u s i o n s .................................................................................................29
L i s t o f Equipment
..............................................................................................................32
L i t e r a t u r e C ite d and C o n su lted ..............................................
,
.
.
.
.
33
-3 -
ACKNOWLSDGMENT
T h is t h e s i s work was u n d e rta k e n a t th e s u g g e s tio n o f P r o fe s s o r
R. c.
S e lb e l o f th e E l e e t r l o a l E n g in eerin g D epartm ent, o f M ontana S ta te
C o lle g e a t Boseman, M ontana.
The a u th o r w ish es to e x p re ss h ie s in c e r e
a p p r e c ia tio n to th e members o f th e E l e c t r i c a l E h g ln eerln g s t a f f f o r th e
h e lp and s u g g e s tio n s o f f e r e d , and In p a r t i c u l a r to P ro fe s s o r S e ib e l under
whose s u p e r v is io n t h i s work was com pleted.
103057
ABSTRACT
The purpose o f t h i s t h e s i s i s to d e s c rib e th e d e sig n and o p e r a tio n
c h a r a c t e r i s t i c s o f a c o n s ta n t- c u r r e n t g e n e r a to r . The g e n e ra to r was
d e sig n e d to have a c o n s ta n t lo a d c u r r e n t as th e lo a d r e s is ta n c e c h an g e s.
The c i r c u i t was d e sig n e d to have a s ig n a l v o lta g e g a in t h a t v a r ie s i n a
l i n e a r manner a s th e lo a d r e s i s t a n c e i s changed. The o u tp u t s ig n a l v o lta g e
changes i n a d i r e c t r e l a t i o n s h i p w ith th e lo a d r e s i s t a n c e , m a in ta in in g a
c o n s ta n t p r o p o r tio n a l ity betw een th e two. T his r a t i o o f p r o p o r tio n a l ity i s
th e d e s ir e d c o n s ta n t c u r r e n t.
The d e s ig n method u sed was t o ta k e an a m p lif ie r c i r c u i t w ith d e s ir a b le
c h a r a c t e r i s t i c s and improve upon i t s g a in c h a r a c t e r i s t i c . Good g a in , la r g e
power c a p a c ity , and b e in g c a p a b le o f h a n d lin g la r g e s ig n a l in p u t w ith o u t
d i s t o r t i o n a re th e d e s ir e d c h a r a c t e r i s t i c s . A 6L6 beam-power tu b e w ith an
u n b y -p assed c ath o d e r e s i s t o r was s e le c te d f o r th e a m p lif ie r c i r c u i t . A
second 6 Lb beam tu b e was p la c e d i n th e p l a t e c i r c u i t o f th e a m p lif ie r to
a c t as a c o n s ta n t d -c c u r r e n t d e v ic e . T his laqiroves th e g a in c h a r a c te r ­
i s t i c o f th e a m p lif ie r a t th e h ig h e r v a lu e s o f lo a d r e s is ta n c e by making
th e a m p lif ie r p l a t e v o lta g e m o re -o r -le s s In d ep en d en t o f i t s p l a t e c u r r e n t.
The g a in c h a r a c t e r i s t i c o f th e a m p lif ie r was th e n made l i n e a r by p ro v id in g
a p o s itiv e fe e d b ac k to th e g r i d . I t was n e c e s sa ry to p ro v id e a n o n lin e a r
fe e d b ac k v o lta g e which In c re a s e d e x p o n e n tia lly w ith th e lo a d r e s i s t a n c e .
T his was accom plished by u s in g th e in v e rs e c u r r e n t c h a r a c t e r i s t i c s o f two
m atched germanium d io d e s . A s i g n a l v o lta g e from t h i s d io d e c i r c u i t was
a m p lifie d by a 6J5 tr io d e tu b e . The feed b ack s ig n a l was o b ta in e d from th e
p l a t e c i r c u i t o f t h i s a m p lif ie r w ith th e p ro p e r p h a sin g f o r p o s i t i v e fe e d ­
b a ck .
The c o n s ta n t- c u r r e n t g e n e r a to r works v e ry w e ll f o r a la r g e ra n g e o f
lo a d r e s i s t a n c e v a r i a t i o n f o r s m a ll v a lu e s o f lo a d c u r r e n t, b u t th e range
i s somewhat li m i t e d f o r lo a d c u r r e n ts o v e r 4 ma. The freq u en cy re sp o n se
i s v ery good I n th e ra n g e from 500 to 5 ,0 0 0 o p s . The 3 -d b . l o s s l e v e l s
a re o u ts id e th e d e s ir e d freq u e n cy ran g e o f 100 to 10,000 o p s. e x c e p t on th e
lo w -freq u en cy s id e a t h ig h v a lu e s o f lo a d r e s i s t a n c e . The lo w -freq u en cy
re s p o n se co u ld have been im proved by p ro p e r d e s ig n o f th e IWI c i r c u i t s .
Two exam ples o f u s e s t h a t may be mude o f t h i s d e v ice a re d is c u s s e d a t
th e end o f th e p a p e r.
IOT RCDDCTION
The problem s e le c te d was to d e sig n end c o n s tr u c t a c o n s ta n t- c u r r e n t
g e n e r a to r to o p e ra te i n th e a u d io -fre q u e n c y ran g e from 100 to 10,000
c y c le s p e r sec o n d .
The problem can be d iv id e d in to two ty p e s t
(a ) The c o n s ta n t- c u r r e n t d e v ic e which produces a c o n s ta n t- c u r r e n t
o u tp u t when th e in p u t v a r i e s .
(b ) The c o n s ta n t- c u r r e n t d e v ice which g iv e s a c o n s ta n t- c u r r e n t
o u tp u t as th e o u tp u t lo a d impedance v a r i e s .
The second ty p e I e th e one developed and d is c u s s e d i n t h i s p a p e r.
C o n sid e ra b le r e s e a r c h h as been done and a la r g e number o f papers
have been w r itt e n on c o n s ta n t- c u r r e n t d e v ic e s u s in g d i r e c t - c u r r e n t , b u t,
to th e a u th o r 's know ledge, no work co m p arativ e to t h i s t h e s i s has been
done w ith th e a lt e r n a t i n g - c u r r e n t s o v e r th e freq u en cy ran g e u se d h e re .
Due to th e in c r e a s in g tr e n d tow ard th e u se o f e l e c t r o n i c computing
and c o n tr o l d e v ic e s , t h i s in v e s t ig a ti o n was th o u g h t to be w o rth w h ile.
A lthough th e a u th o r knows o f no p a r t i c u l a r c i r c u i t where a d e v ic e o f t h i s
ty p e i s u sed a t th e p r e s e n t tim e , i t s f u tu r e p o s s i b i l i t i e s sh o u ld n o t be
o v e rlo o k e d i n c o n n e c tio n w ith th e afo rem en tio n ed ty p e s o f e le c tr o n i c
c irc u its .
The a b b re v ia tio n s and te c h n ic a l term s u sed i n t h i s p a p er a re th e
coBnon and a c c e p ted term s a s u sed i n th e r a d io e n g in e e rin g l i t e r a t u r e .
The e x p e rim e n ta l d a ta was o b ta in e d from a m p lifie rs b u i l t by the
a u th o r.
s ig n a l.
A com m ercial ty p e o f s ig n a l g e n e r a to r was used f o r th e in p u t
PROBLEM ANALYSIS
S e v e ra l ty p e s o f a m p lif ie r c i r c u i t s were c o n sid e re d and th e f i r s t
one s e le c te d was a d o u b le -c a th o d e -fo llo w e r c i r c u i t w hich, when analyzed
m a th e m a tic a lly , seemed to p r e s e n t th e p ro p e r c h a r a c t e r i s t i c s .
were ru n on t h i s c i r c u i t i t d id n o t o p e ra te as e x p e c te d .
When t e s t s
F u r th e r a n a ly s is
showed t h a t th e tra n sc o n d u c ta n c e and a m p lif ic a tio n f a c t o r o f th e tu b e
v a r ie d in a n o n lin e a r f a s h io n as th e lo a d r e s i s t a n c e was in c r e a s e d . There
was n o t a l i n e a r c o r r e l a t i o n betw een th e o u tp u t v o lta g e and lo a d r e s i s t a n c e
as d e s ir e d and th e c i r c u i t was d is c a rd e d .
The f i r s t a tte m p t to produce a c o n s ta n t- c u r r e n t g e n e r a to r u sin g th e
c a th o d e -fo llo w e r c i r c u i t gave th e w r i t e r a b e t t e r i n s i g h t i n t o th e problem
by showing th e way to th e f i n a l c i r c u i t and th e method by which to approach
it.
The problem was one o f d e v e lo p in g an a m p lif ie r c i r c u i t w ith a g a in
t h a t in c re a s e d i n d i r e c t p ro p o r tio n to th e lo a d r e s i s t a n c e .
The method o f
approach was to do th e e x p e rim e n ta l work p r i o r to th e a n a l y t i c a l work.
I t i s w e ll known t h a t th e g a in o f an a m p lif ie r in c r e a s e s as th e lo a d
r e s i s t a n c e i s in c r e a s e d .
T his g a i n - c h a r a c t e r i s t i c i s ap p ro x im ately l i n e a r
o v e r a v e ry s m a ll ran g e o f lo a d r e s i s t a n c e v a r i a t i o n .
The improvement o f
t h i s g a i n - c h a r a c t e r i s t i c was th e b a s ic problem to be c o n s id e re d .
To improve th e g a i n - c h a r a c t e r i s t i c o f th e a m p lif ie r a c o n s ta n t d -c
c u r r e n t d e v ic e was p la c e d i n th e p l a t e c i r c u i t as shown i n F ig . I .
The
a d d itio n o f t h i s c i r c u i t improved th e g a i n - c h a r a c t e r i s t i c b u t i t was s t i l l
f a r from th e d e s ir e d l i n e a r g a in -lo a d r e l a t i o n s h i p .
The c i r c u i t g a i n - c h a r a c t e r i s t i c co u ld be f u r t h e r improved by a non­
l i n e a r p o s i t i v e fe e d b a c k .
I t was n e c e s sa ry to have more g a in a t th e
h ig h e r v a lu e s o f lo a d r e s i s t a n c e w h ile th e g a in a t th e lo v e r v a lu es o f
lo a d r e s i s t a n c e rem ained unchanged.
Diode vacuum tu b e s have p l a t e c u r r e n t -
p l a t e v o lta g e c h a r a c t e r i s t i c s such t h a t th e p l a t e c u r r e n t in c re a s e s as an
e x p o n e n tia l fu n c tio n o f th e p l a t e v o lta g e .
The p o s s i b i l i t y o f u s in g t h i s
type o f c h a r a c t e r i s t i c curve was in v e s t ig a te d .
I t was d eterm in ed t h a t th e
amount o f r e s i s t a n c e needed i n s e r i e s w ith th e d io d es was a t l e a s t
100,000 ohms.
T his amount o f r e s i s t a n c e caused th e diode c h a r a c t e r i s t i c
to f l a t t e n o u t u n t i l i t was no lo n g e r th e d e s ir e d c u r r e n t- v o lta g e r e l a t i o n ­
s h ip .
A T h y rite type o f v a r i s t o r was known to have a h ig h r e s i s t a n c e and
th e d e s ir e d s y m e tric a l c u r r e n t- v o lta g e c h a r a c t e r i s t i c .
The u n s u c c e s s fu l
e f f o r t t h a t was made to o b ta in t h i s type o f v a r i s t o r le d to th e c o n sid e r­
a tio n o f th e in v e rs e c h a r a c t e r i s t i c s o f germanium c r y s t a l d io d e s .
The
in v e r s e c u r r e n t- v o lta g e r e l a t i o n s h i p o f germanium d io d es e x h ib its h ig h
r e s i s t a n c e and a p p ro x im ately th e d e s ir e d e x p o n e n tia l v a r i a t i o n needed f o r
th e feed b ack c i r c u i t .
To g e t a s y m e tric a l c h a r a c t e r i s t i c from th e
germanium d io d e f o r a l t e r n a t i n g v o lta g e s , a m atched p a i r o f germanium
d io d e s were co n n ected c a th o d e -to -c a th o d e .
These d io d e s , p a r a l l e l e d by a
r e s i s t o r , and r e s i s t a n c e , were p la c e d i n p a r a l l e l w ith th e lo a d as shown
i n F ig . I .
T hat p o r tio n o f th e o u tp u t s ig n a l v o lta g e a p p e a rin g a c ro ss
i s a p p lie d to th e g r id o f a t r i o d e a m p lif ie r .
The s ig n a l o u tp u t from
th e p l a t e c i r c u i t o f t h i s a m p lif ie r i s f e d back to th e g r id o f th e beamtu b e a m p lif ie r as p o s itiv e fe e d b a c k .
When th e e x p e rim e n ta l ru n v&a made on t h i s com plete c i r c u i t i t was
found t o be a v e ry good a p p ro x im atio n to th e d e s ir e d l i n e a r r e l a t i o n
betw een o u tp u t v o lta g e and o u tp u t lo a d r e s i s t a n c e .
The d e v ia tio n s frost
l i n e a r i t y can be c o n tr o lle d w ith good accu racy by c o n tr o llin g th e amount
o f r e s i s t a n c e in s e r i e s w ith th e d io d e s .
T h is r e s is ta n c e c o n tr o ls th e
c h a r a c t e r i s t i c end th e amount o f feed b ack v o lta g e .
The amount o f c u r r e n t in th e o u tp u t lo a d can be v a r ie d by changing
th e amount o f in p u t s i g n a l .
T1- C onstant
d- g current d evice .
T2- B e a m - T ube amplifier
T3- F eedback a m p l if ie r .
Gl BGf-GERMANlUM DIODES.
Rl - L oad resistance .
+
R ki Z ' . 000^
1.0 JJFD.
O
Ru
O
F ig . I. Constant - current generator circuit .
-1 0 -
CIRCUIT AiiaLXSIS
This cons t a n t- c u r r a n t g e n e r a to r i s u d e v ice which has a d i r e c t l y
in c r e a s in g o u tp u t v o lta g e us th e o u tp u t r e s i s t a n c e i s in c re a s e d r e s u l t i n g
In a c o n s ta n t c u r r e n t.
a m p litu d e .
The in p u t s ig n a l v o lta g e m ust rem ain c o n s ta n t in
The method by which th e l i n e a r o u tp u t i s o b ta in e d i s ,
e s s e n t i a l l y , one o f com bining n o n lin e a r d e v ic e s i n such & f a s h io n as to
r e s u l t i n a l i n e a r o u tp u t,
a m a th e m atica l a n a ly s is o f su ch a com bination
i s ex trem ely d i f f i c u l t b ecause o f th e n o n lin e a r c h a r a c t e r i s t i c s o f c e r t a i n
components o f the c i r c u i t .
When th e lo a d r e s i s t a n c e i s v a r ie d th e tr a n e conductance and p la te
r e s is ta n c e o f th e beam tu b e changes i n a n o n lld e a r f a s h i o n I t may be
seen from F ig . I t h a t th e lo a d impedance o f th e be am-tube a m p lifie r
c o n s is ts o f th e lo a d r e s i s t a n c e i n p a r a l l e l w ith th e c o n s ta n t d -o c u r r e n t
d e v ic e and th e feedback a m p lif ie r in p u t c i r c u i t .
A change i n th e lo a d
r e s i s t a n c e , I ^ , a ls o a f f e c t s th e c o n s ta n t d - c c u r r e n t d e v ic e c au sin g i t s
c h a r a c t e r i s t i c s t o change i n a n o n lin e a r m anner.
The germanium d io d es in
th e feed b ack in p u t c i r c u i t were u sed because o f t h e i r d e s ir a b le n o n lin e a r
c h a ra c te ris tic s .
T h e re fo re , r a t h e r th a n a tte m p t a rig o r o u s m ath em atical
a n a ly s is , w hich i s im p ra c tic a b le because o f th e n o n lin e a r c i r c u i t elem en ts
u s e d , a nonm&thematlcal approach w i l l be u sed i n e x p la in in g th e f u n c tio n
o f th e v a rio u s p a r t s .
I . Schade, 0 . H ., 1949, ELECTRON TUBES, 7 c l . I , p . 360, RCA Review,
Kadlo C o rp o ra tio n o f A m e r i c a , P rin c e to n , New J e r s e y .
The c i r c u i t shown i n F ig . I i s th e com plete c i r c u i t .
The beem­
power tu b e s were u sed f o r t h e i r d e s ir a b le p l a t e c u r r e n t- p la te v o lta g e
c h a r a c t e r i s t i c s and to o b ta in as h ig h power o u tp u t as p o s s ib le .
The
f u n c tio n o f th e unbypassed c ath o d e r e s i s t o r , Rk 2 * *a
feed b ack a
d e g e n e ra tiv e component o f s ig n a l to th e c o n tr o l g r i d .
When th e lo a d
r e s i s t a n c e , R^, i s z e ro , o r v e ry s m a ll, th e e n t i r e s ig n a l v o lta g e i s
dev elo p ed a c ro s s Rk2, which i s th e o n ly lo a d to a -c v o lta g e i n th e c i r c u i t .
T his v o lta g e i s fe d back t o th e g r id and alm o st e n t i r e l y d e g e n e ra te s th e
g r id s ig n a l v o lta g e .
Ag
in c r e a s e s a g r e a t e r p ro p o rtio n o f th e s ig n a l
i s developed a c ro ss th e lo a d r e s i s t o r , Ii^t and th e c o n s ta n t d -c c u r r e n t
d e v ic e , which a re i n p a r a l l e l .
The c o n s ta n t d -c c u r r e n t d e v ic e has a
v e ry h ig h impedance to th e a -c lo a d v o lta g e , th u s
impedance o v e r th e ran g e to be u s e d .
i s th e c o n tr o llin g
As th e s i g n a l d eveloped a c ro ss
in c r e a s e s th e amount o f s ig n a l v o lta g e a c ro s s th e cathode r e s i s t o r , Rk2,
d e crea se s.
T his e f f e c t a llo w s th e g a in o f th e s ta g e to in c r e a s e p ro ducing
more o u tp u t v o lta g e a c ro s s Rji, th u s te n d in g to keep th e c u r r e n t th ro u g h
Rji c o n s ta n t.
The s i g n a l g e n e ra to r u sed in th e g r id c i r c u i t has a low impedance
o u tp u t.
The re a so n f o r u s in g t h i s s ig n a l g e n e ra to r i s to p re v e n t any
a p p re c ia b le l o s s i n feed b ack s ig n a l v o lta g e which i s fe d back to th e beamtu b e a m p lif ie r g r id thro u g h th e s ig n a l g e n e r a to r .
The s c re e n g r id h as a 30,000 ohm d ro p p in g r e s i s t o r w ith a 0.25
m ic ro fa ra d bypass c a p a c ito r con n ected d i r e c t l y to th e c a th o d e .
These
components i s o l a t e th e s c re e n g r id from s i g n a l v o lta g e changes w ith
—1 2 —
r e s p e c t to th e c a th o d e .
T his h ig h r e s i s t a n c e a ls o p re v e n ts th e sc re e n
g r id from draw ing e x c e s s iv e c u r r e n t on n e g a tiv e p l a t e s ig n a l v o lta g e peaks
and allo w s th e s c re e n g r id power su p p ly v o lta g e to be q u ite h ig h .
The
feed b ack a m p lif ie r p l a t e v o lta g e may a ls o be o b ta in e d from th e same power
so u rc e t h a t s u p p lie s th e s c re e n g r id pow er.
In th e p l a t e c i r c u i t o f th e a m p lif ie r th e c o n s ta n t d - c c u r r e n t d e v ic e
e lim in a te s th e e f f e c t o f p l a t e c u r r e n t v a r i a t i o n s on th e p l a t e v o lta g e
w h ile p r e s e n tin g a h ig h impedance to any a -c s ig n a l o u tp u t.
th e g a in a t th e h ig h e r v a lu e s o f lo a d r e s i s t a n c e .
T his in c r e a s e s
The e f f e c t o f t h i s
c i r c u i t may be see n from a com parison o f th e graphs p l o t t e d i n F ig . 2
w ith th o se o f F ig . 3 .
F ig u re 2 i s a p l o t o f th e o u tp u t v o lta g e v e rsu s
lo a d r e s i s t a n c e f o r th e beam -tube a m p lif ie r alo n e w ith o u t th e m o d ific a tio n
o f th e c o n s ta n t d -c c u r r e n t d e v ic e .
F ig u re 3 i s a p l o t o f th e o u tp u t
c h a r a c t e r i s t i c s f o r th e a m p lif ie r w ith th e c o n s ta n t d -c c u r r e n t d e v ic e in
th e p l a t e c i r c u i t .
A com parison o f th e g rap h s shows th e in c re a s e d g a in
a t th e h ig h e r v a lu e s o f lo a d r e s i s t a n c e produced by th e c o n s ta n t- c u r r e n t
d e v ic e .
The c u rv e s o f F ig . 3 a re p l o t t e d f o r d i f f e r e n t b ia s in g arrangem ents
on th e two 6L6 tu b e s showing how th e g a in c h a r a c t e r i s t i c s may be c o n tr o l­
le d to o b ta in th e n e c e s sa ry f i n a l o u tp u t c h a r a c t e r i s t i c .
The 1000 ohm
r e s i s t o r s , Rk l and Rk2, u se d in th e cath o d e c i r c u i t s produced th e m ost
n e a r ly l i n e a r c u rv es and th e optimum b ia s f o r p ro p e r o p e ra tio n o f th e
tu b e s .
A A .5 - v o lt b ia s in g b a tte r y i s n e c e s s a ry i n th e g r id c i r c u i t o f th e
c o n s ta n t- c u r r e n t d e v ic e t o c o n tr o l th e c u r r e n t th ro u g h th e tu b e and cau se
IO1OOC
L oad
F ig. 2. O utput
resistance
R
12 ,OOC
14,000
OHM S
characteristic of beam - tube amplifier .
*
575 w
590 -Tl
!,000-/'- - 4 .5 Y
10,000
2 ,OOC
Fig. 3.
Load resistance Rl— o h m s
Output characteristics of modified amplifier
-1 5 -
th e power su p p ly v o lta g e to d iv id e e q u a lly a c ro s s th e two tu b e s g iv in g
250 v o l t s o f p l a t e to c ath o d e v o lta g e on each tu b e .
The c i r c u i t w i l l n o t
o p e ra te a s a c o n s ta n t c u r r e n t d e v ic e w ith o u t t h i s b ia s in g b a t t e r y . 1
It
produces a c o n s ta n t In crem en t o f b ia s which I s n e c e ssa ry e s p e c i a l l y a t
la r g e v a lu e s o f lo a d r e s i s t a n c e where th e r e I s c o n s id e ra b le s ig n a l v o lta g e
d ev elo p ed a c ro s s th e unbypassed cath o d e r e s i s t o r , RjtJl.
The u n ty p a sse d
c ath o d e r e s i s t o r , Rjt^* produces th e h ig h Impedance to th e a -c component
o f s ig n a l o u tp u t v o lta g e p re s e n te d Ty th e c o n s ta n t- c u r r e n t d e v ic e .
This
re d u c e s th e sh u n tin g e f f e c t o f th e c o n s ta n t- c u r r e n t s ta g e a t th e h ig h e r
v a lu e s o f lo a d r e s i s t a n c e .
The p l a t e and s c re e n g r id a re co n n ected d i r e c t l y to th e power s u p p ly .
W ithout a lo a d r e s i s t o r In th e p la te c i r c u i t th e p la te and s c r e e n g rid
w i l l b o th be a t 250 v o lte p o s i t i v e w ith r e s p e c t to th e c a th o d e .
W ith
t h i s c o n n e c tio n th e tu b e a c ts as a r e g u la r beam power tu b e which has th e
d e s ir e d c o n s ta n t- c u r r e n t c h a r a c t e r i s t i c s .
The power su p p ly v o lta g e i s
545 v o l t s , o f w hich, 45 v o l t s i s dropped a c ro s s th e b ia s in g r e s i s t o r s ,
RkI and Rjt2 and th e rem ain in g v o lta g e d iv id e s e q u a lly a c ro s s th e two 6L6
tu b e s .
The o u tp u t i s ta k en d i r e c t l y from th e p l a t e o f T2 , th e b east-tu b e.
The
d -c v o lta g e o f th e p l a t e c i r c u i t I s b lo ck ed by th e I m ic ro fa ra d co n d en ser.
I . V a lle y , G. E. and Wailman, H ., 1948, VACOUM TUBE AMPLIFIERS, p . UO
McOraw-Hlll Book Company, I n c . , New I o r k and London
—1C—
I The la r g e c a p a c ita n c e I s n e c e s sa ry to g iv e good low fre q u e n c y re sp o n se a t
s m a ll v a lu e s o f R j.
The lo a d r e s i s t a n c e , - ^ , c o n s is ts o f two n o n ln d u c tlv e ,
10,000 ohm, decade r e s i s t a n c e u n its I n s e r i e s .
This lo a d c o u ld r e p r e s e n t
any v a r ia b le r e s i s t i v e lo a d .
The o u tp u t v o lta g e I s a ls o a p p lie d a c ro s s th e feed b ack In p u t c i r c u i t ,
t h i s c i r c u i t , cs shown In F ig . I , c o n s is ts o f th e two germanium c r y s t a l
d io d e s , Gj and Gg, and th e r e s i s t o r s R2 , R^, and R^.
The two d io d e s
were
m atched f o r In v e rs e c h a r a c t e r i s t i c s by a p p ly in g a d -c v o lta g e a c ro ss each
d io d e ,in s e r i e s w ith 100,000 ohms o f r e s i s t a n c e .
d ir e c te d to make th e c a th o d e p o s i t i v e .
The v o lta g e p o l a r i t y was
In t h i s m anner, each o f th e e l*
germanium d io d e s a v a ila b le were t e s t e d and th e p a i r m ost n e a r ly matched
were s e l e c t e d .
% p la c in g a r e s is ta n c e I n p a r a l l e l w ith th e d io d es th e
o v e r a l l c h a r a c t e r i s t i c Impedance o f th e d io d e s can be made more s u ita b le
fo r th is c ir c u it.
be 150,000 ohms.
The optimum v alu e f o r th e r e s i s t a n c e , S g, was found to
T his r e s i s t o r red u ced th e impedance o f f e r e d by th e d io d e s
from 250,000 ohms t o 100,000 ohms a t 30 v o l t s .
Impedance o f th e com bination I s 40,0 0 0 ohms.
w ith Rj and
At 60 v o l t s th e t o t a l
T his Impedance i n s e r i e s
i s la r g e enough t h a t th e s h u n tin g e f f e c t on Uie lo a d , R j,
I s n o t s e r io u s o v e r th e ra n g e o f v a lu e s u se d f o r lo a d r e s i s t a n c e .
The v a r ia b le r e s i s t o r , R j, s e rv e s a d u a l p u rp o se .
I t c o n tr o ls th e
amount o f th e o u tp u t s i g n a l v o lta g e t h a t ap p ears a c ro s s R^.
v o lta g e I e th e In p u t to th e tr i o d e feed b ack a m p lif ie r .
th e amount o f feedback t o th e beam -tube a m p lif ie r .
T his s ig n a l
Thus, Rj c o n tr o ls
I t a ls o c o n tr o ls th e
d io d e c h a r a c t e r i s t i c by v a ry in g th e p o r tio n o f th e o u tp u t s ig n a l v o lta g e
d ro p a c ro s s th e d io d e s .
T h is c o n tr o llin g a c tio n can be se e n i n F ig . 4*
F ig u re 4 i s a p l o t o f th e v o lta g e d ev elo p ed a c ro s s
v o lta g e a p p lie d a c ro s s th e d io d e c i r c u i t .
a g a in s t th e t o t a l
In c re a s in g th e r e s i s t a n c e o f
Rj d e c re a s e s th e s ig n a l v o lta g e a c ro s s
and a ls o d e c re a s e s th e s lo p e o f
th e curve a t th e h ig h e r v a lu e s o f r e s i s t a n c e .
The a c tio n o f th e s lo p e can
be see n by draw ing ta n g e n t l i n e s to th e c u rv e s a t c o rre sp o n d in g a p p lie d
v o lta g e p o i n t s .
The a n g le betw een th e ta n g e n ts in c re a s e s a s th e a p p lie d
v o lta g e i n c r e a s e s .
T his ty p e o f c o n t r o llin g a c tio n by iy was found to be
d e s ir a b le t o e x ten d th e ra n g e o f lo a d r e s i s t a n c e v a r ia tio n s f o r d i f f e r e n t
lo a d c u r r e n ts w ith o u t c a u s in g an undue amount o f d i s t o r t i o n i n th e wave
shape o f th e o u tp u t v o lta g e .
Tto optimum s i z e o f
was d eterm in ed to be 3100 ohms to ap p ly th e
p ro p e r amount o f s ig n a l v o lta g e to th e g r id o f th e feed b ack a m p lif ie r .
T his v a lu e o f R^ allo w s R j t o be v a rie d o v e r th e ran g e from 50,000 to
80,000 ohms.
T his ran g e o f v a lu e s i s n e c e s s a ry to s e l e c t th e p ro p e r
o p e ra tin g c o n d itio n s f o r d i f f e r e n t v a lu e s o f lo a d c u r r e n t.
The 6 J5 t r l o d e tu b e was s e le c te d f o r th e feedback a m p lif ie r .
This
tu b e has good o p e ra tin g c h a r a c t e r i s t i c s f o r th e power su p p ly a v a ila b le
and s t i l l m a in ta in s a f a i r l y h ig h g a in .
The feed b ack a m p lif ie r c i r c u i t
was d e sig n e d t o have a g a in o f 1 2 , which i s more th a n am ple.
g a in th e beam tu b e can e a s i l y be o v e rd riv e n .
a m p lif ie r I e ta k en from th e p l a t e c i r c u i t .
f o r p o s i t i v e fe e d b a c k .
W ith t h i s
The o u tp u t o f th e feedback
T h is g iv e s th e p ro p e r p h asin g
The 100,000 ohm p l a t e lo a d r e s i s t o r , R j, p ro v id e s
h ig h r e s i s t a n c e i n th e p l a t e c i r c u i t to m a in ta in a h ig h g a in f o r th e
ViQLTAGt
APPUEO
•91»
t
VOLTS
♦
rtrv iz :
Fig. 4. Germanium
diode circuit characteristics
4— r -
-1 9 -
t r l o d a a m p lif ie r .
The c o u p lin g r e s i s t o r , R6 , may be d e sig n e d to c o n tr o l
th e amount o f feedback s ig n a l r e tu r n e d to th e beam -tube a m p lif ie r .
A
p o te n tio m e te r was t r i e d f o r t h i s purpose b u t e x p e rim e n ta l ru n s proved t h a t
Hg was th e more d e s ir a b le and e f f e c t i v e c o n tr o l f o r th e feed b ack v o lta g e .
-2 0 DISCUS3 ION CF RESULTS
The r e s u l t * o f th e e x p erim en t* ! t e a t s on th e c i r c u i t e r e p l o t t e d i n
F ig . 5 .
The c u rv e s have e l l been c o rre c te d by a d ju s tin g th e feed b ack
c o n tr o l, Rg o f F ig . I , to produce th e m ost l i n e a r r e l a t i o n s h i p between
o u tp u t v o lta g e and o u tp u t lo a d r e s i s t a n c e .
The cu rv es a r e a d ju s te d to
r e p r e s e n t th e I , 2 , 3 , 4# 5# 6 , and 7 o lllla m p e r e v a lu e s o f lo a d c u r r e n t.
The I ma. cu rv e was made m ost l i n e a r by a d ju s tin g R- to 50 ,0 0 0 ohms
and th e 2 ma. cu rv e i s c o r r e c te d by s e t t i n g Hg to 64,000 ohms.
Both o f
th e s e e u rv s s a re good ap p ro x im atio n s to l i n e a r c o n s ta n t c u r r e n t o u tp u t
beyond th e maximum v a lu e o f 2 2 ,0 0 0 ohms o f lo a d r e s i s t a n c e u s e d .
The 3 ma.
and 4 ma. c u rv e s e re c o r r e c te d by s e t t i n g Rg to 76,000 and 8 0 ,0 0 0 ohms
re s p e c tiv e ly .
The d e v ia tio n s from l i n e s r l t y Sn th e s e two c u rv e s , a t th e
h ig h e r v a lu e s o f lo a d r e s i s t a n c e , Ie p o s i t i v e .
To com pensate f o r t h i s
d e v ia tio n more r e s i s t a n c e I s needed i n Hg th a n I s r e q u ire d f o r I ma. and
2 ma.
Fo
th e I ma, and 2 ma. c u rv es I t was n e c e s sa ry to c o r r e c t f o r a
n e g a tiv e d e v ia tio n a t th e h ig h e r v a lu e s o f lo a d r e s i s t a n c e .
The same
r e l a t i v e sag o f th e s e c u r r e n t c u rv es can be n o tic e d on th e 3 ma. and 4 ma.
c u rv e s a t 1 1 ,0 0 0 ohms o f lo a d r e s i s t a n c e .
At t h i s lo a d r e s i s t a n c e th e
beam -tube a m p lif ie r g a in i s th e c o n tr o llin g f a c t o r f o r th e 3 ma. and 4 ma.
c u rv e s and th u s th e sag c a n n o t be c o r r e c te d by a d ju s tin g th e feed b ack
c o n tr o l, Rg.
The 3 ma. and 4 ma. c u rv e s a re good ap p ro x im atio n s
l i n e a r i t y up to 2 0 ,0 0 0 ohms o f lo a d r e s i s t a n c e .
to
The u s e f u l ru n g s co u ld
p ro b a b ly be ex ten d ed by p la c in g a v a r ia b le r e s i s t a n c e a c ro s s th e germanium
d io d e s i n p la c e o f R2 I n F ig . I .
T h is c i r c u i t change was n o t t r i e d by th e
T
vo lts
voltage —
L oad
8,000
L oad
Fig. 5. O utput
10,000
12,000
resistance
iTpoo
Rl —
iepoo
ispoo
20,000
22,000
ohms
characteristics of the constant- current generator.
-2 2 -
a u th o r, b u t e x p e rim e n ta l d a t a ta k en on th e d io d e c h a r a c t e r i s t i c s w ith and
w ith o u t th e p a r a l l e l i n g r e s i s t o r I n d ic a te d t h i s p o s s i b i l i t y .
I t I s doubt­
f u l t h a t th e lo a d r e s i s t a n c e ra n g e can be ex ten d ed a p p re c ia b ly f o r th e U ma.
curve b ecau se u n d e s ir a b le waveshape d i s t o r t i o n a p p a re n tly b e g in s a t
s l i g h t l y o v e r 22,000 ohms o f lo a d r e s i s t a n c e .
This e f f e c t was n o t v e r i f i e d
by e x p e rim e n ta l d a ta b u t I s in d ic a te d by th e i n t e r s e c t i o n o f th e dashed
d is to rtio n - b o u n d a r y curve on F ig . 5 .
The d is to rtio n - b o u n d a r y curve was d eterm in ed by o b e rv ln g th e o u tp u t
v o lta g e wave form on a c a th o d e -ra y o s c il lo s c o p e .
The o b s e rv a tio n s were
compared w ith th e c u r r e n t c u rv e s and I t was found t h a t above t h i s boundary
th e c u rv e s sagged beyond th e c o n tr o l o f Hg.
T h is curve i n d i c a t e s th e
approxim ate l a r g e s t lo a d r e s i s t a n c e t h a t can be u sed w ith e ac h c u r r e n t
w ith o u t n o tic e a b ly d i s t o r t i n g th e wave sh a p e .
The optimum v a lu e s o f
maximum lo a d r e s i s t a n c e a r e d eterm in ed by th e i n t e r s e c t i o n o f t h i s
boundary c u rv e w ith th e r e s p e c tiv e c u r r e n t c u rv e s .
The 5 m a ., 6 m a,, and 7 ma. cu rv es were o b ta in e d w ith Hg s e t a t
72,000 ohms.
The d i s t o r t i o n l i m i t s th e ra n g e o f lo a d r e s i s t a n c e f o r th e se
c u r r e n t v a lu e s to such an e x te n t t h a t fe e d b ac k c o n tr o l need n o t be v a rie d
f o r each in d iv id u a l c u r r e n t c u rv e .
The maximum lo a d r e s i s t a n c e t h a t can
be u sed w ith th e 5 ma. curve l a 16,000 ohms.
F or th e 6 ma. and 7 ma,
c u rv es 12 ,0 0 0 and 9 ,0 0 0 ohms r e s p e c tiv e ly a re th e maximum v a lu e s o f lo a d
r e s i s t a n c e u s a b le .
Ttwse v a lu e s a re e ac h lim it e d by th e d i s t o r t i o n
boundary.
No e x p e rim e n ta l d a ta was ta k e n f o r c u rv e s o f 8 ma. o r more because
-2 3 -
th a s ig n a l g e n e r a to r was In c a p a b le o f su p p ly in g enough in p u t s ig n a l v o lta g e .
Th* d is to rtio n - b o u n d a r y cu rv e i n d ic a te s t h a t t h i s c i r c u i t would have o n ly
a sm a ll ra n g e o f u s e f u ln e s s f o r c u r r e n ts above 8 a a .
The d i s t o r t i o n curve
in d ic a te s t h a t good l i n e a r i t y c o u ld be approxim ated up t o 6 ,0 0 0 ohms f o r
8 ma. and o n ly 4 ,0 0 0 ohms f o r 9 ma. o f lo a d c u r r e n t.
The o p e r a tio n f o r e ac h o f th e c u rv e s was s e le c te d f i r s t by t r i a l and
e r r o r and th e n , a f t e r th e u s e f u l ran g e o f o p e r a tio n was fo u n d , th e optimum
a d ju stm e n ts were made a s f o llo w s t
(1) The approxim ate m id -p o in t o f th e u s e f u l lo a d r e s i s t a n c e ran g e i s
d e term in e d f o r th e lo a d c u r r a n t d e s i r e d ,
(2 ) The lo a d i s s e t a t t h i s d eterm in ed v a lu e and th e in p u t s ig n a l
l e v e l a d ju s te d to produce th e p ro p e r o u tp u t v o lta g e to g iv e th e
d e s ir e d lo a d c u r r e n t .
(3 ) The lo a d i s th e n s e t to th e ra n g e maximum and th e feed b ack a d ju s te d
by Rj u n t i l th e d e s ir e d o u tp u t v o lta g e i s re a ch e d a t t h i s p o in t to
produce th e p ro p e r c u r r e n t th ro u g h th e r e s i s t a n c e .
These a d ju stm e n ts a re u s u a lly s u f f i c i e n t , how ever, i t was found d e s ir a b le
to check th e d e v ia tio n from l i n e a r i t y a t th e th r e e - q u a r te r p o in t o f lo a d
r e s i s t a n c e ra n g e .
I f th e d e v ia tio n i s to o l a r g e th e fe e d b ac k may be r e a d j­
u s te d to re d u c e i t to w ith in a to l e r a b l e v a lu e .
The c u rv e s a l l have e p o s i t i v e d e v ia tio n a t th e lo w er v a lu e s o f lo a d
re s is ta n c e .
T his d e v ia tio n can be red u ced i f i t ' s e f f e c t s a re u n d e s ir a b le
by p la c in g a 1500 ohm v a r ia b le r e s i s t a n c e i n th e cath o d e c i r c u i t o f th e
beam a m p lif ie r i n p la c e o f
o f F ig . I .
In c re a s in g t h i s r e s i s t a n c e
-24*
lo v e r s th e g a in o f th e beam a m p lif ie r as shown in F ig . 3 .
I f th is
r e s i s t a n c e i s in c re a s e d th e power su p p ly v o lta g e s m ust be r e a d ju s te d to
keep th e beam -tube p l a t e v o lta g e down and th e c o n s ta n t- c u r r e n t tube p l a t e
v o lta g e up to where th e power su p p ly v o lta g e d iv id e s e q u a lly .
t a i n s th e d e s ir e d o p e ra tin g p r o p e r tie s o f th e s e two tu b e s .
T his main­
I f th e
o p e ra tin g ra n g e o f lo a d r e s i s t a n c e i s s m a ll, say 8 ,0 0 0 ohms, th e c i r c u i t
can be a d ju s te d as i n ( l ) , (2 ) , and ( 3 ) above f o r t h i s ra n g e and th e t o t a l
o v e r a ll d e v ia tio n w i l l be i n s i g n i f i c a n t .
An e x p e rim e n ta l v e r i f i c a t i o n o f
t h i s was ta k e n f o r th e 5-raa. curv e end found to g iv e good l i n e a r i t y f o r an
8 ,0 0 0 ohm ra n g e o f lo a d r e s i s t a n c e .
The p l o t o f t h i s cu rv e i s F ig . 6 .
The fre q u e n c y u sed f o r th e cu rv es o f F ig . 5 was 2 ,0 0 0 c p s ,
This
freq u e n cy was s e le c te d b ecau se a t 2 ,0 0 0 c p s . th e c i r c u i t h as h ig h g a in
and i t i s n e a r th e c e n te r o f th e lo g rlth m ic freq u en cy ra n g e .
The fre q u e n c y re s p o n se s f o r th e 5-ma. and 2-m a, lo a d c u r r e n ts a re
shown i n F ig . 7 . and F ig . 8 . r e s p e c t i v e l y .
The c u rv es r e p r e s e n t th e
d i f f e r e n t v a lu e s o f lo a d r e s i s t a n c e f o r th e s e c u r r e n t v a lu e s .
Load
r e s i s t a n c e s f o r th e s e c u rv e s were s e le c te d to r e p r e s e n t th e e n t i r e u s e f u l
ran g e f o r each o f th e c u r r e n t v a lu e s u s e d .
I t was th o u g h t, by th e a u th o r,
to be u n n e c e ssa ry to p l o t freq u e n cy c h a r a c t e r i s t i c s f o r each c u r r e n t v a lu e ,
th e r e f o r e , th e 5-m a. and 2-m a, c u r r e n ts were a e le c te d as r e p r e s e n ta tiv e o f
th e h ig h e r and low er v a lu e s o f lo a d c u r r e n ts r e s p e c tiv e ly
The d a ta f o r th e freq u e n cy re sp o n se cu rv es was ta k e n by making th e
a d ju stm e n ts , f o r th e c u r r e n t v a lu e s e le c te d , as s t a t e d on page 2 3 .
The
in p u t s ig n a l v o lta g e from th e s i g n a l g e n e r a to r was m easured and m a in ta in e d
4 ,0 0 0
Load
Fig. 6. O utput
resistance
Rl —
ohms
characteristic of constant - current generator for small load range
Rl =I 2,000
Load v o l t a g e — volts
level
T
t'
R1= 1,000
Dv
F ig .
7
F requency
2
I ,OOC
FREvUI NCY
-
S ,OOf
IOpOV
20,000
CPS
characteristics
for
5 m a . load c u r r e n t .
•i 11
-*
R, =2 0 ,0 0 0 -n.
■
<
3 DB.
•
••
LEVEL
20
iopoo
20,000
FREQUENCY— CPS.
F ig. 8.
F requency cha ra ct e ri s ti cs for 2 m a . load cu r r e n t .
-2 8 -
c o n s ta n t as th e freq u en cy was v a r ie d th ro u g h o u t th e e n t i r e ra n g e .
The 5-ma. cu rv es o f F ig . 7 show a v e ry good f l a t fre q u e n c y re sp o n se
o v e r th e e n t i r e freq u e n cy ran g e f o r th e lo w er v a lu e s o f lo a d r e s i s t a n c e .
At th e h ig h e r v a lu e s o f lo a d r e s i s t a n c e th e f l a t p o rtio n o f th e re sp o n se
cu rv e becomes more n arrow .
was 100 to 10,000 c p s .
The freq u en cy ra n g e d e s ir e d f o r t h i s c i r c u i t
The 3 -d b . l e v e l i s w e ll o u ts id e th e s e freq u en cy
l i m i t s e x c e p t f o r th e h ig h e r lo a d r e s i s t a n c e s .
The p o in ts a t which th e
o u tp u t v o lta g e i s 3—d b s , below th e v o lta g e l e v e l a t 2C00 c p s , i s in d ic a te d
on each curve as th e 3 -d b . l e v e l .
At 10,000 ohms th e lo w -freq u en cy 3 -d b .
l e v e l i s r i g h t a t 100 c p s . and th e lo w -freq u en cy 3- d b . l e v e l f o r 12,000
ohms i s a t 103 c p s .
T his lo w -freq u en cy re sp o n se co u ld be improved by
u s in g a l a r g e r r e s is ta n c e f o r
and a l a r g e r b lo c k in g co n d en ser i n th e
feed b ack c o u p lin g c i r c u i t o f F ig . I .
The 2-m a. cu rv es o f F ig . 8 show th e same g e n e ra l c h a r a c t e r i s t i c s as
th e cu rv es f o r th e 5-ma. lo a d c u r r e n t.
The lo w -freq u en cy re sp o n se co u ld
be improved i n th e same way as f o r th e 5-®&. c u rv e s .
The cu rv es f o r th e
5 .000 ohm i n t e r v a l s a re q u ite e q u a lly spaced in th e ran g e from 200 to
10 .0 0 0 c p s .
T his shows t h a t th e c i r c u i t m a in ta in s i t s c o n s ta n t- c u r r e n t
c h a r a c t e r i s t i c s v ery w e ll o v e r t h i s ra n g e o f fr e q u e n c ie s .
freq u e n cy ra n g e f o r th e 2-m a. cu rv e i s 500 to 7000 c p s .
n e a r ly f l a t p o r tio n o f each c u rv e .
a p p ro x im a te ly 300 to 7000 c p s .
The b e s t
'IM s i s Lhe most
The b e s t ran g e f o r th e 5-ma. cu rv e i s
-2 9 -
StiKKtiiX AND COMCLtiSIONS
The problem o f d e v e lo p in g & c o n s ta n t- c u r r e n t g e n e r a to r was e s­
s e n t i a l l y worked o u t by s t a r t i n g w ith an a m p lif ie r w ith d e s ir a b le c h a r a c te r ­
i s t i c s and im proving i t s g a in - lo a d c h a r a c t e r i s t i c to be a c o n s ta n t r a t i o .
T his r a t i o i s th e lo a d c u r r e n t .
The feed b ack c i r c u i t a lo n e improved th e
c o n stan cy o f th e g a in - lo a d r a t i o b u t th e r e s u l t d e v ia te d p o s itiv e ly and
n e g a tiv e ly ab o u t th e d e s ir e d c o n s ta n t v a lu e o f lo a d c u r r e n t .
This e f f e c t
was red u ced by adding th e c o n s ta n t d -c c u r r e n t d e v ic e i n th e p l a t e c i r c u i t
o f th e a m p lif ie r .
The f i n a l r e s u l t s s t i l l d e v ia te a l t e r n a t e l y above and
below th e d e s ir e d r a t i o a s th e lo a d r e s i s t a n c e i s in c r e a s e d , b u t th e s e
e f f e c t s can be lim it e d to s m a ll v a lu e s by u se o f th e fe e d b ac k c h a r a c t e r i s t i c
and o u tp u t c o n t r o l .
U sing t h i s c o n tr o l and th e p ro ceed u re o u tlin e d w ith ­
i n th e t h e s i s a very good a p p ro x im atio n to c o n s ta n t c u r r e n t can be o b ta in e d
w ith t h i s c i r c u i t .
The lo a d r e s is ta n c e ra n g e i s sm a ll f o r th e l a r g e r v a lu e s o f c u r r e n t
b ecause o f th e lim it e d o u tp u t v o lta g e t h a t can be o b ta in e d from an
a m p lif ie r w ith o u t d i s t o r t i o n .
th e a m p lif ie r tu b e .
T his d i s t o r t i o n o c cu rs from o v e r-d riv in g
The d i s t o r t i o n may be red u ced by u s in g a l a r g e r b ia s
and a h ig h e r p l a t e v o lta g e b u t t h i s would n e c e s s i t a t e u s in g a n o th e r power
su p p ly f o r th e p l a t e v o lta g e s o u rc e .
The in c re a s e i n lo a d r e s is ta n c e
ra n g e would n o t be s u f f i c i e n t l y la r g e to w a rra n t t h i s i n m ost c a s e s .
The l i n e a r i t y o f th e o u tp u t v o lta g e t o lo a d r e s is ta n c e i s v ery good
c o n s id e rin g th e r e s i s t a n c e ran g e f o r which t h i s c i r c u i t i s d e sig n e d .
Improvements i n l i n e a r i t y co u ld u n d o u b ted ly be made f o r any p a r t i c u l a r
-3 0 -
c u r r e n t v alu e a t some c o n s ta n t In p u t fre q u e n c y and v o lta g e .
The freq u en cy re sp o n se o f t h i s c i r c u i t le a v e s som ething to be d e s ir e d
f o r th e low fr e q u e n c ie s a t h ig h lo a d r e s i s t a n c e .
This c o u ld be c o rre c te d
by c a r e f u l d e sig n o f th e r e s is ta n c e - c a p a c ita n c e c o u p lin g c i r c u i t s and th e
a m p lif ie r s t a g e s .
The fre q u e n c y re sp o n se I e v ery good a t th e m idrange o f
fr e q u e n c ie s and a v a r i a t i o n o f one o r two thousand c y c le s p e r second
would n o t change th e c u r r e n t l i n e a r i t y c h a r a c t e r i s t i c s a g r e a t d e a l,
A p r a c t i c a l u s e o f t h i s c i r c u i t c o u ld be made as shown i n P ig . 9
where a s ig n a l v o lta g e o f c o n s ta n t a m p litu d e I s d e s ir e d f o r in p u t f o r a
v a r ia b le number o f o th e r d e v ic e s .
F o r exam ple, suppose i t I s d e s ir e d to
have a c o n s ta n t s ig n a l v o lta g e o u tp u t from a c ro s s
r e s i s t o r s a re added to th e c i r c u i t .
and Rg as o th e r
The c o n s ta n t- c u r r e n t g e n e r a to r , as
d e sig n e d , would work v ery w e ll f o r t h i s ty p e o f o p e ra tio n a s lo n g as th e
t o t a l r e s i s t a n c e v a r i a t i o n was w ith in th e ra n g e c a p a b i l i t i e s o f th e
g e n e r a to r .
A nother p o s s ib le u se o f t h i s c i r c u i t may b e , as an example o f
N o rto n 's theorem .
N orton' a theorem s t a t e s t h a t a netw ork, c o n ta in in g
g e n e r a to r s and l i n e a r im pedances, mey be re p re s e n te d as a c o n s ta n t- c u r r e n t
so u rc e g e n e ra to r In p a r a l l e l w ith th e i n t e r n a l ad m ittan ce o f th e g e n e r a to r .
An example o f N o rto n 's theorem I s shown I n P ig . 10 where th e box c o n ta in s
th e o r i g i n a l g e n e r a to r which I e re g a rd e d as a c o n s ta n t c u r r e n t so u rce I
In p a r a l l e l w ith i t s I n t e r n a l a d m ittan ce G0 ,
r e p r e s e n ts a lo a d
conductance which may be connected t o th e netw ork.
-3 1 -
o—
\
OUTPUT
TERMINALS OF
CONSTANTCURRENT
GENERATOR
Fig. 9. A
t y p e of load for use with t h e constant -
current
GENERATOR.
«
CONSTANT-CURRENT GENERATOR
F ig. 10. E x a m pl e
of
N o r t o n ’s
theorem.
*
-3<iLIST OF EyUIBENT
Audio O s c i l l a t o r - H ew lett-Iuckfcrdj Kodel 200 BH,
Cathode-Rsy O sc illo sc o p e - DuMont, Type 208, No. 2836.
D-C Power S upply, 0-600 v o l t .
Decade .te s is ta n c e Boxes - U eneral itadlo Company,
Type 602J , Noe. 14060 sud 14040.
‘‘
:
Germanium D iodes - S y lv a n ia E l e c t r i c Company,
Type l-N -3 4 , Nos. 4 and 5 .
Vacuum Tube V o ltm e te r - H e w lett-P a ck a rd , Model 410-fi.
e
-3 3 -
LITERATURE CITED AND CONSULTED
C r u f t L ab o rato ry S t a f f , 1947, ELECTRONIC CIRCUITS AND TUBES,
p p . 325-350, M cG rw -H ill Book Company,
Nev Tork and London.
Eastm an, A. 7 . , 1941, FUNDAMENTALS OF VACUUM TUBES
p p . 264-294, 361-382, McGraw-Hill Book Company,
I n c . , New York and London.
E. K. S t a f f , M. I . T „ 1947, APPLIED ELECTRONICS,
p p. 155-165, 1 7 4-196, 378-418, 525-538,
John W iley and S o n s, I n c . , New Y ork.
RCA, 1940, RBCIEVING TUBE MANUAL, pp. 5 , 8 , 1 2 -1 5 , 20-23,
111-116, 122-125, RCA M anufacturing Company, I n c .,
Nev J e r s e y .
RCA Review, 1949, ELECTRON TUBES, V ol. I , p p . 320-364, Radio C o rp o ra tio n
o f America, P r in c e to n , New J e r s e y .
S m ith , F . L angford, 1940, RADIOTRON DESIGNER'S HANDBOOK,
C h ap ter 8 , The W lre le e e P r e s s .
S ta n s e l , F . R ., "The C h a r a c t e r i s t i c s and Some A p p lic a tio n s o f V a r is to r s " ,
PROCEEDINGS OF THE INSTIIUTE OF RADIO ENGINEERS,
V o l. 39, p p . 3 42-358, A p ril, 1951.
T em an , F , Ees 1937, RADIO ENGINEERING, p p . 174-188, 248-256,
McGraw-Hill Book Company, I n c ., New York and
London.
V a lle y , G. E. and Wallman, K ., 1948, VACUUM TUBE AMPLIFIERS, p p . 409-441,
467-479, McGraw-Hill Book Company, I n c . ,
New York and London.
103057
3 1762 10015478 8
Sh58c
c o p .2
S h e ru n o d , W ill jam D.
A. c o n s ta n t- c u r r e n t g e n e ra to r
in th e a u d io -fre q u e n c y ra n g e .
date"
J
J
i s s u e d to
//
N S rT S
/2 2 /
ShS'ScC ,O D . “
2*-
103057
IyJ /